Re: AW: Define Continuous DC Voltage

[Rich Nute]

Hi John:


   SELV can protect under single fault conditions. BUT, as I tried to
   explain, under some conditions, it can allow a single fault *to persist
   undetected*, until eventually a second, unrelated fault occurs which
   then results in a serious hazard.

This is a problem of the double-insulation scheme: one
cannot know when the first insulation has failed.  So, 
your argument not only applies to SELV but also to 
ungrounded accessible metal parts and any other double-
insulation scheme.

If we pursue your argument, then we should outlaw double
insulation as an acceptable scheme of protection against
electric shock, independent of SELV.

And, we should add a new criterion that failure of any
safeguard should be obvious to the operator *without*
presenting a hazard to the operator.  An interesting 
design problem.

   With PELV, this does not happen: the grounding ensures that the
   protective device operates. 

This scheme requires that the path between the ungrounded
PELV pole and the grounded PELV pole be capable of carrying
the fault current until the protective device operates.  In
other words, the ungrounded PELV pole must carry 25 amps for
1 minute (or appropriate criteria).  In turn, this means the
fault current, 25 amps, must flow from the ungrounded PELV
pole through the PELV source to the grounded PELV pole.  In 
my experience, there are few PELV circuits that can meet this 
criterion.  In the PELV circuits I have worked with, the 25
A would cause the PELV source to open before the operation
of a protective device, and the mains voltage would appear on 
the PELV ungrounded pole.

   Well, perhaps I have made it clearer now. My beef with SELV is the ban
   on grounding, whereas PELV which is grounded AND double/reinforced
   insulated is clearly safer for systems extended in space.

Agreed.  In the products I deal with, this is our construction.
However, we do not test the capability of the ungrounded PELV
pole to carry fault current.


Best regards,
Rich




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Re: Define Continuous DC Voltage

[Rich Nute]

Hi John:


   Tetanus is a disease caused by a bacillus. Muscles spasm is tetany.

Not according to my (American) dictionary.

Tetanus has two definitions.  

The first is the disease or the bacterium that causes
the disease.

The second is a prolonged contraction of a muscle
resulting from rapidly applied repeated motor impulses.
This is what happens when a 50-60 Hertz current is 
applied to the muscle.

Reilly uses the word tetanus thusly:

...maximum muscle tension is achieved at an AP
(Action Potential) rate of about 80/s, leading to a
condition of maximum fusion termed tetanus.

Tetany is defined as a condition of physiologic calcium
imbalance marked by tonic spasm of muscles and often
associated with deficient parathyroid secretion.


Best regards,
Rich


ref:  J. Patrick Reilly, Applied Biolectricity from 
  Electrical Stimlation to Electropathology.
  ISBN 0-387-98407-0 Springer-Verlag, New York 





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Re: [Fwd: User Warning Signal Words]

[Rich Nute]

   According to ANSI Z35.4 the following definitions are provided:
   
   
   
   DANGER - Indicates an imminently hazardous situation which, if not
   avoided will result in death or serious injury. This signal word is to
   be limited to the most extreme situations.
   
   WARNING - Indicates a potentially hazardous situation which, if not
   avoided may result in minor or moderate injury. It may also be used to
   alert against unsafe practices.

   CAUTION - Indicates a potentially hazardous situation which, if not
   avoided may result in minor or moderate injury. It may also be used to
   alert against unsafe practices. 
   
   Note: DANGER or WARNING should not be considered for property damage
   accidents unless personal injury risk appropriate to these levels is
   also involved. CAUTION is permitted for property-damage-only accidents.

I feel the authors of these definitions neither consulted
a dictionary nor the users of warnings.  According to my 
Webster's Collegiate Dictionary:

warn (verb):  1a: to give notice to beforehand, especially
of danger or evil; 1b: to give admonishing advice to; 1c:
to call to one's attention; 2: to order to go or stay away:
to give a warning.

So, all of the ANSI definitions serve to warn.

warning (noun):  1: the act of warning: the state of being 
warned; 2: something that warns or serves to warn, especially
a notice or bulletin that alerts the public that a tornado
has been reported in the immediate vicinity or that the 
approach of a severe storm is imminent.

So, all of the ANSI definitions are warnings.

danger (noun):  1:  (archaic);  2:  (obsolete); 3:  exposure
or liability to injury, pain, harm, or loss a place where
children could play without danger;  4:  a case or cause of 
danger the dangers of mining.

So, the word danger refers to a thing.  

caution (noun):  1: warning, admonishment; 2: precaution; 3:
prudent forethought to minimize risk; 4: one that astonishes
or commands attention some shoes you see these days are a 
caution.

So, the word caution is defined as a warning.

The so-called signal words are fabrications unrelated to the
definitions of the words.  This is a shame because it dilutes the
power of the words.

In my experience, users do not understand the subtle differences
intended by the signal words.


Best regards,
Rich


Richard Nute
Hewlett-Packard Company
San Diego





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Re: HiPot testing of DC mains powered products

[Rich Nute]

Hi Chris:


I wonder why you need to hi-pot test a SELV-powered
product?  (I presume your external 48-volt dc source
is SELV.)

We make products intended for connection to an external
dc source (SELV).  We have never been required to 
perform a hi-pot test on such products.

The production-line hi-pot is a test that tests the 
insulation between the two input leads (in parallel) 
and the chassis (or accessible metal).  The insulation 
provides protection against electric shock.  Since no 
electric shock is possible from 48 V dc (according to 
the standards), then there is no requirement for the 
insulation to withstand the transient overvoltages (if 
any) on the dc mains.  Indeed, if the insulation were 
to fail, there would be no electric shock.


Best regards,
Rich




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Re: CLASS 11(DOUBLE INSULATED) 2/3-CORE CABLE

[Rich Nute]

Hi John:


   No; France and Norway have 'IT' systems, in which grounding is only to
   prevent static charge build-up; the neutral is grounded through an
   impedance at the sub-station. AIUI, this is used in mountainous
   districts where ground paths are long and of low conductivity. There are
   significant differences between the French and Norwegian systems.

Can you please explain what those differences are?


Thanks, and best regards,
Rich


Richard Nute
Hewlett-Packard Company
San Diego






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Re: Define Continuous DC Voltage

[Rich Nute]

Hi Dan:


   Does anyone know what voltage is used in electric chairs?  Just Curious.

I'm sorry you asked.  The electric chair is one of
the more barbarous methods of execution.

The voltage is a function of time, and varies with
the execution authority.  The voltage is in the range
of 1000-2000 volts, sometimes more, sometimes less.

For more than you would ever want to know, including
the voltage specs, see:

http://www.theelectricchair.com/

You will need to use search the site to find voltage
and other details.  Be sure to read biology of 
electrocution.  Also check out the botched 
electrocutions.

Here are other sites I found as the result of a web
search.  The descriptions and pictures are gruesome
and are not recommended.

http://www.geocities.com/CapitolHill/6142/chair.html
http://www.albany.edu/~brandon/sparky.html
http://hypertextbook.com/facts/NancyRyan.shtml
http://hypertextbook.com/facts/AprilDunetz.shtml
http://members.aol.com/karlkeys/chair.htm
http://www.pdimages.com/X0029.html-ssi
http://www.suburbanchicagonews.com/joliet/prisons/chair2.html
http://noop.rotten.com/chair/
http://library.thinkquest.org/23685/data/chair.html
http://www.capitalcentury.com/1907.html
http://www.fcc.state.fl.us/fcc/reports/methods/emappa.html
http://www.hatchoo.com/deathrow/
http://www.ariel.com/au/jokes/The_Electric_Chair.html
http://northstargallery.com/pages/Electric01.htm

This site has some body impedance data taken during 
several executions.  The descriptions and arguments
are explicit and gruesome.

http://www.dc.state.fl.us/oth/deathrow/drorder.html

Calculate the power (E x I) dissipated in the body.


Regards,
Rich












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Re: [Fwd: User Warning Signal Words]

[Rich Nute]

Hi John:


   I feel the authors of these definitions neither consulted
   a dictionary nor the users of warnings.  
   
   I'm sure that you are right, but what is the alternative to using
   existing words but with special definitions that make their meanings
   more precise? If you solve this one, you clear up a significant problem
   in standards-writing.

Well... I guess I didn't make my point.

The ANSI standard defines three classes of signal 
words.  

My point is that the actual signal word is largely 
unimportant to warning (the verb) the user.  The 
signal word calls attention to the warning.  The 
classes of attention-getting simply are not 
recognized by users (and are not consistent with 
dictionary definitions of the words).  

One could just as well use any of the described 
signal words (and maybe some others as already 
suggested here) or various suitable symbols for 
any of the severity classes of warnings.

The degree of detail in specifying classes for
signal words is not warranted.  Delete these 
specific requirements for signal words from the 
standards.  (We are over-standardized in this
case.)  Instead, concentrate on effective content 
of the warning.


Best regards,
Rich





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Re: Define Continuous DC Voltage

[Rich Nute]

Hi Gregg:


   There was also a very good (but short) article by Tektronix in the 70's
   called The Lethal Current.
   
   It concluded that currents between 100 mA and 3 Amps were more lethal that
   currents of more than 3 Amps because those high currents tended to 'restart'
   the heart.

Hmm.  Having been the manager of product safety at Tektronix in 
the '70's, I don't recall such an article.  At least not by that
name.

Electric energy causes various injuries to the body depending 
on the magnitude of the energy.  Only two of the injuries can 
lead to a fatality.

The two injuries are fibrillation of the heart, and overheating 
of internal organs, especially the liver.

Fibrillation is caused by ac current in the range of 50 mA to
500 mA (external connections) where the current pathway through 
the body includes the chest (and the heart).  Above 500 mA, 
fibrillation is not a likely consequence.  (And, I believe I
am correct in asserting that dc cannot cause fibrillation.)

Overheating of internal organs is a function of power dissipated
in the body, where the body impedance can be taken as 1000 ohms.
The power required depends on the time of contact.  Electric
utility linemen are subject to such injury.  Consider 1 ampere
through 1000 ohms is 1000 watts!  (The electric chair kills by
over-heating the internal organs, not by fibrillation.)

So, Gregg's statement that there is both a lower and upper limit 
for fibrillation is correct (although I do not agree with Gregg's 
values).


Best regards,
Rich






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Re: Define Continuous DC Voltage

[Rich Nute]

Hi Chris:


I'll attempt to answer the question as to the effect of
ac and dc current on the body (the hazard).

The discussion is in regard to three waveforms:

1)  ac sinusoidal -- 50-60 Hz.
2)  dc
3)  dc interrupted (equal on and off times) up to 200 Hz.

Each has a different effect on the body. 

For each waveform, the magnitudes of voltage and current 
at which the effect takes place are different.

The body is most sensitive to ac, where the current
reverses through the body.  Such currents can cause both
tetanus of various muscles, and fibrillation of the heart.

Dc does not cause either tetanus or fibrillation.  Dc 
with ripple or superimposed ac is still dc because the
current does not reverse direction.  From my reading of
research papers, there is no significant effect on the
body due to ac riding on a dc bias provided the current
does not change direction.

Interrupted dc (50% duty cycle, 0 mA off, up to 200 Hz) 
is surmised to have similar effect to that of ac.  I 
believe that UL modeled this, and came to the conclusion 
that such interruption could cause fibrillation.  (I 
don't believe any tests on animals or people were 
actually performed.)  Hence, the limitation on voltage 
for such waveforms.

Most of the research on live humans (grad students) was 
performed by Charles Dalziel, UC Berkeley, during the 
late 40's and early 50's.  Dalziel published numerous 
papers on his tests, most in IRE and AIEE journals.

Dalziel gave us the tetanus values for ac, and determined
there was no tetanus for dc.  Dalziel also gave us the
effect of frequency on humans.

During the 30's, 40's and 50's, UL also did some 
measurements on live humans (UL employees) to determine 
body impedance.

Most recently, Beigelmeier (Vienna) has measured himself.
His research is the basis for much of the data in IEC
60479, effects of current on the human body.

Almost all other research was either on live (anesthesized)
animals or on cadavers.

When discussing waveforms that are beyond the research,
we must identify the injury we wish to prevent.  If we
are considering 40 V dc which has an on/off period of 1
second, then the person can disconnect himself from the
source during the 0.5-second off period.  So, this would
be the same as a steady-state 40 V dc source which is
deemed non-hazardous.


Best regards,
Rich


ps:  Charles Dalziel is the inventor of the GFCI.



   Subject: RE: Define Continuous DC Voltage
   Date: Fri, 9 Nov 2001 16:10:29 -0500
   From: Chris Maxwell chris.maxw...@nettest.com
   To: Ken Javor ken.ja...@emccompliance.com,
   Tania Grant taniagr...@msn.com,
   Doug McKean dmck...@corp.auspex.com,
   EMC-PSTC Discussion Group emc-p...@majordomo.ieee.org
   
   
   Sadly, I can't give that frequency; but I think I know the reasoning
   behind Eric's question...
   
   AC signals under 200Hz are especially dangerous to humans because AC
   currents really screw up our nervous system and cause death by heart
   attack at very low currents.  It only takes milliamps of 60Hz AC current
   to kill a human being.
   
   On the other hand, people can withstand far more current from a DC
   source because it doesn't have the same effect on our nervous system.
   (Come on, who hasn't put a 9V battery on their tongue to test it out?)
   
   I think that this is the reasoning that the referenced standard uses to
   give two limits for AC and DC.  My GUESS is that someone (who loved
   to torture living organisms) must have performed tests to figure out how
   DC current affected people (or monkeys, or rats... something).  They
   then must have performed tests with different AC frequencies.  Perhaps
   they even plotted a graph of hazardous voltage/current versus
   frequency.  I would imagine that this is the type of data used by the
   IEC or any other safety organizationn to set hazardous voltage levels.
   Problem is...the standards don't give a graph or table of hazardous
   voltage vs. frequency, it just says DC and AC.   Since we don't have
   access to the graph we really don't know what happens at ultra low
   frequencies.   (Although I have a few rodents in my basement who are
   just asking to be test samples.)
   
   Of course, now there is the gray area of interpretation. (which keeps us
   all employed)
   
   For example, how would a safety engineer classify a 40V thermostat
   control signal (non current limited) with a five second hysteresis that
   prevents it from switching any faster than once every five seconds (0.2
   Hz).   Under normal conditions, this signal would switch once every
   couple of hours (0.00014Hz).  Is this hazardous AC (after all it is
   40V, and it does vary with time)?  Or is it non-hazardous DC.  
   
   Anybody want to tackle that question?  It may help us to figure out
   Eric's initial problem.  Remember to show your work...partial credit
   will be given :-)
   
   Just to show that I'm game... I'll take a stab.  My opinion 

Re: Safety Critical etc - the future

[Rich Nute]

Hi Peter:


As I have already written, I feel that safety-critical
component as well as safety-related component are terms
that require more consideration than is necessary.

Basically, as I understand what you have said, a safety-
critical component is one where its failure creates a
hazardous situation.  A safety-related component is one
where its failure does not create a hazardous situation,
and a second component now provides protection.

Rather, I prefer the term safeguard.  A safeguard is a
device or scheme that is specifically installed in a 
product to provide protection against a specific injury.

Unless we know how the injury occurs, we cannot prevent 
the injury.  If we know how the injury occurs, then we can
install a safeguard to prevent injury.

   Those components that encapsulate into one single component the 2 safety
   layers
   that are normally used to isolate the operator (and others) from a hazard.

I suggest that each of the two layers are safeguards.  
These safeguards cannot be encapsulated into a single
component because each safeguard must be independent of
the other such that it is not subject to the same failure
mechanism.

   All components that -by there function- may create a hazardous situation
   when defective, direct or indirect.

If we have a safeguard, then the product is safe as long
as the safeguard is functional.  The safeguard must be
independent of equipment functional failure. 

So, I do not accept the thesis of safety-critical
component and safety-related component.

   Both layers of a double insulation are in themselves not a safety critical
   component; once they are integrated into one part -called reinforced- they
   are.

I disagree.  Each insulation within a double-insulation 
scheme provides a safeguard function.  Because it is a
safeguard, I consider it safety-critical.  

The fact that most safety standards require protection in
the event of a fault in Basic insulation does not denigrate
Basic insulation to a non-safety-critical function.

Double-insulation is distinctly different from reinforced
insulation.  Double insulation is a scheme employing two,
independent insulations, Basic and Supplementary.  

Reinforced insulation is a single insulation whose performance
is equivalent to double insulation.

   A supply transformer of a not grounded SELV is a safety critical component.
   A supply transformer of a grounded SELV is a safety related component.

For me, whether or not the SELV output of a safety-isolating
transformer is grounded is irrelevant.  Two safeguards must
be interposed between the mains and the SELV.  In some 
situations, the grounding of the SELV output winding can
serve as the required grounded barrier (a supplemental 
safeguard to the Basic insulation, the principal safeguard).

   The art of safety thinking is finding and recognizing these double
   protection layers
   in equipment, processes and concepts (or the lack thereof).

I disagree.  I especially disagree with characterizing
safety thinking as an art.  If it is an art, then only
artists can know safety.

Safety is a legitimate engineering discipline, although not
yet developed to the point of being included in engineering
curricula.

Within HP, we think of safety in terms of the 3-block model:

+---++--++---+
| hazardous || energy   || body  |
| energy|---| transfer |---| susceptibilty |
| source|| mechanism||   |
+---++--++---+

A hazardous energy source is any energy source whose magnitude
exceeds the body susceptibility to that energy.  In engineering
terms:

hazardous energybody susceptibility

non-hazardous energybody susceptibility

The energy transfer mechanism is the way that energy is 
transferred to the body (usually by contact with the energy
source).

A safeguard is a device that replaces the energy transfer
mechanism and prevents energy transfer.  Usually, this is
an energy attenuator.  (Electrical insulation is an energy
attenuator that prevents sufficient energy from being 
transferred to the body.)

This is one way in which safety can be treated as an 
engineering discipline.  Using this model, energy sources
and transfer mechanisms can be quantified, and energy 
attenuators can be quantified.  Safety in any given 
situation can be an engineering problem of interposing a
safeguard between the hazardous energy source and the
body.

When we think of safeguards as being interposed between a
hazardous energy source and the body, then we can easily
identify the protection layers.

This is a too-short and unfortunately incomplete overview 
of our view of product safety.


Best regards,
Rich


Richard Nute
Hewlett-Packard Company
San Diego





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Re: Safety Critical Input Summary

[Rich Nute]

Hi Lauren:


There are different definitions for safety-critical 
component based on the different needs for identification
of such components.  I'd like to list a few, and then
offer yet a different definition of a safety-critical 
component.

1.  A safety-critical component is a component which 
appears in a safety report such as those produced
by third-party safety certification houses.  (Doug
McKean)

2a. A safety-critical component is any component the 
failure of which would lead to a hazardous condition 
of either the product or the system.  (Richard Woods, 
John Allen) 

2b. A safety-critical component is either component in
a two-component scheme intended to provide protection 
(against a hazardous condition) in the event of a 
fault in one of the components.  (George Alspaugh)

3.  A safety-critical component may be a specific 
construction, e.g., smooth, rounded edges, or color
of wire insulation, or a specific rating, or a 
warning, etc., rather than a component.  (Gregg
Kervill, George Alspaugh)

If you can't identify the hazard against which the safety-
critical component provides protection AND how it provides
the protection, then either the component is not safety-
critical, or the safety function it provides is not known.

There is one word that rarely appears in the discussion 
of safety:  safeguard.

A safeguard is a device or a scheme of construction which 
renders a product or system as safe.  A safeguard has
certain parameters which are critical to its effectiveness
as a safeguard.

Rather than identifying safety-critical components, we 
should be identifying the safeguards and their parameters
applicable to the specific scheme or product.

For example, Basic Insulation is a safeguard against 
electric shock.  Its safety-critical parameters include 
voltage rating, dielectric withstand rating, and 
temperature rating.

An enclosure may or may not be a safeguard, depending on
the construction of the product.  If the enclosure is a
safeguard against anything, then one of its parameters 
is robustness sufficient to withstand the impact test.  
Depending on the hazard against which the enclosure is 
providing protection, there may be other applicable 
parameters.

Another safeguard is the two, independent fixings of 
wire terminations where the wire carries a hazardous
voltage (or is adjacent to hazardous voltage).  

So, which is a better description?  

   The wire terminal is a critical component.  

Or: 

   The wire terminal is provided with two, independent 
   fixings.

A warning is a safeguard in that it imposes a specified
action on the part of the user.  Its parameters include
color, font, and size.  (Gregg Kervill)

A list of safety-critical components is largely useless
because the safety function of the component and its 
safety-critical parameters are rarely identified.

On the other hand, a list of safeguards must necessarily
identify the hazard which the safeguard protects 
against.  (And, safeguard avoids the problem of safety-
critical feature and compliance-critical component 
identified by John Allen and Oscar Overton.)

Best regards,
Rich





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Re: skinny power cords.

[Rich Nute]

Hi Jack:


   I'm having a problem with Rich's explanation in this particular case (I know
   it's often true, though).  How did resisitive heating occur *without*
   current flow?  It was clearly stated that the heater was switched OFF.

I believe that the process I described takes a 
lot of time.  It starts when the heater is first
used, i.e., a heavy current through the plug and
socket.  The heating due to the contact resistance
degrades the material between the blades of the 
plug due to pyrolysis, the decomposition of a 
material by heat alone.

The decomposition results in unknown materials 
between the blades.  Plastics are carbon-based. 
Decomposition of carbon-based materials tends to
reduce the size of the molecule, and the material
approaches pure carbon, a resistor.

So, we can assume that these unknown materials 
are resistive.  We will have a leakage current 
through the resistance.  

Once the leakage path is established, the heater 
does not need to be on for the process to continue.

Since this isn't a good resistance, some elements
will open, and micro-arcs will occur.  These micro-
arcs create new resistances, and the leakage current
will continue to increase.  And the arcs get bigger.

Etc.

I could be wrong...


Best regards,
Rich






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Re: skinny power cords.

[Rich Nute]

Hi Jim:


   Thanks Rich:  I suspect you're right. Isn't that mechanism exactly what the
   tracking index tests are meant to address?  I thought that any UL-approved
   wiring device like this would have a material that is designed to resist
   tracking, hence my speculation that contamination might be involved.  

No, I believe the UL tracking index tests do not address 
the scenario I described.

My scenario starts with heating the insulating material
to the point where it begins to pyrolyze, i.e., decompose
by heat alone.

The UL tracking index test starts with a drop of saline
solution to provide a resistive path on the surface of
the plastic insulator.  The micro-arcs occur in the saline 
solution.

In my scenario, pyrolysis, not pollution, leads to the 
micro-arcs.  

So, I don't believe the tracking index is necessarily a
predictor of tracking due to pyrolysis.

I could be wrong...


Best regards,
Rich





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Re: NEC Question BUT REMEMBER OSHA

[Rich Nute]

Hi Jack:


   You are implying, but not stating, that NEC has the force of law
   regarding the domestic environment.

I meant to state that the NEC does indeed have the force
of law not only for the domestic environment but for all
electrical installations within the scope of the NEC.

   This differs with my understanding, or lack thereof. I have
   always regarded the National Electric Code as a recommended set
   of standards and practices which enabled localities to reference
   NEC in their local building codes, rather than develop their own
   from scratch.

The NEC as published by the NFPA is indeed a recommended 
code.  It is specifically offered to authorities for adoption
as their Code.  For example, the States of Oregon and Washington
adopt each edition of the Code.  The adoption is NOT a reference,
but a true establishment of the NEC as the local Electrical Code, 
i.e., a regulation under the law.

(Most authorities adopting the Code also have a few variations
as well as identification of accepted safety certification
houses.  Sometimes, this is a pamphlet that supplements the
NEC book.)

However, various governments do indeed develop their own
electrical code.  The cities of Chicago and Los Angeles are
two examples.  

   Perhaps you can expand on where the force of law applies to the
   NEC with regard to portable, plug-in (not permanently wired) home
   appliances and such?

The adoption of the Code makes the Code a regulation under 
the law.  Usually the law is the one that establishes the
Building Code, of which the Electrical Code is a part.

I recently posted a message specifically identifying the NEC
Articles that specify third-party safety certification of 
appliances.  

I hope this answers your question!


Best regards,
Rich






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Re: skinny power cords.

[Rich Nute]

Hi Jim:


   I'm curious:  given that North American plug blades are 1/2 apart, there
   must have been substantial contamination to aid in 120Vac jumping that far
   (arcing).  Did you identify any sort of contamination or moisture?

I don't believe contamination is a significant factor
in events such as this one.

I believe such events start with a loose connection
between the plug and the socket (or between the wire
and the socket parts).  A loose connection means 
that the contact area is relatively small.  In turn, 
this means high current density at the point of 
contact.  

The smaller the contact area, the greater the 
resistance of the contact.

The smaller the contact area, the greater the current 
density at the point of contact.

These two factors contribute to heating of the two
parts, the plug blade and the socket.  Heating tends
to reduce the springiness of the socket part, and
of the connection between the supply wire and the
socket (because they are thermally connected).

The heating also tends to degrade the surface of the
insulating material in which the conductors are mounted.

Heating also enhances oxidation of the plating on the
parts, which further increases the resistance of the
connections.

If the plug-connected appliance is ON, arcing can
occur as the parts expand due to heating and make
various intermittant connections.  Arc temperatures
are very high, and can burn the surface of nearby 
insulating materials via radiation.

As the surface degrades, leakages occur across the
surfaces.  At this point, whether or not the appliance
is on or even connected is not a factor.  There is a
current path between the two poles along the surface
of the insulator.  This can either be between the 
socket parts, or between the wired parts.  The leakage
current causes further heating and micro-arcs where
the leakage path opens due to current density.  The
micro-arcs further damage the insulator until there
is nearly continuous micro-arcing.  I suggest this
is the source of the noise.  The heat from the micro-
arcing and the resistance of the carbonized surface 
of the insulator eventually lead to ignition and 
flames.

I admit that this is a hypothesis.  I believe that 
the process is more-or-less correct, but the details 
may not be correct.


Best regards,
Rich






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Re: NEC Question BUT REMEMBER OSHA

[Rich Nute]

Hi Gregg:


   Just to ensure that I have my understanding right - if the equipment is used
   where OSHA applies then it must be approved by a third party like UL

Yes.  More specifically:

   If... the product is used by an employee in the workplace... 

   Then... the product must be certified by an NRTL, 
   of which UL is one.

   If it is domestic then it does not (in most states.

No and yes.  

No, OSHA rules do not apply to a domestic place.  

Yes, NEC rules apply to a domestic place and do 
require third-party safety certification.

OSHA rules apply to the workplace, not to domestic places.
So, domestic places are not required -by OSHA- to have 
NRTL-certified products.

HOWEVER, the National Electrical Code applies everywhere,
including domestic places.  The NEC requires products,
including domestic products, to be listed by a third-
party engaged in the safety evaluation of products.  

The NEC does not specify the third-party.  During the 
process of adoption of the NEC by various city, county, 
or state governments, the government agency decides 
which certification houses are acceptable to them.  The
acceptable certification houses are published locally.

For a third-party certifier, this means the certifier
must not only apply to OSHA for NRTL, but must also
apply to every jurisdiction in the USA for acceptance
under the NEC.

Many, but not all NRTLs are also accepted by the various
city, county, or state governments under the local version
of the NEC.

Likewise, there are some certifiers who are accepted by
one or more governments under the NEC, but are not NRTLs.

There are a few pockets where local governments do not
require listing under the NEC.

In summary:

OSHA requires products used in the workplace to be
certified by an NRTL.

The NEC requires products used in an installation 
(including domestic places) to be certified by an 
organization designated by the local government 
agency charged with enforcing the NEC.

These are independent functions.

For all practical purposes, third-party safety certification
is required throughout the USA.  

Enforcement of both OSHA and NEC for cord-connected products 
is spotty at best.

Since virtually all products are NRTL-certified, OSHA spends 
its time addressing more immediate workplace safety issues.

Since cord-connected products are installed AFTER the 
electrical installation is complete and approved, and since
virtually all products are safety-certified, there is little 
or no enforcement of NEC-required certification.

   AND, does anyone have a list of States where certification is mandated?

I would be easier to come up with a list of where certification
is NOT required!  :-)  It would be a one-page list of cities
or counties which have very low population densities.


Best regards,
Rich







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Re: skinny power cords.

[Rich Nute]

Hi Gary:


Somewhere in my past, I've heard the rationale for
this conundrum.  I'm just guessing here.

Power cords and similar mains devices are sized 
based on rated load, and are not sized based on
fault-condition load.

The requirement should be that, under fault 
conditions, the device is capable of withstanding 
the fault until the overcurrent device operates 
without igniting or otherwise causing a hazard.  
It can get hot; indeed, it can exceed rated 
temperature under the fault, and it can fail, 
but it should not ignite or otherwise cause a 
hazardous condition.

A power cord is supposed to be sufficiently robust 
as to withstand the rigors of use.  There are 
different degrees of robustness according to use.
In other words, the power cord itself is not
expected to fail under normal conditions of use.  

So, the power cord should only be subject to load
faults.  Since the load is protected against 
faults, the fault-protection in the load also 
provides fault-protection for the power cord.


Best regards,
Rich






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Re: NEC Question

[Rich Nute]

Hi Mike:


   Can someone point me to a section in the NEC that says a piece of
   RESIDENTIAL computer equipment must be listed (NEC definition).  Article
   645  which requires a listed piece of equipment appears to apply to a
   computer room and not a residence.

In terms of the NEC, a computer is an appliance.
(Article 100, definitions.)  Also see the definition
for utilization equipment.

An appliance is also an equipment.  (Article 
100, definitions.)

Article 110-2 requires equipment to be approved.

Approved means acceptable to the authority 
having jurisdiction.  (Article 100, definitions.)

Article 90-7 states that ...equipment need not be
inspected at the time of installation... if the
equipment has been listed...

Listed is defined in Article 100.

So, the NEC says that a residential computer must 
be Listed.

   Basically I've been asked where its says a piece of computer equipment must
   listed/approved by a NRTL.  I'm ignoring the obvious liability implications
   should someone get injured for the purposes of this question..

The NEC does not require listing by a NRTL.  (NRTL 
is an OSHA concept, not a NEC concept.)  

The acceptable certification houses are defined by 
the authority having jurisdiction (the government
that is enforcing the code, i.e, the city, county, 
or state).  Many NRTLs are also accepted by all 
jurisdictions, but not necessarily so.


Best regards,
Rich



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Re: Light Emission from Professional Photography Flashes

[Rich Nute]

Hi Peter:


   Any limitations/requirements for amount of light emitted from professional
   photography flashes? Any UL, IEC or European standards which specify
   limitations?
   
   I know how many you like being photographed - but imagine what harm one of
   these flashes can cause to your eyes!

The root question is what is the maximum safe optical energy 
as a function of time for the eye?

I suspect there are many research documents for this eye 
parameter.  Check out this optical radiation safety calculator:

http://vision.arc.nasa.gov/personnel/jbm/home/exps/java/safe_txt.html

The calculator is described for situations such as when the eye 
is illuminated for photography.  I suspect this is for steady-
state and not for flash.  But, it should provide some references.


Good luck, and best regards,
Rich

 


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Re: UL - marking - gentle warning not to extrapolate.

[Rich Nute]

Hi Peter:


Thanks for sending the UL letter and UL Listing Mark page.

I stand corrected.

For Information Technology Equipment, UL's letter of March 31,
1995, authorizes the optional use of the file number as the
control number.

Apparently, this option is not available for non-ITE products.


Best regards,
Rich




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Re: Printable calendar for 2002

[Rich Nute]

Hi Robert:


   I got two NAV (Norton Antivirus) msgs back from the ieee.org that said my
   attachment had a virus in it.
   
   If so, please let me know, because there is nothing but text files (that I
   know of) and a simple batch program in that attachment.

The IEEE listserver checks all messages for viruses 
before sending them to our subscribers.

Your files that were sent to subscribers were zipped.  
I checked both the zipped and unzipped files for 
viruses (with up-to-date Norton AV, 7.03, scan engine
4.1.0.2, virus definition file 31017e) and found no 
viruses.  So, the file distributed by the IEEE server
seems to be clean.

So, it may be that the IEEE server removed the virus,
forwarded the message to our subscribers, and notified
you of the virus.


Good luck, and best regards,
Rich


Richard Nute
Administrator, IEEE emc-pstc listserver
c/o Hewlett-Packard Company
San Diego

Tel:1-858-655-3329
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e-mail: ri...@ieee.org




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Re: UL - marking - gentle warning not to extrapolate.

[Rich Nute]

Hi Gregg:


   The File number 'may' be used on things like cable (it is common to see UL
   and CSA file numbers) and other components - Control numbers (I forget the
   correct terminology) is required on some UR components but not all Listed
   products.

UL Listed wire:

According to the UL Yellow Book, UL-certified 
Appliance Wiring Material must bear the UR mark.

If the wire bears the UL file number, then it is
either in lieu of the manufacturer's name or is 
in addition to the manufacturer's name; the file 
number is NOT required.

UL Listed products:

The UL 4-character control number is ALWAYS 
required on a Listed product.  

UR Recognized Components:

There is no control number for UL Recognized 
Components (UR).  

The usual UL component certification mark is 
the manufacturer's name and model number.  

For some components, e.g., connectors, the UR 
mark either is not required or is optional.  

For some components, the UR mark is required.  

The UL Yellow Book identifies the required 
markings for components, including those 
components that must bear the UR mark.

I hope this answers your question.


Best regards,
Rich





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Re: UL - marking - gentle warning not to extrapolate.

[Rich Nute]

Hi Ron:


   1. UL in a circle (the famous UL Listing mark symbol),
   2. The word LISTED in capital letters,
   3. The assigned control number*, and
   4. Product identity (e.g., NWGQ, I.T.E., etc.)
   
   * The assigned control number is a designation that UL assigns to a 
 manufacturer, or manufacuring
   location (I have also seen UL file numbers used here, although I'm not sure 
 that UL objects to
   that).

The file number cannot replace the control number.
The control number is always required.

The file number can replace the manufacturer's name,
or can be used in addition to the manufacturer's
name.  

   So, given the above and IMHO, I would say that just a generic UL Listing 
 mark would neither be
   acceptable nor authorized by UL. However, I recommend contacting UL for 
 their official position on
   the use of their Listing mark.

The generic UL mark (the UL in a circle -- without the
other three items) may be used in advertising or on 
the product carton.  I believe UL has some guidelines 
for this use.


Best regards,
Rich




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Re: UL - marking - gentle warning not to extrapolate.

[Rich Nute]

Hi Chris:


   Does the file number need to be on the label?  Can the manufacturer
   leave the file number off of the label and simply put a generic UL (or
   CSA) mark on the product along with their nameplate identifying the
   manufacturer, model and serial number?

In my response to Amund, I indicated that UL requires
a manufacturer identifier, which is usually the 
manufacturer's name, but alternatively can be the UL 
file number.

UL also requires a product identifier such as the model
number, but does not require a serial number.

A generic UL label is not possible.  Section General of
your UL FUS Procedure identifies 4 elements that comprise
the UL certification mark:

1.  The copyrighted UL mark itself (UL in a circle).
2.  The word Listed.
3.  The category of equipment, e.g., ITE or PRINTER.
4.  The UL control number assigned to the manufacturer
for this specific category of equipment.

While you may be able to buy Item 1, or even Items 1 and 
2, on a generic pre-printed label, you must necessarily 
provide Item 3 because it describes your equipment, and 
you must necessarily provide Item 4 because it is unique 
to you.  All of these elements must be in reasonable 
proximity of each other.

I don't believe CSA has the same four requirements for its 
mark.  So, I believe you can buy the generic CSA mark.  Be
sure to verify this statement with CSA before taking any 
action.


Good luck, and best regards,
Rich






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Re: Power Plugs

[Rich Nute]

Hi John:


   Not to Continental countries, AIUI, because rewirable plugs are not
   available (maybe in Denmark still). 

Oh?  In April, 2001, I bought a re-wirable plug in Grenoble,
France, at a major chain store.  They had a nice selection!


Best regards,
Rich




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Re: agency seminar quality

[Rich Nute]

Hi Brian:


   I have, w/in previous 6 mos, attended seminars offered by (2) agencies. I
   learned much from both, and appreciated being able to talk to the gurus.
   
   But upon review of seminar notes and manuals from previous Product Safety
   Engineers, it would seem that issued materials  syllabus do not compare
   with seminars of years past.
   
   Is this a bottom-line issue, or is it just my perception? Comments?

Your question is almost too general to answer.

In my experience, the seminars are continuously
updated to reflect the most current versions of
the various safety standards and the must current
interpretations of the various certification 
houses.  

And, I have found that current seminars reflect much
better safety engineering than past seminars (which
were more oriented to strict conformity without 
rationale).

So, I would not expect current seminar notes to 
agree with older seminar notes.  I expect that you
got a much better course than the previous product
safety engineers.


Best regards,
Rich




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Generic safety standard

[Rich Nute]

Hi Peter:


   I agree with you with your statement rather than address products, safety
   standards need  to address hazards. However, the hazard standards must be
   made taking into consideration the environment in which the product is used
   light industry, heavy industry, medical, office, outdoor, home, etc.. For
   example, a product's leakage current should be stricter if product is used
   in outdoor non-protected envieonments or as medical equipment. Similarly, a
   product's mechanical hazards should be guarded more if used by ordinary
   persons and less if product is used by professionally trained persons. 

I agree that the limit values for various safety
parameters depend on the situation.

And that is my point.  The situation defines the
limit, not necessarily the product.  (However,
often, specific products are likely to be used in
specific environments.)

Furthermore, the standard needs to give guidance 
as to how the situation contributes to a limit
value.  

Using your example, exactly why should leakage 
current be lower for an outdoor environment than
for an indoor environment?  

This is not an easy question, although everyone
knows that leakage current must be lower for the
outdoor environment.  If 0.5 mA leakage current 
through the body is deemed acceptable in an
indoor environment, why is that same current 
through the body deemed unacceptable in an 
outdoor environment?  We have been taught that
the current through the body is the dangerous
parameter.  So, if 0.5 mA body current is 
acceptable, why does this change with environment?

We presume the presence of moisture makes a given
situation more dangerous.  Yet, the current is
established by the product, not by the environment.

Some would suggest that the presence of moisture
reduces the resistance of the contacts.  I would
agree.  Yet, still, the current is established
by the product (as a current source), and is
largely independent of the body resistance, with
or without moisture.

According to Whitaker (1939), the dc body 
resistance drops significantly under wet 
conditions.  This is probably due to improved 
electrical contact with the skin.  

Nevertheless, leakage current is largely a 
constant-current source, and is therefore 
independent of body resistance.

If we assume 0.5 mA from a 120-V source, then the
source resistance is 240 kohms.

Using Whitaker's data, the nominal dry body 
resistance is 11.45 kohms, and the nominal wet
body resistance is 1.86 kilohms.  For the dry
body, the current from a 120-V source through
250 + 11.45 kohms is 0.459 mA.  For the wet body, 
the current through 250 + 1.86 kohms is 0.4765 mA.  
The current is nearly the same!  

So, we still have the same question:  Why should 
0.5 mA be considered harmful in a wet environment?
There must be some data somewhere that explains 
why the body current in a wet environment must be 
less than that of a dry environment.


Best regards,
Rich








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Re: Applying the appropriate ENs

[Rich Nute]

Hi Gregg:


   Consider the number of PRODUCT standards written - these reflect the
   accepted degree of protection (Operator or User) for each product under
   specified operating conditions and accepted uses.

I invite consideration of INJURY, and the means by
which an injury can occur from a product.  It is
NOT the product that causes the injury, but an 
ENERGY SOURCE that is the cause of the injury. 

For an injury to occur, energy must be transferred
to a body part.  

Prevention of injury is by means of prevention of
transfer of energy to a body part.

Consider a thermal (burn) injury.  A burn injury 
will occur if sufficient energy is transferred from
a hot part to a body part.  For example, consider
aluminum foil and an aluminum bar, both at the same
high temperature.  The aluminum foil will not cause 
a burn, while an aluminum bar will cause a burn.  
The difference between the two is the amount of
energy stored in the aluminum and available to be 
transferred to a body part.

   Simplistically USE will change  safety. (Indoor - Outdoor and Underwater
   lights). Using the SAME STANDARD approach there is a VERY REAL risk of over
   design being forced upon manufacturers and product costs soaring.

I would suggest that the CONDITIONS under which the
transfer of energy occurs is the salient point, not
the product or its use.  These conditions may enhance
energy transfer or they may impede energy transfer.

Clearly, electric shock energy transfer is enhanced
by water or moisture contributing to the energy
transfer mechanism.  The mechanism is that water is
not an insulator, is a fluid, can displace air 
insulation, and can provide a conductive coating to
a solid insulator.  So, a generic standard would
specify that where moisture or water is present, then
safeguards must be employed to prevent the water from
compromising the insulation.

If the generic standard is written correctly, it will
not force overdesign.  Requirements can be written in
the from of If moisture or water is present, then...
Such conditional statements prevent overdesign.

   More subtle situations exist. We have recently obtained a UL Listing for a
   UK product (The First Pocket CDR) - the test lab suggested '065 but I
   insisted upon '950 because (amongst other things, the Creepage and clearance
   distances between the two standards is not compatible and as a result '950
   provides a higher level of protection for the user). There is no issue when
   two '950 products are interconnected, but the user safety of the '950
   product MAY BE REDUCED if a non'950 product is connected.

The presumption is that the larger the creepage and
clearance distances, the safer the product.

A clearance is really the use of air as the insulator.
As with all insulators, the state of being an insulator
is a function of the applied voltage.  At low voltages,
the material is an insulator.  At some high voltage, 
the material breaks down and becomes a conductor.  The
breakdown voltage is well-known.  

For the purposes of safety, the question is, how much
margin should be between the maximum applied voltage
and the breakdown voltage?  At some point, the margin
is excessive, and overdesign is forced on the 
manufacturer.  For the purposes of safety, there is no
good rationale for having different margins in different
safety standards, especially '065 and '950.

I contend that we must understand the physics of 
safeguards in order to use them effectively.  A generic
safety standard can address such attributes.


Best regards,
Rich



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Generic safety standards

[Rich Nute]

Hi John:


   But the standards for these products have to be written by experts in
   the design of these products - non-experts would not know the problems
   that can arise. To produce generic safety standards, experts in the
   design of ALL products would be required. These exist only on the
   newsgroups.(;-)

I agree that a generic standard does indeed require input 
from product experts.  Indeed, such requirements are already
available in existing product standards.  But, this does not 
require that a safety standard be a *product* safety 
standard.

   should be no unnecessary differences between product standards, but some
   differences are fully justified. For example, some products must be
   allowed to get very hot, or to have exposed moving parts, otherwise they
   would not work.

If we consider that a hot part will produce a burn injury
regardless of product, then appropriate generic requirements
can be written.  For example, an iron must be hot in order 
for the iron to do its job.  However, the handle must not be 
hot in order for the iron to do its job.  So, from a 
standards viewpoint, a generic standard must be written to 
accomodate hot accessible parts and require cool parts for 
those that must be manipulated in the normal operation of 
the product.  This is not so difficult!

   You are doubtless aware that IEC/EN60950 (and, I suppose, UL1950) has an
   extensive text on hazards.

Yes.  The text is a notable beginning (but hardly extensive)
towards a generic safety standard.  

   While the products I deal with are computer 
   peripherals, I had the opportunity the other day to 
   attend a seminar on insulation diagrams used for 
   medical products.  The interesting fact is that the 
   seminar did not address anything unique to medical 
   products. 
   
   This may be down to the presenter, rather than to the question of
   whether medical products require special safety standards (which they
   do).

The presenter deliberately focussed on medical products.

I disgree that medical products require special safety standards.
Having experience in the safety of medical products, the 
principal differences between a medical product safety standard 
and other product safety standards are the limits for the various 
safety parameters.  Leakage currents for applied parts are very 
much lower than for other parts.  The means for accomplishing 
such lower leakage currents is the same as for other products -- 
suitable insulation.  

I envision generic safety standard in which only the limit 
values are a function of the product.


Best regards,
Rich



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Re: Applying the appropriate ENs

[Rich Nute]

   It is the primary end use of the product that dictates the standard(s)
   required.

Unfortunately, this archaic and provincial view on 
the part of standards organizations that standards 
should be per product has created problems that 
most of us would like to avoid.

Consider product safety.  The safeguards required
for protection against electric shock, for example,
are independent of the product.  Although we have
product standards for hair dryers and for computers, 
the requirements for safety are common.

Rather than address products, safety standards need 
to address hazards.  

While the products I deal with are computer 
peripherals, I had the opportunity the other day to 
attend a seminar on insulation diagrams used for 
medical products.  The interesting fact is that the 
seminar did not address anything unique to medical 
products.  Rather, it addressed a tool -- 
insulation diagrams -- that is equally applicable 
to my products and all other products where 
protection against electric shock is required.  
Indeed, the insulation diagrams of some of my 
products are identical to the medical product 
insulation diagrams presented in the seminar!

(For the purposes of electric shock, the only 
differences between a computer peripheral and a
medical product are the limit values used for the
various parts of the equipment.)

I would like to see safety standards based on the
hazards.  I would like to see separate, independent
safety standards for electric shock, electrically-
caused fire, thermal injury, moving parts (kinetic
energy), etc.

(Note that the USA and Canada already have 
independent safety standards for x-radiation and 
electromagnetic radiations.  These standards are 
based on the hazard, not on the product.)

Doing this job is not easy.  If you compare 
product safety standards, you will find much in
common, but you will also find differences.  It 
is these differences that cause difficulties in 
writing a generic safety standard.  

Committees are reluctant to discard any 
requirement on the basis that products built to 
the standard have a good record.  

Likewise, committees are reluctant to introduce 
a new requirement because it may cost 
manufacturers more money in the product design,
and products built to existing requirements have
a good record.

Virtually no one is willing to invest in
research in product safety in order to make 
decisions on whether or not a safety requirement
is an effective safety requirement.  

There are a few -- very few -- exceptions.  The
basis of IEC 664 (dimensioning of insulation) is 
research.  More recently, the CES has published
research data on TV fires.

Our EMC colleagues don't appear to be so hampered.
They have peer-reviewed journals, and annual
symposia reporting on the results of research.

A bunch of IEEE folks are doing their best to set
up an IEEE Product Safety Society.  I would hope
that this society will serve to improve product
safety, to bring it to the same level as EMC.


Best regards,
Rich






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Re: Photo Sensitive Epilepsy. (PSE)

[Rich Nute]

Hi John C. and John W.:


   Would anyone have any guidelines on how to design computer graphics
   in such a way to avoid inducing Photo Sensitive Epilepsy in anyone
   who suffers from that complaint ?
..
..
..
   A web search for 'epilepsy AND photosensitivity' will probably give you
   more information that you will ever need.

I did this (photosensitive epilepsy) and did indeed find
more information than I would ever need.  I did find a very 
informative site that seems to answer John C's questions.  
See:

http://www.epilepsytoronto.org/people/eaupdate/vol9-3.html

Interestingly, the flicker rate of computer displays is not 
considered provacative in terms of photosensitive epilepsy.

However, the article does discuss the effects of spatial
contrast in a display, whether TV or computer.


Best regards,
Rich





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Re: Limited Current

[Rich Nute]

Hi John:


   Are you sure that you can get appreciable X-radiation at 8 kV? In the
   days of monochrome TV with 9 and 12 inch tubes running at 8 kV, we never
   bothered about X rays, but projection sets running at 25 kV were known
   as a radiation hazard. Maybe tolerance levels have been reduced greatly
   over 50 years? 8 kV seems high for 1 inch tubes, anyway. The 1CP1 used
   to require less than 1 kV, IIRC.

X-radiation is generated whenever an electron strikes
a target.  Some target materials are more efficient
at generating x-radiation than others, given the same
electron energy.  In a crt, the space inside the crt
is filled with x-radiation resulting from the bombardment
of the screen (or the shadow mask) by the electron beam.

The faceplate and funnel glass are x-radiation-attenuating
glass.  For conventional crts operating at 15 kV or less,
the thickness of ordinary glass necessary for the 
structure also provides adequate x-radiation attenuation.  
Above 15 kV, the glass usually needs to include some
specific x-radiation-absorbing materials such as lead or
boron.  (Lead glass is subject to x-ray browning so it is
not used for faceplate glass.)

The x-radiation frequency and magnitude is a function 
of the target material and the electron beam energy, 
keV.

As a general rule, x-radiation energies from electrons
at less than 5 keV cannot penetrate paper.  So, they 
cannot escape the glass envelope of the crt.  However,
because x-radiation energy (for a given target material)
is a power function of the voltage (as high as 20), the 
increase of x-radiation with increase of voltage can be 
dramatic.

Assuming a 1-inch crt is relatively thin glass, 8 kV
*may* be enough to produce measurable x-radiation at
the crt faceplate (rather than the standard 50 mm). 
Since the device is in the form of a goggle, and since
the eye is much closer to the crt than 50 mm, one would
want to measure the x-radiation at the crt faceplate.
(X-radiation drops as a function of the square of the
distance from the source, assuming a point source.)

No, I am not sure that there would be measurable 
x-radiation at the faceplate of an 8 kV 1-inch crt.
But, based on the above technical facts, I think it
would be prudent to consider the possibility of such
x-radiation.  The crt manufacturer should be able to
provide an isoexposure curve for the crt.


Best regards,
Rich





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Re: UL requirement for driver

[Rich Nute]

[quote]Hi,
I am basically an EMC guy and do not know much about UL requirements.  I
would like to know if there are any UL requirements for a driver with
output voltage swing of +/- 30V peak-to-peak.  The RMS voltage will be much
smaller, and the power will be less than 0.5 watt.  The driver is to be
used to activate an off-card electromagnetic coil, and a flex cable will be
used as interconnect.

I will appreciate your replies.

Regards, Ravinder
PCB Development and Design Department
IBM Corporation
Email: ajm...@us.ibm.com
***
Always do right.  This will gratify some people and astonish the rest.
. Mark Twain



[/quote]
Hi Ravinder:


I am assuming that the 30 V peak-to-peak will be accessible to the user.  There 
are two UL issues regarding your 30 V peak-to-peak voltage:

1.  Isolation of the voltage from hazardous voltages, e.g. mains voltage.  The 
isolation must be comprised of two safeguards:

-basic insulation plus a grounded barrier;
-basic insulation plus supplementary insulation;
-reinforced insulation (a single insulation equivalent to double insulation.

Each insulation must be a UL-Recognized insulation or plastic.  The 
grounded barrier must be constructed in accordance with the multitude of 
requirements specified in the applicable standard.

2.  The voltage must not exceed:

-30 volts rms;
-42.4 volts peak;
-60 volts dc.

Assuming the waveform is symmetrical, 30 volts peak-to-peak is less than 42 
volts peak.  So, your voltage meets the requirements.

If the 30 volts peak-to-peak is not isolated from the mains as described above, 
then the insulation of the 30 volts peak-to-peak must not be accessible and the 
insulation interposed between the operator and the 30 volts p-p must be at 
least basic insulation, and *may* need to be the same insulation as mains 
insulation (listed above).

The answer to your question is well beyond what we can do via e-mail.  I've 
only touched on the basic construction; there are many more applicable 
requirements.  


Best regards,
Rich


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Re: Limited Current

[Rich Nute]

Hi Rick:


   I am working on a project that delivers 8 KV to 1 CRTs mounted on a
   helmet mounted display. The output of the high voltage supply is limited
   to 100uA and assuming that the supply complies with paragraph 2.4 (2.4.4
   in particular) Paragraph 2.4.1 states that:

Note that the output must not exceed 2 mA dc in both normal 
condition and single-fault condition in the source.

You're okay for normal condition, but what is the output
with the worst-case single fault in the supply?  I would
guess that the supply is quite simple, and any fault will
result in zero output.

   
   Except as permitted in 2.4.6, segregation of parts of LIMITED
   CURRENT CIRCUITS from other circuits shall be as
 described in 2.3 for SELV Circuits
   
   So what this tells me is if the stored charge is less than 45 uC
   (2.4.6), it is considered safe with only basic insulation? If over the
   stored charge limit, it must be treated as an SELV circuit and isolated
   accordingly. 

No.  The circuit is either a Limited Current Circuit or
it is not.  If yes, then it can be accessible.  If not,
then it must be treated as a hazardous voltage circuit.

There are two sets of insulation which you must consider:

1.  The insulation between the energy source and the 8 kV
source.  I would guess that the energy source is an
oscillator operating from SELV.  I would guess that 
the 8 kV is from a transformer and possibly a voltage
multiplier.  So, the insulation between the energy
source (SELV) and the 8 kV is that of the transformer.
If the supply is SELV and the output is Limited Current,
then no safety insulation is required between the two
circuits.  If the supply is not SELV, then the insulation
between the source and the 8 kV must be the same as for
the insulation between the source and SELV, except that
the voltage rating must be the sum of the 8 kV and the
supply source voltage.

2.  The insulation between the body and the 8 kV.  If the
circuit is a Limited Current Circuit, then it is 
treated just as a SELV circuit, i.e., no insulation is
necessary between the body and the 8 kV.  A Limited
Current Circuit can be accessible; it is the current-
limited image of the voltage-limited SELV. 

   My application mandates that the anode wire be as small as possible, as
   it is bundled inside a larger cable grouping and is limited by the
   design of the helmet mount. In lieu of a standard HV anode wire I have
   found that a coax cable provides excellent performance when tested for
   dielectric strength. Upwards of 2 KV is possible without breakdown. I am
   considering grounding the shield to prevent static buildup and the
   possibility of an potential breakdown in the coax. So if this is in fact
   LIMITED CURRENT it seems to me the application is valid.

This is a different issue.  You are asking whether or not an
insulation rated less than 8 kV can be used in this application.
For the purposes of safety, no insulation is required.  However,
for reliability, it would be unwise to use an insulation rated
less than 8 kV as the insulation would be subject to early
failure.

Furthermore, despite the fact that the standard does not require
insulation for safety purposes, UL and most other certification
houses would require suitably rated insulation.

   My questions now are:
   1. If this in fact LIMITED CURRENT, is basic insulation such as the coax
   adequate?

See above.

   2. If the requirement is SELV, is it reasonable that the insulation
   between the center conductor and shield may meet the requirement for
   reinforced insulation?l

Yes and no.  For safety purposes, a Limited Current Circuit is
equivalent to a SELV circuit; no insulation is required.  A 
Limited Current Circuit can be accessible.  However, for 
reliability -- especially with such high voltage -- the insulation 
should be rated 8 kV minimum.  (The long-term effects of partial 
discharge will eventually destroy the insulation.)

   3. To add additional safety should the coax shield be tied to chassis
   ground or should the shield be tied to the anode return?

If the coax is rated 8 kV or more center-to-shield, then the
shield can be tied to ground (anode return).  If the coax is
not rated 8 kV, then don't use it (per above remark).

   I am especially concerned  because this cable routes against the body
   between the helmet and supply, and is in close approximation to the
   head. Any thoughts or comments you may have would be appreciated.

I would guess that the head might be especially sensitive to
partial discharges that might occur in the air between the 8 kV
and the skin.  I would certainly apply 8 kV insulation between 
the body and the 8 kV.  Even that may not be acceptable because
of the capacitive divider that can exist.  The best solution is
a grounded barrier (e.g., coax) between the 8 kV and the body.


Best regards,
Rich


ps:  Beware of 

RE: Need electrical characteristics of epidermis (skin)

[Rich Nute]

Sorry to post here but am running into a brick wall at trying to find out
something as simple as the resistivity and dielectric constant for skin!

Does anybody have a source?  Need quick, please.

   - Robert -


 



Hi Robert:


The skin resistivity and dielectric constant is NOT simple.

My reference is Reilly, Applied Bioelectricity from Electrical Stimulation
to Electropathology.

Resistivity varies with 
body location;
wet/dry;
electrode contact area;
electrode contact perimeter;
uniformity of current distribution below the contact;
the kind of tissue below the skin (muscle, fat);
thickness of the corneum (outermost layer of dead skin cells);
duration of current;
activity of sweat ducts;
etc.

Having said this, here are some resistivity values in
kilohm-centimeters-squared for dry contacts:

Arm dc 100-1,000
Arm 1.5-10 Hz  600-1,200
Fingertip   0-1 Hz 120-130
Palm30-65 Hz60-80

While Reilly has information on skin capacitance, he has no information on
dielectric constant.


Best regards,
Rich




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Re: UL vs CSA (IT product)

[Rich Nute]

Hi Amund:


   UL and/or CSA certification are mandatory within the electrical safety 
 area, to 
   have access to the US and Canadian marked. Right ?

Yes -- sort of.  Safety certification of IT equipment 
is mandatory in both Canada and the USA.  However,
both countries accept safety certification from a
number of certification houses, including but not
limited to CSA and UL.  So, certification is required
in both Canada and the USA, but not necessarily by 
CSA and UL.

Both UL and CSA can certify IT products for both
the USA and Canada.

Canada  USA
--  ---

UL  cUL markUL mark

CSA CSA markCSA-NRTL mark

(The marks are combination marks, i.e., they are
base logo marks with additions that indicate the 
certification applies to both countries.)

   1.Do they have the same status?

UL and CSA (and their certification marks) have the 
same status in both Canada and the USA.

   2.What requirements do the end users/ buyers have, do most of them prefer 
 one 
   of the approvals?

Not many customers pay attention to who issues the
certification mark.  However, among those that have
an interest, in general, Canadian folks prefer CSA,
and USA folks prefer UL.

   3.Do we have to go for both of them?

Not necessarily.  Your product must have safety
certification in both Canada and the USA.  

1)  Either CSA or UL can issue one certification 
that is acceptable in both countries.  

Or,

2)  CSA and UL can issue individual certifications
for their respective countries, i.e., you can
submit to CSA for Canadian certification and to
UL for USA certification.

Most of us in the USA use one certification house 
for both countries.  Some of us use CSA, some of us
use UL, and some of us use other certification
houses.


Best regards,
Rich




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Re: Component and material traceability

[Rich Nute]

Hi Jim:


There are a number of ways to prove traceability
of bulk-labelled components, i.e., components
that are not indivually marked.

   - unmarked components - may not be able to tell mfr and cat. no, let alone
   agency status and ratings

These are really bulk-labelled components.  

If you deliver the components to the production-
line in the original shipping container, then 
there should be sufficient identification on the 
shipping container label.  

If you re-package the components, and discard
the original carton, then I take the inspector
to the re-package site so that he can see the
original shipping carton and its label.

If we don't happen be re-packaging at that time,
I determine when the next shipment will arrive
and invite the inspector to return at that time.

(There is no requirement to retain the original
shipping carton for the inspector.  It certainly
doesn't make sense to warehouse used and empty
shipping cartons or even the labels cut from 
those cartons as some folks have done.  Inspectors
will sometimes jawbone such a requirement.)

   - wiring harnesses - agencies won't rely on surface markings - spool tags
   are at the harness mfr's plant not ours

If you are spec'ing and buying certified wiring
harnesses, then they are either individually
labelled or bulk labelled.  If bulk labelled, 
then I use the preceding process.

If you are not buying certified wiring harnesses,
then you're really non-compliant (at least for 
UL).  (Most wiring harness makers are in the UL
program -- all you have to do is include UL on
your harness diagram.)

(With regard to spool marking versus surface
marking... UL requires surface marking but does
not rely on it!  @#$%!!!)

   - plastic parts - can't tell what the raw material was and whether or not
   it's approved without seeing the label on the bulk package of pellets -
   that's located an ocean away in the injection moulding vendor's plant

There are two ways of doing this.

1.  Require your molder to provide parts under 
the UL molder's program.  This way you get a 
UL certification on the plastic part itself, 
or a bulk-label on the shipping carton, or a 
cert sheet in the shipping carton.  See UL 
746D.

Most molders are in this program, or will 
readily agree to join the program if that is
a contingency for getting your business.  I've
done this a number of times, with little or
no resistance on the part of the molder.  
(Some of our folks are engaged in this process
at this moment with molders in a country far, 
far away, and are getting good cooperation.)  
A molder in the UL program has a distinct 
marketing advantage over a non-UL molder.

Sometimes the molder will ask us to pay for
the program.  We agree, but he cannot use the
program for any other customer.  So, we have
yet to pay the molder's UL program costs!

2.  Set up a split inspection.  UL will lift those
items that are to be inspected at a separate
site, and set up a procedure for that site.
You get to pay for two inspections instead of
one.

   - boards stuffed out-of-house by sub-contractors

I'm not sure what you mean by this.  So, I'll 
answer both possibilities:

1.  Traceability of the board itself.  This is done
by the UL mark on the board.  In some cases, the 
mark may be under a component, in which case you
will destroy a board to prove the board is UL.

2.  Traceability of bulk-labelled components on the
board itself.  One way to do this is the split
inspection.  Another way is to set up an 
Unlisted Component at the board manufacturer's 
site.  Both are simply look for the mark type
inspections.

   - transformers and other sub-assemblies where the material in question may
   be buried deep inside

Same as the above descriptions, as applicable to 
your particular situation.

I've used all of these techniques -- successfully --
at one time or another in my career.

Your contract obligates you to prove that you are
using the components specified in the FUS Procedure
or equivalent.  

Bulk-labelled parts present a real challenge, 
especially if you don't have a bulk label to show 
the inspector.  In such a case, even if you invite 
him back at a different time, he can write a
Variation Notice which will simply state that he 
could not determine compliance of a specific part 
or parts.  If he does so, make sure he mentions 
that you invited him back on a specific date to 
perform that inspection.  Then, you will probably
need to work out a plan with a cert house supervisor
so that you don't incur undue expense, and so that
the inspector will be satisfied that you are using
the correct parts.  This is fully negotiable.

One item that you did not ask, and I have no answer
for:  cut-to-shape sheet plastic insulating material.
It loses its traceability at the sheet-cutting shop,
and there is no specific program by UL that I know
of that 

Re: UL approval IT equipment

[Rich Nute]

Hi Amund:


   We want our IT product to be UL approved. We purchase a modified power 
 supply 
   (PS) which is not UL appoved. The original PS is UL approved. The only 
   difference in the PS is the value of one resistor which means that we now 
 can 
   take 2.3A/28VDC out instead of 1.9/28VDC. It exist av TUV CB report on the 
   original PS.

1.  Even though only one resistor is changed, the
modified power supply is *NOT* UL certified.  
The modified power supply needs to be re-
evaluated for safety.  

Some of the original tests can be applied to 
the new power supply, and some tests must be 
repeated.

Generally, the electric shock requirements 
and tests can be carried over from the 
original power supply.  For example, the 
spacings and dielectrics of the original 
power supply can be applied to the new power 
supply.  

However, the supply is changed from 53.2 watts
output to 64.4 watts output.  This means that
the heating, short-circuit, overload tests, 
and similar tests must be repeated.

   A local test lab tells us that thay can approve the total IT product 
 (including 
   the PS), but I feel we can get trouble with UL during the audits (4 times a 
   year).

2a. The end-product can be safety-certified as package,
where the cert house does the applicable power
supply tests in the end-product.  The cert house
can rely on *some* of the testing of the original
power supply.

This, however, has nothing to do with UL, which 
is your objective.

2b. Since you want your end-product certified by UL, 
then you *MUST* have a UL-certified power supply 
(or UL must have tested the power supply as a 
part of your product just as indicated by your 
local test lab).


Best regards,
Rich




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Re: Sound Engineering Practice

[Rich Nute]

Hi Lauren:


   Does anyone have a practical or authoritative definition of Sound
   Engineering Practice?. 

The key word is sound.  Presumably we all know
what engineering practice is.

I checked Merriam-Webster's on-line Collegiate
Dictionary and found the following definition.
The 3rd definition would seem to apply in this
case.
 
Main Entry: sound 
Pronunciation: 'saund
Function: adjective
Etymology: Middle English, from Old English gesund;
akin to Old High German gisunt healthy
Date: 13th century
1 a : free from injury or disease : exhibiting normal
health b : free from flaw, defect, or decay sound
timber
2 : SOLID, FIRM; also : STABLE
3 a : free from error, fallacy, or misapprehension sound
reasoning b : exhibiting or based on thorough
knowledge and experience sound scholarship c :
legally valid a sound title d : logically valid and
having true premises e : agreeing with accepted views :
ORTHODOX
4 a : THOROUGH b : deep and undisturbed a sound
sleep c : HARD, SEVERE a sound whipping
5 : showing good judgment or sense
synonym see HEALTHY, VALID

Presumably, all engineering practice (at least
by the subscribers of this forum) is (or should
be) sound.  Indeed, I suggest that sound 
engineering is nearly a tautology.  (Look that 
up in your dictionary!)

Given the definition, I would suggest that the 
determination of whether a design represents 
sound engineering practice will be in the eyes
of the regulator, not in the eyes of the designer.
Its a lose-win situation.  You lose, the 
regulator wins.

You do the best job you can to soundly design the 
product in accordance with the standard.  This 
may mean that the designer should get up from his 
chair and take specific actions in regard of 3a, 
3b, 3d, and 3e of the definition.  

You said, One aspect of conformance is dependent 
on the application of Sound Engineering Practice.

I think it is smoke.  The statement would be 
nonsense without the word sound.  

Its a weasel-word that says the standard doesn't 
cover everything.  Which they could not say.


Best regards,
Rich




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Re: current carrying conductors

[Rich Nute]

Hi Richard:


A number of good comments have been made.

I'm not sure anyone covered the fact that
two wires make a bundle, and therefore each
single in that bundle must be derated.

So, a single 10 AWG is good for 32 amps at
20 C.  But, two 10 AWG wires in the same 
bundle are only good for 31 amps each at 20 C.

Here's a caculator that might be of interest,
but use cautiously as the calculator may not
apply to your situation:

http://www.geocities.com/CapeCanaveral/Lab/9643/awg.htm


Best regards,
Rich




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Re: virus targeting list?

[Rich Nute]

Hi George and Ravinder:


We really like to discourage off-topic discussions,
including virus warnings.

Here are some points I'd like to make:

1.  The IEEE majordomo listserver includes virus checking
(for known viruses) of all messages posted to the 
listserver.  IEEE virus checking is reasonably up-to-
date.  To our knowledge, the IEEE majordomo software
has never forwarded a virus.  (Also see item 4.)

2.  We discourage all mail formats except ASCII text.
I believe ASCII text cannot transmit a virus.  If you
are using Outlook or a similar mailer, we suggest you
use the plain text option when posting to emc-pstc.
(I'm not sure if Netscape mail has a plain text
feature.)

3.  We discourage attachments of any kind.  As far as I 
know, viruses are in attachments, not in ASCII text.
Majordomo has an attachment limit of 100k, so emc-pstc
messages rarely have attachments.  (We probably should
be discouraging V-card and similar signature
attachments.)

4.  Some viruses will use addresses from saved mail 
messages themselves, not just the address book.  So,
if a message was posted by you to emc-pstc and ends 
up in a mailbox that is attacked by a virus, then 
you will probably get that virus.  

5.  Some viruses insert bogus addresses into the From 
line.  So, you can't rely on the virus as having been
sent from the address on the message header.

6.  A virus cannot post a message to emc-pstc (except 
from your mailbox).  We have a closed list; only 
subscribers can post messages to emc-pstc.

7.  If you have not posted a message to emc-pstc, then
no one (except majordomo and the admins) know your
e-mail address.  This means that if you have not
posted a message to emc-pstc, then the address used
by the virus did not come from emc-pstc.

We feel we have some pretty good controls on virus-
spreading via emc-pstc.

We have no control over what happens when your posted
message with your address ends up in a mailbox that is 
subject to a virus attack.  Your address can be forwarded 
to anyone anywhere in the world.  That's one of the risks 
of e-mail, and it is multiplied if you post messages to a 
listserver such as emc-pstc.

If you have received an .exe attachment to an emc-pstc
message, please contact me or one of the other admins at
the footer to the emc-pstc messages.  We don't want even
a new virus to be forwarded by majordomo.  Armed with 
this info, we'll see what we can do to prevent any
such future distribution.  

If you want to discuss this further, please do so off-
line.  I, as well as our admins, would be happy to discuss
further any virus issues and the emc-pstc listserver.


With best regards,
Rich




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Re: CE test suite for computers

[Rich Nute]

Hi Ghery:


   Also EN 61000-3-3 needs to be considered.  Useless standards, if you ask me.

Agreed!

EN 61000-3-2 is driven by Euro power distributors
who don't want to correct for non-linear loads.
(But who have no quarrel with correcting for 
phase angle.)

I don't understand the drive for EN 61000-3-3.


Best regards,
Rich



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Re: Hot Chassis?

[Rich Nute]

Hi Dan:


   with a three prong NA cord set.  I have found that if I bypass the earth
   ground plug I can measure a 80 VAC potential from my chassis to earth

This is normal.

Consider the circuit:

  L o- 120 V rms
 |
 |
   2200 pf -
   -
 |
 | 
 o---   ~60 V rms
 |   |  (depending on the
 |   |  tolerances of the 
   2200 pf - |  capacitors)
   - |
 |   |
 |   |
  N o---)-   0 V rms
 |
 |
 PE o
 (open)  |
 |
 |
   - 
   / / /


The capacitors form a 2:1 voltage divider.  So, 
if the chassis is not grounded, then about 1/2 
of the supply voltage appears on the chassis.
(You need a 10-megohm input meter to measure
this voltage; otherwise, the meter impedance 
affects the measurement.)

The current is:

I  =  E / Xc

Xc  =  1/(2*pi*f*C)

=  1.206 megohms (for 60 Hz)

I  =  120/(1.206 x 10*6) or  ~100 uA

This confirms what the manufacturer told you.

   grounded bench and got zapped.  Is there guidelines regarding this?   I see
   the UL mark on this power supply.   I want to use a two prong NA cord set

This current is well below the two typical
values permitted by safety standards:

500 uA
   3500 uA

Some people can feel this current (i.e. 100 uA)
when they are solidly grounded and they lightly 
touch the chassis.  If they hold on firmly, most 
people cannot feel the current.  Disclaimer:  I 
am not suggesting that you do this.  

(We had a discussion about a month ago as to 
the physiology of the light touch.)

   the UL mark on this power supply.   I want to use a two prong NA cord set
   not a three prong NA cord set and I have been told OK by the manufacturer.

I disagree with the manufacturer on this point.
The safety of the product was designed on the
condition that it be connected to ground.  If
the unit is used without a ground, then one of
the two safeguards against electric shock is 
defeated.

With respect to electric shock, safety standards
require a principal safeguard and a supplemental
safeguard.  The ground is one of several 
supplemental safeguard schemes.  Without a ground,
then the customer or user has only the principal
safeguard providing protection.  If that principal
safeguard should fail, then there is a risk of 
electric shock.

If you want a two-wire product, then I urge you to
use a product whose safety is expressly designed 
for two wire connection.  Such products are known
as double-insulated and bear the square within
a square mark.


Best regards,
Rich




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emc-pstc archive and RCIC

[Rich Nute]

The RCIC archive for emc-pstc messages will be shut
down, permanently, some time this week.

The archive itself is preserved.

We are working on a new web site for the archive.
We expect to announce the new web site along with
a number of other web-based features some time this 
Fall.

We apologize that there is a gap in access to the
emc-pstc archives.  However, in the long run, we
believe you will like the new site.

At this new site, you will be able to access all 
emc-pstc messages, and you will be able to post
messages from that site.  You will be able to
customize your subscription for the topics and
authors of interest to you.  You will be able to 
read the messages at the new site, or have them 
sent to your regular e-mail address.

And, we have a number of other features that will
enhance the value of this forum.

If you are attending the IEEE EMC Symposium in 
Montreal, you will have an opportunity to preview
our new web site.  Check the bookmarks on the PCs
in the Internet Cafe or look for Jim Bacher.

If you have any questions, please contact me or
Jim Bacher off-line.


Best regards,
Rich





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TV nostalgia

[Rich Nute]

Okay... more nostalgia and a bit on safety
back in those days... so that we don't stray
too far from the subject matter of this
forum.

My first TV was a Motorola 7-inch round in
a Bakelite cabinet.  The speaker was the
same size as the CRT.

My second was the famous RCA 10-inch round
chassis with 32 tubes.  I could pull out
15 tubes and still have a usable picture.

Kyle mentions Packard Bell, which I considered
a straight-forward, good product.  It used the
Standard Coil turret tuner.

The one that won my respect was Muntz TV.
It was CHEAP!  When you looked inside the
chassis, there was nothing there compared to
the other TVs.  They really knew how to take
the cost out of the TV!  Amazingly enough,
its picture was among the best, and its 
reliability was indeed the best -- no parts
to go bad!  The company was owned by Mad
Man Muntz, the classic Los Angeles used 
car dealer.

In the mid-fifties, GE came out with a 
transformerless 17-inch TV.  One side of
the power line was tied to the chassis
(2-wire plug back in those days).  The
only protection was the plastic knob on
the shafts of the various controls.  When
servicing this TV, you quickly learned
never to touch the chassis!

The power supply was a simple full-wave 
rectified power line.  The tube heaters 
were connected in a series-parallel 
arrangement.

These sets were the initiation of UL's 
investigation into antenna coupling
capacitors.  These capacitors provided 
the isolation between the TV antenna 
terminals and the mains voltage.

TV sets of those days consumed between 
400 and 600 watts.  When they were turned 
on, the cold filaments were a very low 
impedance, so the turn-on current was 
very high.  The off-on switch was often 
mounted on the back of the volume control.  
Eventually, the contact resistance of the 
switch would grow to the point where the 
I**2*R power would melt the solder and 
the power wires would come loose.  It was 
common to have a customer report that his 
TV was dead, and it was due to the lack 
of a good connection to the switch.

At one company, we had metal bat-handle
toggles blow out of the switch due to the
cold filament load.

Out of this experience, UL developed the
requirements for the TV-rated switch,
which had specially-designed contacts 
that would not overheat when used in a
TV or similar application.


Best regards,
Rich







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Re: Creepage dist. for more than 1000V ?

[Rich Nute]

Hi Terry:


   I don't recall the Sony but do recall the Philco and that Zenith held out 
 with the `hand wired' chassis.

Now that you mention it... I do indeed
recall that campaign.  But, I did not --
then -- realize the context.

Today, looking back, that campaign was
really quite absurd!  But it worked!


Best regards,
Rich



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Re: Creepage dist. for more than 1000V ?

[Rich Nute]

   I saw the first PCB show up in TVs,
   
   Would you care to put a date on that?
   
   You can't change the facts.  So yes!  In the Middle to late 50's.  :-) 

Having been a TV serviceman until 1960 (end of
my college days), I saw no PCBs in USA TVs.

I do recall PCBs in circa 1963 TVs.  (Anyone 
remember the Sony tummy TV of the time?)  :-)


Best regards,
Rich




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Re: Class III anomoly

[Rich Nute]

Hi John:


   This came up at IEC TC74 WG8 and was the object of several 
   proposals. We could come to no agreement about whether Class III 
   included TNV, earth connections, internally generated voltages of 
   assorted energies, etc., so we dropped the subject. It plays no 
   part in the safety requirements of  IEC 60950, so resolving the 
   issue was unimportant. There is a definition which you may use for 
   what purposes you like, but it will serve mainly as a concept, not 
   a requirement.
   
   The definition of Class III is not the responsibility of the self-
   appointed rulers of the Universe, sorry, I mean IEC TC74, corporately,
   not individually. It is the responsibility of TC64. See IEC 61140.

Thank you for the reference to IEC 61140.

According to IEC 61140:

Class III equipment is equipment relying on 
limitation of voltage to ELV as provision for 
basic protection, and with no provision for 
fault protection.

While battery-operated equipment would nominally
be Class III, most laptop computers would not be
Class III due to the backlight voltage exceeding
ELV.

Furthermore, according to IEC 61140, such laptop
computers would be Class II.

I agree with Robert Johnson that these classes 
serve mainly as a concept, and that the safety 
requirements are largely independent of equipment
class.

My argument with the equipment class concept is
that few equipment are truly fit one of the 
classes.  I prefere to replace the word equipment
with the word circuit.  Now, I can apply the 
different class concepts to various parts of my
equipment and accomplish the safety design.


Best regards,
Rich













   From owner-emc-p...@majordomo.ieee.org Wed Aug  1 13:56:40 PDT 2001
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   Date: Wed, 1 Aug 2001 21:05:37 +0100
   To: emc-p...@majordomo.ieee.org
   From: John Woodgate j...@jmwa.demon.co.uk
   Subject: Re: Class III anomoly
   References: 200107311606.jaa21...@epgc196.sdd.hp.com
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   3b683241.1010...@ma.ultranet.com, Robert Johnson
   robe...@ma.ultranet.com inimitably wrote:
   This came up at IEC TC74 WG8 and was the object of several 
   proposals. We could come to no agreement about whether Class III 
   included TNV, earth connections, internally generated voltages of 
   assorted energies, etc., so we dropped the subject. It plays no 
   part in the safety requirements of  IEC 60950, so resolving the 
   issue was unimportant. There is a definition which you may use for 
   what purposes you like, but it will serve mainly as a concept, not 
   a requirement.
   
   The definition of Class III is not the responsibility of the self-
   appointed rulers of the Universe, sorry, I mean IEC TC74, corporately,
   not individually. It is the responsibility of TC64. See IEC 61140.
   -- 
   Regards, John Woodgate, OOO - Own Opinions Only. 
 http://www.jmwa.demon.co.uk 
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Re: Mains fusing

[Rich Nute]

Hi Kyle:


   I have a new product that includes an off the shelf UPS that is rated for
   230V ac operation and has an internal single pole circuit breaker on the
   mains inlet.  We want to target this product world-wide.  The UPS presently
   is CB and certified to EN60950 european only.  For North America we want it
   to have UL1950, and to obtain this, UL is demanding the breaker be double
   pole.

This is an unusual situation.

On the one hand, the UPS, with single-pole overcurrent
protection, has a CB to EN 60950 for use in Europe where
most mains supply plug configurations are non-polar.
There is no control that the overcurrent protection will 
be in the live conductor.

On the other hand, the UPS, with single-pole overcurrent
protection, is denied UL certification for use in the
North America where UL requires polarization of both
the UPS overcurrent protection and the mains supply plug 
configuration.  There is a reasonable control that the 
overcurrent protection will be in the live conductor.

There is indeed something wrong with this picture.

My guess is that the certification engineer is invoking
Table 1, Case B (UL 1950, 3rd).  (As someone had already
suggested, you should verify this with your certification
engineer.)  Probably, this is because he knows that you 
are marketing your product worldwide.

Since you have a CB, you are qualified for Case B 
independent of your UL certification.  You should point
this out to your certification engineer.

I would ask UL to investigate the product under Case A.
UL can, at its discretion, investigate products to
specific provisions of their standard.  UL can invoke
paragraph D of the UL foreword to the standard.  You
can even ask UL to so note this construction in the
UL report. 

In my experience, these proposals should get you around
this situation.

If you are still unable to use the single-pole overcurrent
protection, I would go to another NRTL.


Good luck, and best regards,
Rich





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Re: Vibration Perceived as Shock

[Rich Nute]

Hi Tania and Richard:

   When you find out any papers on this subject, please share with us.

I stumbled across a short, authoritative discussion
about why a mechanical vibration can feel like a
small electric shock.  Actually, it is the other 
way around.  At small voltages and currents, the 
stimulation is a mechanical stimulation, not an
electrical one.

This quote is from:

Applied electricity from electrical stimulation
to electropathology

by J. Patrick Reilly
Springer Verlag New York
ISBN 0-387-98407-0

*

7.1 Introduction

Sensory sensitivity to electrical stimulation depends on a host of
factors associated with the stimulus waveform, its method of delivery,
and subjective variables.  In most situations involving electrical
safety or acceptability, current is applied to the body by cutaneous
electrodes.  There are also practical applications in which electric
current may be applied subcutaneously or induced internally by external
electromagnetic fields.  Although the emphasis in this chapter is on
electrocutaneous stimulation, many of the principles discussed may be
applied to other modes of stimulation.  The reader is directed to
Chapter 9 for additional discussion of peripheral nerve stimulation by
time-varying magnetic field effects or by induced shock within intense
electric field environments.  In addition to sensory effects described
in this chapter, stimulation by electric current and electromagnetic
fields can also elicit visual and auditory sensations.  These will be
treated in Sect. 9.8. 

7.2 Mechanisms of Electrical Transduction

Current of a fraction of a microampere can be detected when the finger
is gently drawn across a surface charged with small AC potentials
(Grimnes, 1983b).  Such levels are roughly 100 times less than commonly
tested electrical thresholds.  Detection of such small current results
from electromechanical forces arising from electrostatic compression
across the stratum corneum (the outermost layer of dead skin cells).  As
analyzed by Grimnes, the electrostatic force K is

 AEV**2
K =   ---  
 2d**2 

(7.1)

where A is the contact area, E is the dielectric constant of the
corneum, d is its thickness, and v is the instantaneous voltage.  The
compression of the corneum would not normally be sensed.  But when the
skin is moved along the charged surface, there is a vibratory frictional
force on the finger that is maximized on each half-cycle of the
alternating voltage.  This vibrational force stimulates mechanoreceptors
and is responsible for the detection of microampere currents.  Grimnes
estimates that the minimum voltage contributing to a detectable
vibration is about 1.5V at 5OHz.

The detection of microampere currents through mechanical vibration is,
for most purposes, of passing interest, although it may be important for
a researcher to know about it when designing perception tests.  Of
greater significance is the mode of detection when the current level is
raised to roughly 0.1 mA or above.  At that point, perception can be
initiated by the electrical excitation of neural structures, according
to the mechanisms discussed in Chapters 3 and 4.

Exactly what is excited with electrocutaneous stimulation, and what is
the specific site of initiation?  At the lowest levels of stimulation,
it is likely that peripheral structures are involved, because these are
closest to the surface electrode.  Among fiber classes, the larger-
diameter myelinated fibers have the lowest electrical thresholds, and
circumstantial evidence presented in this chapter points to the
involvement of one or another class of mechanoreceptor.  The precise
site of cutaneous electrical stimulation is unknown; whether stimulation
is initiated at the axon proper, at the site of the generator potential
of sensory receptors, or along free nerve endings has not been
demonstrated.  Some evidence, however, exists, as noted in Chapter 4,
that the site of initiation is near neural end structures, including
receptors or free nerve endings.  Electrocutaneous perception is a local
phenomenon; subjects typically report sensation occurring locally at the
electrode site rather than remotely as might be supposed if the
excitation occurred on the axons of deeper-lying nerves.  It is only
when the current is raised substantially above the perception level that
distributed sensations are felt.

If the current is raised sufficiently above the threshold of perception,
excitation of unmyelinated nociceptors becomes possible.  Because of
their higher electrical thresholds and generally deeper sites, these
structures are not likely to be involved at perception threshold levels.
At still higher current levels (some tens of milliamperes for
long-duration stimuli), thermal detection due to tissue heating becomes
possible.  Neuroelectric thresholds may exceed thermal thresholds if the
waveform of the electric current is inefficient for electrical
stimulation, 

Re: IEC60417 Symbols for Power Over the LAN Back-Up Source of Supply

[Rich Nute]

Hi Peter:


   For back-up power supply connections to a device which has means for
   connection to a floating dc input of 44-57 Vdc / 15 A max (to serve as
   back-up source for the internal power over LAN source), what is the proper
   way of marking with symbols to IEC60417

Does the power-consuming device REQUIRE the source
to be floating?

If yes, then I don't believe there is any suitable
symbol in IEC 60417.  I believe you will have to
design a suitable marking indicating that the source
must have NO connection to earth.  Furthermore, I
would guess that you would need to specify a minimum
source-to-earth impedance.

I believe this source requirement to be highly unusual.

I suspect what you mean is that the supply circuit in 
the device is floating (which is the typical case).
If this is the situation, then there is no need to
so identify it as this is a design that will tolerate
any source configuration (i.e., negative pole is 
grounded, or postive pole is grounded, or neither pole
is grounded).

   2) To specify that it is a back-up source of supply (vs. primary source of
   supply).

Again, I don't believe there is any IEC symbol that 
represents the terminals as being for connection to
a back-up source of supply.  

Your situation is opposite that of the conventional
situation.  The conventional situation is where the
external source of supply is the normal supply, and
the internally-derived source of supply is the back-
up.  In your case, the internally-derived (LAN) is 
the normal source of supply, and the external source
of supply is the back-up.

For your product, the back-up terminals should be
suitably marked with rating markings.  However, 
connection of the terminals to a source of supply is
optional as the connection is for a back-up source
of supply.  

Identification of the terminals and their connection
for back-up purposes can be by text on the product
itself, or in the installation instructions.


Best regards,
Rich




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Re: You won't believe this ... Well, maybe you will.

[Rich Nute]

Hi Doug:


   done, the entire safety approval reduced to a simple 
   cfm rating fan for a chip both on the secondary 
   side of the power supply. 

The issue for me is:  What is the safety requirement
that requires cfm (I presume a minimum cfm)?

Reading between the lines...

The fan cools the chip.

The chip runs warm/hot and requires forced-air
cooling.

The chip heats the printed wiring board on which
it is mounted.

The temperature of the printed wiring board is
a function of the power dissipated by the chip 
and by the cooling of the chip.  Without the 
cooling, the PWB temperature would exceed the 
limits specified in the standard.

So, I presume the safety requirement is that of 
temperature of the PWB.  Without the fan, the PWB
temperature would rise above the limit value specified
in the standard.

For the purposes of safety, nobody cares whether the
fan is effective at cooling the chip, or even if the
chip gets so hot as to self-destruct.  We are only
concerned with the temperature of the PWB.

Working with these data, I see a number of ways out 
of this predicament.

1.  Control the fan by manufacturer's name and model
number.  The cfm is not necessary.  We simply 
know by test that the cooling provided by this
specific fan is sufficient to keep the PWB from
exceeding the allowable limit.

(Since the fan is a secondary circuit motor, you
will have to comply with those requirements, but
they are a separate issue to that of the PWB
temperature.)

2.  Control the fan by electrical ratings and physical
size.  The electrical ratings (power) are 
proportional to cfm.

3.  Exempt the secondary PWB from the temperature 
requirements.  Since the PWB is a secondary 
circuit (I presume SELV), then there is no shock
hazard in the event of failure of the PWB insulation.
So, the PWB provides no safety function in terms of
electric shock, and therefore the temperature 
requirements need not apply.

However, failure of the fan is an abnormal/fault
condition for determining fire hazard.  So, the 
unit must be tested for 7 hours without the fan
running to determine no fire.  

   So, I'm wondering some of the following: 
   
   1.  Have any you ever run into something 
like this before? 
   
   2. If you have, what did you do about it? 

I have not answered your questions.  But, I often
invoke these kinds of reasonings so that we don't
get stuck with traditional or single solutions that
otherwise would be onerous or impossible. 


Best regards,
Rich




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Dithering

[Rich Nute]

Hi Ken:


   In conclusion, dithering will help you meet a test requirement, but it might
   not actually reduce potential interferences.

I have come to the same qualitative conclusion,
and the EMC experts with whom I have discussed
this concept agree.

But, is there any evidence, anecdotal, qualitative,
or quantitative, that interference is not reduced?

In my home, AM radio, even for local stations, is 
useless due to interference (or is it due to poor
AM receiver design?).


Best regards,
Rich




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please accept my Apology

[Rich Nute]

EMC Forum, 

I apologize for the Shameless Advertisement on the Technocal Forum.  It was
not my intention to break any rules of the IEEE society.  I have been a
member for quite some time and thought that I have seen similar  FREE OPEN
HOUSE events in the past. I have learned much from this group and believe
that I have contributed positively as well.

After reviewing some past e mails, I realized that these aforementioned
events were in response to requests from members of the group. After
reviewing the rules, I have realized that this is contrary to the EMC-PSTC
rules.

Please accept my apology!  I hope to remain a member of the forum.

Regretfully Yours,

Bill

ps.  For those of you, out there, who know me, Please don't abuse me too
much.  I'll take what I deserve like a man!


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Re: Secondary Grounding

[Rich Nute]

Hi Chris:


Consider a product with two, independent protective
grounding/earthing connections.  This may be by 
means of two power cords (as is done for uptime
reliablity by employing parallel power supplies) or
by means of one power cord and a separate ground
connection (as, for example, by mounting in a 
grounded rack).

   Safety standards call for single fault testing.  For Class I equipment, one
   of the single fault test conditions is removal of the ground connection.

Agreed.  The disconnection of one ground is a single-
fault condition.

   I'm curious how most test labs would reconcile the two statements above.  My
   guess is that they would interpret removal of ground to mean removal of all
   ground connections.  So putting on an extra ground wire wouldn't help.  It
   would just make the safety engineer disconnect another wire to perform the
   test.  

I don't agree.  The requirement is that of a single-
fault condition.  If normal operation employs 
redundant grounding, then a single-fault condition
is that of failure of one ground connection.

   What if the product is used in a building or environment with an unreliable
   ground?  or How can you garantee that the product's ground  potential will
   always be equal to the potential of the floor where the user is standing

If the ground within the building installation is
subject to failure, then the fault is that of the
building installation, not of the product.  So, it
would be nonsense to require a product single-fault
no-ground test on that basis.  

A faulty ground in the building installation allows
cumulation of leakage currents from all equipment to 
be available on each and every grounded equipment, 
a truly dangerous situation because the cumulative
leakage current could be in the hundreds of 
milliamperes!

(Ironically, the equipment with a faulty ground would
be the only safe equipment in such a situation!)

I was recently invited to comment on the subject of
single-fault testing requirements for products with
multiple power cords.  My argument was based on the
idea that any product with multiple power cords is
professional equipment where the advantage of such
equipment is only achieved by connecting to multiple
power sources.  So, this is normal operation.  A
single-fault test is with one ground open (a meaningless
test when there is a second ground in place).  I
recommended a leakage current test with one power
cord connected which would simulate the situation 
where the redundancy was not used, and there was a
fault in the grounding system.


Best regards,
Rich



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Re: UL Mark

[Rich Nute]

Hi Dave:


   Can anybody tell me where it is defined at what point in production it is
   permissible to apply the UL mark? Can it only be done after the hi-pot has
   passed? Or is it OK to have the label applied before the test as long as the
   units are clearly marked as having failed hi-pot?

You should direct this question to UL.  :-)

This is one of those questions that if you 
should ask UL, you must be prepared to live 
with the worst-case answer.  This is a
question that is better not asked because...

Strictly speaking, the UL mark goes on *AFTER* 
the product meets all the UL requirements, 
including passing the hi-pot test.  I believe
this is specified in the front matter of your 
UL FUS Procedure.

But, putting the UL mark on AFTER these processes
implies a stick-on label and a specific production
step. 

These two implications may not be compatible with 
your production sequences.  And, such implications
would prohibit molding the mark into a plastic
part (which many of us do).

In real life, the mark can go on at any step in 
the production process.  In practice, UL looks the 
other way in terms of the sequence step in which
the mark is applied.

If UL had any doubts about your product and the
quality of your production, UL would probably
insist that the mark be applied AFTER the hi-pot
test.


Best regards,
Rich



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HP Product Safety jobs in Boise, ID, and Vancouver, WA

[Rich Nute]

Hewlett-Packard has 3 product safety jobs available.

Requisition   Position   Location
number
---      -

711022Safety EngineerVancouver, Washington
713386Regs/Safety Engineer   Vancouver, Washington
713501Regs/Safety Engineer   Boise, Idaho

For a job description, go to:

http://www.jobs.hp.com/

Click on advanced search.

In keywords, enter the requisition number.
In search in, click on requisition number.
(Ignore the other fields.)
Click on search.

Click on Product Safety Engineer.

If you are interested, send your resume to ME.  I will
forward it to the hiring manager (and I will be eligible
for a referral bonus).  If you apply directly from the
web site, I won't get a bonus for having posted the jobs
to emc-pstc!

If you want to talk about product safety and product
safety jobs at HP, or if you want to talk about HP, 
please feel free to call me, or send e-mail.


Best regards,
Rich
+1-858-655-3329
ri...@sdd.hp.com








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Re: Production Line Test Requirements - Medical Devices

[Rich Nute]

Hi John:


   Because continuity at low current does not ensure that the protective
   circuit will carry a large fault current - it might be 'hanging on by
   one strand'.

Yes, for one strand.  No, for five strands.

Some years ago, I did some experiments on what
problems the 25-amp test would detect.  I 
simulated broken strands by cutting them one
at a time.  With five strands intact, the circuit 
passed the 25-amp, 2-minute test.  It failed at 
4 strands and 1 minute.

(The tested wire was 18 AWG comprised of 36 
strands of 34 AWG.)

The ability of a few strands to carry the 25-amp
current depends on the free length of the 
those few strands, which in turn determines the
heat-sinking provided to those strands.  The 
free length was on the order of 3 mm.  

My experiment assumed the problem was caused by 
an incorrectly set wire stripper, that cut a 
number of strands.  So there was a very small 
free length of strands.

I published this study in the Product Safety
Newsletter, Vol. 10, No.1, January-March, 1997.


Best regards,
Rich





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Re: US Mains Plug/Earthing

[Rich Nute]

Hi Tania:


   I hate to call you an old-timer;-- I would rather state that you might be
   thinking of  UL 114 and UL478 standards that are no longer in force.   B
   ut I don't believe that even they allowed a willy-nilly change from groun
   ded equipment to one that is ungrounded, unless provided with a special g
   rounding plug adapter.  


Well... I *AM* an old-timer!  And you can call me that!

Back in the days of UL 114 and UL 478, there were no
classifications (in North America) equivalent to Class 
I and Class II, and the concept of double-insulation 
was barely touched upon in UL 114.

Likewise, (in North America) there were no insulation 
designations equivalent to Basic, Supplementary, and 
Reinforced.  The rule was simply that all insulations
must be UL-recognized insulations, and that all 
components must be UL-recognized components.

If you submitted a product with a ground wire, it was
tested as if it did not have a ground wire.  The 
justification was that, even though the NEC mandated 
grounded outlets for all new electrical installations, 
there were still many installations with 2-wire sockets. 

Insulations were tested by a hi-pot test.  The hi-pot 
test voltage was 1000 volts.  During the time of UL
114 and UL 478, the test voltage was changed to 2V + 1000,
where V is the maximum rated voltage of the equipment.

The spacings (not clearance or creepage) were HUGE by
comparison to today.  I believe they were based on a
wire strand escaping from a screwed connection.

But, I digress.  

My point is that under UL 114 and UL 478, certified 
two-wire products used a single insulation just as 
we do today for grounded products.  They did not use 
double or reinforced insulation.  The ground was 
something nice, but not necessary.  But, if you 
used a ground, it had to meet all the electrical and
constructional requirements!


Best regards,
Rich





   From owner-emc-p...@majordomo.ieee.org Thu May 17 16:09:50 PDT 2001
   Received: from sanrel1.sdd.hp.com (sanrel1.sdd.hp.com [15.80.36.45])
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   for emc-pstc-resent; Thu, 17 May 2001 18:57:35 -0400 (EDT)
   X-Originating-IP: [63.25.206.200]
   From: Tania Grant taniagr...@msn.com
   To: Allen, John john.al...@uk.thalesgroup.com,
   emc-p...@majordomo.ieee.org
   Subject: Re: US Mains Plug/Earthing
   Date: Thu, 17 May 2001 15:58:02 -0700
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   John,
   
   I hate to call you an old-timer;-- I would rather state that you might be=
thinking of  UL 114 and UL478 standards that are no longer in force.   B=
   ut I don't believe that even they allowed a willy-nilly change from groun=
   ded equipment to one that is ungrounded, unless provided with a special g=
   rounding plug adapter.  The equipment adhering to these standards may sti=
   ll be allowed to be shipped until 2005, I believe, provided that no major=
changes are being made to this equipment;--  at which point, the new sta=
   ndard (UL/CSA 60950) applies.   =20
   
   However, UL no longer allows new equipment to be submitted to these older=
standards.   I forget exactly the cut-off date when that happened.   =20
   
   The key point is that equipment defined as Class I under IEC/EN 60950 wou=
   ld be defined the same under the UL/Canadian 60950 standard and require a=
   n earthed connection.   Thus, short of redesigning completely the stated =
   equipment and making it Class II, there is no way that a 2-pin plug would=
be legal (or sane).
   
   taniagr...@msn.com
   
   
   
   - Original Message -
   From: Allen, John
   Sent: Thursday, May 17, 2001 8:13 AM
   To: emc-p...@majordomo.ieee.org
   Subject: RE: US 

Re: US Mains Plug/Earthing

[Rich Nute]

   Do EU manufacturers have to fit a suitable mains plug
   to appliances when exporting to USA?... or can it
   be supplied without a plug, putting the requirement on the user
   to follow the instructions - in my case, stating that
   a grounding plug must be used ?

There are two answers:

1.  Safety certifiers will require that a plug be fitted
to the equipment before it leaves the factory.

(As previously discussed, safety certification is a
de facto pre-requisite for selling in the USA.)

2.  Regardless of safety certification, in the USA, 
products are NEVER sold without a plug.

(Most Americans would NOT know how to buy a suitable
plug or how to wire a cord to a plug!)


Best regards,
Rich



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http://www.rcic.com/  click on Virtual Conference Hall,

Safety of CD discs at high read rates.

[Rich Nute]

Here's a web site that reports that CDs will come
apart if attempted to be read at 64x.

http://www.qedata.se/en-cdrom.htm

There is no data as to what happens in a real CD
drive.


Enjoy!
Rich







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Re: US Mains Plug/Earthing

[Rich Nute]

   I am in the UK, a customer in USA wants us to fit
   2 pin mains plugs to the Class 1 appliances he
   is going to be buying from us.
   
   He is very firm that there are no regulations in US
   that requires this to be so. Is that true?

The simple answer is that there is no regulation 
requiring a Class 1 appliance be provided with a
grounding-type (Class I) plug.  Indeed, one can 
buy a cheater plug (3-to-2-wire adapter) at any
electrical or hardware store.  However, the use
of the cheater implies an independent connection
of the ground wire to a ground (which is facilitated
by the cheater plug).  No one ever connects the
cheater ground to ground; in practice, it is
simply a means to connect the 3-prong plug to a 
2-wire socket.

Be aware that about 1/3 of the homes in the USA 
were built before the advent of the grounding-type
plug.  These homes, unless remodeled, still have 
only 2-wire mains sockets and wiring.

The more complicated answer is that all electrical
products in the USA must be certified for safety.  
This requirement arises from two, independent sets 
of regulations.

OSHA (Occupational Health and Safety Admiinstration)
regulations require electrical products used by 
employees in a workplace be certified for safety.

NEC (National Electrical Code) regulations require 
electrical products used in installations be 
certified for safety.

Safety certifications require Class I appliances 
be fitted with grounding-type mains plugs.

So, your appliance should bear a NRTL (Nationally
Recognzied Testing Laboratory) safety certification 
mark.  Your obligation under the safety 
certification is to provide the appliance with a
grounding-type power cord and plug.

Your customer does not understand the USA safety
certification regulations (which is not particularly
uncommon).

You should explain to your customer that the fitting
of a 2-wire mains plug requires you to remove the
safety certification mark.  You should then further
explain that the certification mark is required by
OSHA and NEC regulations, and that, while the 
product *can* be sold without the mark, the
regulations forbid the *user* from connected it to a 
source of supply.

In summary, appliance safety certification requires
a Class I product be fitted with a grounding-type
plug.  Safety certification is required for 
workplaces and for electrical installations (both
of which cover all possible uses of electrical 
appliances).

Note that enforcement of safety certification in 
electrical installations usually only occurs during
the construction and remodeling phases.  Appliances
installed after such phases are not subject to 
inspection, so enforcement of regulations on cord-
connected appliances is virtually zero (which doesn't
make non-certification acceptable under the law).

On the other hand, you can satisfy your customer 
by altering the design such that it meets the 
requirements for a Class II product.  This may not
be as difficult as it first seems.  If the appliance
is enclosed in metal, then all primary wiring must
be sleeved.  The other mains components must be 
similarly evaluated.


Best regards,
Rich






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Re: 120V appliance on 240V supply

[Rich Nute]

Hi Ravinder:


   I am interested in knowing if a 120V, 60Hz microwave oven can be safely
   used on a 240V, 50Hz mains supply with a step-down transformer.

In essence, you are asking if a microwave oven
rated for 60 Hz will operate safely at 50 Hz.

I would guess that the microwave oven uses both
a mains-frequency transformer and a mains-frequency
fan.

The problem is that of the quantity of iron in both
the transformer and the fan.  50 Hz requires more
iron to prevent saturation of the core.

If there is insufficient iron, the devices will 
tend to overheat at maximum or even normal load.
The fan will run slower.

If the devices are conservatively designed, they 
will work okay, although without margin, at 50 Hz.

Overheating of the transformer or the fan motor 
will initially produce an odor followed by smoke.
I believe microwave ovens have an internal fuse,
so this should operate before things get too hot.
In addition, microwave ovens are in metal 
enclosures, so if a failure should occur, the fire
is not likely to escape the enclosure.

As a general rule, for electro-magnetic devices 
such as transformers and motors, you run a risk of 
failure when the frequency goes down.


Best regards,
Rich



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EMC-PSTC survey results

[Rich Nute]

A summary of the emc-pstc survey results is now available 
on the Product Safety Newsletter web site:

http://www.ewh.ieee.org/soc/emcs/pstc/

The results are in a 16k pdf file.  If you are unable to 
access our web site, send e-mail to either Jim or Rich 
and we will be happy to send you a copy via e-mail.

We had 255 participants (from 850 subscribers) from 22 
countries.

While the summary does not include comments, we have 
read all of the comments and will be addressing many of
the issues that you have identified.  

From your responses and comments, the emc-pstc 
e-Community is highly successful in responding to your
needs for discussion of emc and product safety issues.
Not only are we good at the technical side, we're also
one of the best listservers (eCommunity) anywhere!  We
thank you for your contributions to our eCommunity.  Our 
subscribers' contributions make us successful!

The IEEE is developing new resources for eCommunities 
such as emc-pstc.  Jim and Rich are participating in 
this work, and are working to develop these exciting new 
resources to meet the needs (and resolve the issues you
identified in the survey) of the emc-pstc eCommunity.

We thank Dan Roman for putting the survey together, and 
for providing the results.

And, thank YOU very much for your participation.  


Best regards,
Jim Bacher  jim_bac...@monarch.com
Richard Nuteri...@ieee.org





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Re: UL 1419 Earth Leakage

[Rich Nute]

Hell Duncan:


   -   Would they use S1 when leakage testing our type of equipment?

Your equipment is cord-connected equipment, not
direct-plug-in equipment.  Therefore, S1 is not
used when testing leakage current from your
equipment.

   -   What is the definition of 'direct plug in' (it is not mentioned in the
definitions section)

Direct-plug-in equipment is the wall wart type
of product where the product is integrated into
the plug.  Examples are:

Small plug-in power supplies.
CO detectors
Some electronic room deodorizers
Night lights
Some electronic bug killers

   -   For 'direct plug in' equipment what is the reason for operating this 
 switch
i.e what is the rationale behind the provision of S1.

UL is the instigator of the S1 (neutral switch)
when measuring leakage current.

The story, as I know it, is simply a convenience
switch during product testing.  For products with
a single-pole power switch, it simulates the 
situation of a two-wire product being connected 
to the supply where the polarity is such that the 
single-pole switch is in the neutral pole of the 
supply.  

UL designed a test fixture for their technicians
such that during the leakage current test, the 
test technician need not reverse the plug and 
operate the power switch, but instead operate 
two switches on the his leakage current test 
fixture, one to reverse the supply and the other,
S1, to open the neutral.

The situation of a single-pole power switch and
reverse polarity (i.e., the power switch in the
neutral conductor) has the effect of doubling the
leakage current.

When USA manufacturers started moving into the
international market where polarity was not
assured, they started using double-pole power
switches.  These were hi-tech products with line
filters that caused relatively high leakage 
currents.  When tested with S1 open, the leakage
current exceeded the 3.5 mA limit.  So, the
manufacturers lobbied UL to change the standard
such that S1 was not operated if the product had
a double-pole switch.

This was accepted for the high-tech equipment.

As for why the test applies to direct-plug-in
units and not for cord-connected units... I can
only guess.  My guess is that the standard
presumes an overcurrent failure of the unit, in
which case the overcurrent device can be in either
pole of the supply, and thus the leakage current
can be double the normal leakage current.

Of course, this also applies to cord-connected
products, so my guess must be wrong.

Unless all products that are not direct-plug-in
are two-wire plus ground, in which case both a
fuse operation and an open ground is a double-
fault condition and would not be tested.


Best regards,
Rich



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Reminder: IEEE emc-pstc subscriber survey

[Rich Nute]

If you haven't filled out the IEEE emc-pstc survey,
please do so no later than 05 May.

Please go to this URL for the survey:

http://www.ewh.ieee.org/soc/emcs/pstc/Survey/

At my last check, only about 20% of our subscribers
have completed the survey.  I'd really like to get
80% or better!  

The results of the survey will provide the direction
for the future of the emc-pstc listserver; please
don't let a minority dominate where we are going.

If you are unable to access the web site, or if you
do not want to complete the survey, I would appreciate
a private e-mail as to the reasons.  I'll keep your
response anonymous, but I would like these reasons
be considered in determining our direction.  

If you've already completed our survey, thank you!
We have some good results that will help us determine 
the future of the emc-pstc listserver.


Thanks, and best regards,
Rich




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Re: EN60950 (UL1950, IEC 60950) On off switch marking.

[Rich Nute]

Hi Rick:


There are three issues which we must address:

1.  The safety function of the switch.

2.  The safety function of all-pole disconnect
versus one-pole disconnect.

3.  The marking of the function of the switch.

*

The safety function of a disconnect device (for
which a switch is acceptable) is:

* to disconnect power in the event of some
  sort of safety incident within the 
  equipment;

* to disconnect power for the situation of
  servicing the equipment.

For cord-connected equipment, I alway designate
the plug or appliance coupler as the disconnect 
device.  This means that the power switch is not
an isolating switch.  This solves the problem of 
one-pole versus all-pole switch requirements, and 
contact separation requirements.

For the first function, a one-pole switch will
satisfy all safety incident situations except 
(50-50 chance) phase-to-ground fault (in which
case the building overcurrent devices provides
the protection).

For the second function, a one-pole switch 
satisfies the servicing situation for a 
polarized supply system with a polarized plug
and socket.  A one-pole switch does not satisfy
the servicing situation for a non-polarized 
supply system.  (However, the plug does satisfy
the servicing situation.)

In the situation you describe, you can designate
the plug (or appliance coupler) as the disconnect 
device, and the on-off switch as a functional 
(i.e. not a safety) switch.

According to the requirements you quoted, the 0
and 1 symbols may be used on *any* primary power 
switch.  So, the symbols may be used on either a 
functional switch or an isolating switch.

For marking, there is no requirement that the 0 
and 1 symbols are restricted to isolating 
switches.  However, isolating switches must be
marked with the 0 and 1 symbols.

Most of our products do not have a primary power
switch.  We use a secondary circuit functional 
on-off switch.  Since we do not switch primary 
power, we do not use the 0 symbol, but the 
stand-by symbol.


Best regards,
Rich




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Re: Dielectric Voltage-Withstand Test

[Rich Nute]

Dan Kinney asks about pole-to-pole hi-pot testing.

The hi-pot test determines that the product's 
insulation (both air and solid) is sufficient to 
withstand the normally-occuring transient over-
voltages on a power line.  Transient over-voltages 
are both differential-mode (pole-to-pole) and 
common mode (both-poles-to-ground).

Some safety standards require two hi-pot tests:

pole-to-pole, and 
both-poles-to-ground.

Some safety standards only require one hi-pot test: 

both-poles-to-ground. 

Pole-to-pole insulation is functional insulation.
That is, the insulation is used for the proper
functioning of the circuit.  It does not provide
protection against electric shock.  Pole-to-pole
insulation failure is mitigated by an overcurrent
device either in the building installation or in
the product itself.

Pole-to-ground insulation is a safeguard (i.e.,
basic insulation).  That is, the insulation is
used for protection against electric shock
(although it may also serve a functional purpose).
Pole-to-ground insulation failure is mitigated by 
a second safeguard, typically a grounded part or
a second, independent insulation.

Pole-to-pole insulation is comprised of the
insulation between the terminals of components,
and the insulation provided by the construction
of the conductors connecting the components to
the circuit.

Except for the terminal-to-terminal construction,
components are not considered insulation; rather,
they are impedances and therefore are not subject
to insulation requirements, i.e., the hi-pot test.

On the other hand, the wiring that connects the
components into a circuit does have pole-to-pole
insulation.  The electric strength of this 
insulation is determined by the product manufacturer
by means of the way in which the conductors are
designed and arranged, whether individual wires or 
a printed wiring board or both.

The pole-to-pole hi-pot test is to determine that
the wiring -- independent of the components -- is
sufficient to withstand the differential-mode
over-voltages.

In some standards, pole-to-pole hi-pot testing is
not done in lieu of constructional requirements --
minimum physical distances through air (clearance)
and along the surface of solid insulation 
creepage distance).

In yet other standards, pole-to-pole insulation is
not considered to affect the safety of the product
since the overcurrent device provides protection
in the event of failure of pole-to-pole insulation.

   circuitry is unaffected by the potential and thus is untested.  Is the
   proper method then to lift the ground leg of the regulator and then work on
   down the line lifting each current conducting component to ground?

Yes.  (Although, it doesn't make much difference
which leg is lifted.)


Best regards,
Rich





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Re: 2. 4 GHz cordless telephone, question of general interest

[Rich Nute]

Hi Ken:


Here are some quotes:

http://www.eskimo.com/~billb/weird/microexp.html#demo:

Q:  Aren't these ovens tuned to a special frequency so they only heat
water?

A:  No.  The usual operating frequency of a microwave oven is nowhere
near the resonant frequency of water, and the RF energy will heat
other substances.  For example, drops of grease on a plastic
microwave dish can be heated far hotter than 100C, and this causes
the mysterious scarring which frequently occurs on plastic utensils.
Any molecule which is polar and has positive and negative ends
will be rotated to align with the electric field of the radio waves
in the oven.  The vibrating electric field rotates (vibrates) the
water molecules (and any other polar molecules) within the food.

Microwave ovens have difficulty melting ice, presumably because the
water molecules are bound together and cannot be easily rotated by
the e-fields.  If the oven was tuned to the water resonance
frequency, then the water would become far more opaque to the wave
energy.  The water in the food's thin surface would absorb all the
energy, and only the outside surface of foods would be heated.  The
thin outer surface of meat would become a blast of steam, and the
inside would remain ice cold.  But because water does not resonate
with the microwave frequency, the waves can travel an inch or so
into the meat before being absorbed.

See also:

http://hypertextbook.com/facts/HowardCheung.shtml

Here's another quote:

http://rabi.phys.virginia.edu/HTW//microwave_ovens.html

My science book said that a microwave oven uses a laser resonating
at the natural frequency of water.  Does such a laser exist or was
that a major typo?

It's a common misconception that the microwaves in a microwave oven
excite a natural resonance in water.  The frequency of a microwave
oven is well below any natural resonance in an isolated water
molecule, and in liquid water those resonances are so smeared out
that they're barely noticeable anyway.  It's kind of like playing a
violin under water--the strings won't emit well-defined tones in
water because the water impedes their vibrations.  Similarly, water
molecules don't emit (or absorb) well-defined tones in liquid water
because their clinging neighbors impede their vibrations.

Instead of trying to interact through a natural resonance in water,
a microwave oven just exposes the water molecules to the intense
electromagnetic fields in strong, non-resonant microwaves.  The
frequency used in microwave ovens (2,450,000,000 cycles per second
or 2.45 GHz) is a sensible but not unique choice.  Waves of that
frequency penetrate well into foods of reasonable size so that the
heating is relatively uniform throughout the foods.  Since leakage
from these ovens makes the radio spectrum near 2.45 GHz unusable for
communications, the frequency was chosen in part because it would
not interfere with existing communication systems.

As for there being a laser in a microwave oven, there isn't.  Lasers
are not the answer to all problems and so the source for microwaves
in a microwave oven is a magnetron.  This high-powered vacuum tube
emits a beam of coherent microwaves while a laser emits a beam of
coherent light waves.  While microwaves and light waves are both
electromagnetic waves, they have quite different frequencies.  A
laser produces much higher frequency waves than the magnetron.  And
the techniques these devices use to create their electromagnetic
waves are entirely different.  Both are wonderful inventions, but
they work in very different ways.

The fact that this misleading information appears in a science book,
presumably used in schools, is a bit discouraging.  It just goes to
show you that you shouldn't believe everything read in books or on
the web (even this web site, because I make mistakes, too).

On the other hand:

http://www.sciencenet.org.uk/database/Physics/EMandLight/p00571b.html

How does a microwave oven work?

Everything has what is called a natural frequency.  When you hold a
ruler over the edge of a table and ping it, it will bounce up and
down at a certain rate.  If the length of ruler is kept the same,
the frequency of the bounce will be the same however hard the ruler
is struck.  This frequency is called the natural frequency.  A swing
in a children's playground also has a preferred frequency.  In fact,
it is extremely difficult to make it swing at any other frequency.

On a much smaller scale, water molecules also have a natural
frequency at which they prefer to rotate from side to side.

One way to cook a potato, is to stick it into a hot oven.  Heat
energy from the oven is transferred to the potato and the particles
inside the potato 

Re: SELV vs ELV

[Rich Nute]

Hi Dave:


   the UL report for the PSU, it states that the output is ELV, not SELV,
   because of the connector interfacing the modules to the rack.

I don't see how a connector makes the difference
between ELV and SELV.

The terms ELV and SELV imply the energy is
derived from a hazardous voltage source, i.e., a 
source that could cause electric shock.  

ELV has only one safeguard (i.e., Basic Insulation)
interposed between it and the hazardous voltage 
source.

+---+  +---+  +---+
|   |  |   |  |   |
|Hazardous  |  |Basic  |  |   |
|Voltage|-|Insulation |-|ELV|
|   |  |   |  |(not   |
|   |  |   |  |accessible)|
+---+  +---+  +---+

SELV has two safeguards (i.e., Basic Insulation 
PLUS another safeguard, e.g., Supplementary
Insulation, grounded barrier, etc.) interposed 
between it and the hazardous voltage source.

+---+  +---+  +---+  +---+
|   |  |   |  |   |  |   |
|Hazardous  |  |Basic  |  |Supplementary |   |
|Voltage|-|Insulation |-|Safeguard  |-|SELV   |
|   |  |   |  |   |  |(accessible|
|   |  |   |  |   |  |part)  |
+---+  +---+  +---+  +---+

In this modern age, the cost of the supplementary
safeguard is negligible.  I cannot understand why
a PSU manufacturer would produce a 48-volt supply
that is *NOT* SELV.

ELV cannot be accessible.  This requires that a
supplementary safeguard be interposed between the
ELV circuits and accessible parts.

+---+  +---+  +---+  +---+
|   |  |   |  |   |  |   |
|Hazardous  |  |Basic  |  |   |  |Supplementary
|Voltage|-|Insulation |-|ELV|-|Safeguard  |
|   |  |   |  |(not   |  |(accessible|
|   |  |   |  |accessible)|  |part)  |
+---+  +---+  +---+  +---+

However, my understanding of TNV circuits is that 
TNV circuits are more-or-less treated as an ELV
circuit.

+---+  +---+  +---+
|   |  |   |  |   |
|   |  |Supplementary |   |  
|TNV|-|Safeguard  |-|Accessible |  
|   |  |   |  |part   |
|   |  |   |  |   |
+---+  +---+  +---+

+---+  +---+  +---+   
|   |  |Basic or   |  |   |   
|   |  |Supplementary |   |  
|ELV|-|Safeguard  |-|Accessible |  
|   |  |   |  |part   |
|   |  |   |  |   |
+---+  +---+  +---+  

If this is the case, then the 48 volts supply may
be ELV without causing undue construction 
requirements in the CO.  

(Note that ELV allows either Basic or Supplementary 
safeguards, but TNV must use a Supplementary 
safeguard.  This is an interesting way of thinking 
about ELV and TNV circuits.)


Best regards,
Rich


ps:  Note that one supplemental safeguard is that of
 grounding the SELV circuit.  See IEC 60950, 2nd,
 sub-clause 2.3.3.3.  So, if you ground the 48 V
 in your equipment, then you change the ELV to 
 SELV (if you meet the relative impedance 
 requirements of 2.3.3.3.)




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Re: Flicker problem

[Rich Nute]

   expressed as 'dmax', the maximum relative voltage change, measured as an
   r.m.s. value over the worst half-cycle, starting from the zero-crossing.

While I suppose there are sophisticated oscilloscopes
that will make this measurement at the touch of a button,
I wonder how many colleagues would know how to make this
measurement without such a tool?

I don't know how to do it without an oscilloscope.  A
storage oscilloscope would be nice.

The first problem is how to identify and capture that 
particular half-cycle.  I would first need to look at
the start-up waveform for the first 5 cycles or so to
identify the specific worst half-cycle.  I would 
probably put the current waveform on the other channel
and observe both traces; the maximum current should 
correspond to the maximum relative voltage change.  

This effort alone requires either a storage scope or a 
scope camera.  Then, I would want to change the sweep 
speed so as to capture just the desired half-cycle and 
make it full-screen (for the best resolution).

With a digital scope, this part should be easy.  It is
also possible with an analog scope, but it would take 
a bit of trial-and-error.

Doing the RMS requires manipulating a graphic image of
the waveform.  To get the image, I would use a digital
scope with a printer, or an analog scope with a camera.

Then comes the fun of graphically determining the rms
value of the half-cycle.  I would divide the half-wave
into equal increments.  Then, measure the amplitude at
the center of each increment.  How many increments 
before entering the realm of diminishing returns?

Then, square each value, sum them, and take the square
root.  Does anyone remember how to do this by hand?  
Anyone still have the CRC math tables for getting the
square root?

Using such methodology and now having a value, is the 
value within the resolution required by the standard?  
Or, is the measurement fully dependent on a sophisticated 
oscilloscope?

I often wonder whether being able to measure a
phenomenon means that we should therefore control 
that phenomenon.  Phenomena such as flicker and EMC 
have been observable long before the advent of suitable
measuring equipment.  Clearly, in the absence of the 
ability to measure any phenomenon, we cannot control it.  
But, do we sometimes control it because we can measure 
it?


Best regards,
Rich





   From owner-emc-p...@ieee.org Wed Mar 14 13:51:14 PST 2001
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   Message-ID: mp9gsaamk7r6e...@jmwa.demon.co.uk
   Date: Wed, 14 Mar 2001 18:03:56 +
   To: Wagner, John P (John) johnwag...@avaya.com
   Cc: EMC-PSTC emc-p...@ieee.org,
   Colgan, Chris chris.col...@tagmclarenaudio.com
   From: John Woodgate j...@jmwa.demon.co.uk
   Subject: Re: Flicker problem
   References: 
 4203d61676d0ae468aa5cea90a891c13235...@cof110avexu4.global.avaya.com
   In-Reply-To: 
 4203d61676d0ae468aa5cea90a891c13235...@cof110avexu4.global.avaya.com
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   4203d61676d0ae468aa5cea90a891c13235...@cof110avexu4.global.avaya.com,
   Wagner, John P (John) johnwag...@avaya.com wrote:
   I think this refers to Amendment 1of IEC 61000-3-3 published January 
 2001.  
   Apparently it has not yet been transposed into an EN.
   It was dual-voted, so it will be.
     The amendment (at 
   least as it effects me) deals primarily with requirements and limits 
 for 
   inrush current.
   
   Well, inrush current is dealt with in the unamended standard, but
   neither that or the amendment deal with it directly. Limits are
   expressed as 'dmax', the maximum relative voltage change, measured as an
   r.m.s. value over the worst half-cycle, starting from the zero-crossing.
   
   The amendment goes into much more detail about this, and gives relaxed
   limits for some types of equipment. The main problem with inrush current
   is where there is a lot of equipment in 

Re: Repeat Postings

[Rich Nute]

Hi George and all:


Yes, we are having some listserver multiple send
problems.  Please bear with us while we fix the
problem.

Please direct all such questions to me or to one
of our admins at the bottom of the message.  Even
your message could get sent multiple times, so we
want to keep the extranneous messsages to a 
minimum.  :-)


Best regards,
Rich




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Chris' humorous story is true!

[Rich Nute]

Hello from San Diego:


One of my colleagues has added material of interest to
one of Chris Allen's humorous stories.


Best regards,
Rich




This is actually a true story (with some adjustment).  It happened at a Ford
plant back in the 1930's (which makes the $50K fee all the more
significant).  The engineer was brought over from Germany to fix the
problem, and Henry Ford was appreciative of the service, but outraged with
the bill.  Thus he demanded an itemization of it thinking he could out one
over on this engineer and reduce the amount.  And he did, in fact get the
itemization below.  Henry Ford paid the bill.

Mario Raia
10319 SE 15th Street, Suite 100
Vancouver, WA 98664
360-891-6113
360-891-6114 Fax
mr...@ipinc.net Email





 Comprehending Engineers-Take 2
   
   There was an engineer who had an exceptional gift for fixing all things
   mechanical.  After serving his company loyally for over 30 years, he happily
   retired. Several years later the company contacted him regarding a seemingly
   impossible problem they were having with one of their multimillion dollar
   machines. They had tried everything and everyone else to get the machine to
   work but to no avail. In desperation, they called on the retired engineer
   who had solved so many of their problems in the past.
   The engineer reluctantly took the challenge.  He spent a day studying the
   huge machine.  At the end of the day, he marked a small x in chalk on a
   particular component of the machine and stated,  This is where your problem
   is.
   The part was replaced and the machine worked perfectly again. The company
   received a bill for $50,000 from the engineer for his service. They demanded
   an itemised accounting of his charges. The engineer responded briefly:
One chalk mark   $1
Knowing where to put it $49,999





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Re: Harmonics -- WSJ opinion.

[Rich Nute]

Hi John:


   Of course, no one has shown that unacceptable
   overheating will actually occur. 
   
   Do you have any more such gems to contribute? What do you think happens
   to the total current through a capacitor when the applied voltage
   contains harmonics? What happens to the I^2R loss and the dielectric
   loss? What happens to hysteresis loss in motors and transformers?

My assertion is based on the original reason for the 
harmonic current emission standard, not the general
case for problems caused by harmonic currents.  I
apologize for writing in such a way as to confuse the 
general case of overheating due to harmonic currents
with the specific case of overheating in distribution 
transformers.

My recollection of the original reason for the 
harmonic current standard was to prevent overheating
of distribution transformers on the public power
network.

Perhaps you can correct this recollection.  If this
is not correct, then kindly disregard the following
remarks.  

Based on my probably incorrect recollection, my 
assertion is that no distribution transformer in the 
public power network has failed due to harmonic 
current.

I further recall that such failures were a prediction
based on the expected proliferation of products with 
full-wave rectifiers, especially SMPS.  The electric 
power distribution representatives to the committee 
predicted massive distribution transformer failures 
due to harmonic currents by the year 2000 or 
thereabouts.  Therefore, the committee operated with 
a high sense of urgency.

Perhaps you can correct me on this recollection.

Can you tell us whether, at the time the work on the 
standard was initiated, any such transformers had 
indeed failed due to harmonic current overheating?
Or, have any such transformers failed due to harmonic
current since the work has been undertaken?


Best regards,
Rich







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Re: Harmonics -- WSJ opinion.

[Rich Nute]

Hi John:


I must admit to several motives for posting my
message regarding the WSJ-E opinion article.

1.  I wanted our subscribers to know that the
issue rated comment in the WSJ-E, a high-
level, respected newspaper.

2.  I wanted our subscribers to know that the
technical arguments are bolstered by some
political arguments.

As for knowing that my posting would generate
further discussion... well, that is up to our
subscribers and whether they want to move from
the technical arena to the political arena!

As for your request for comment...

   But the comment is 'non-technical' . . . can anyone in this forum offer 
   any 'technical' arguments that would a)Back-up such a statement as 
   Mr. Hunter's or b) FAVOR the harmonic standard? 

With respect to your first question (a) I 
believe you refer to Hunter's assertion that 
the European electricity distributors benefit
from the standard.  

I don't know that this statement is subject to
a technical argument.

With respect to your second question (b), the 
technical argument in favor of the standard is 
that triplen harmonic currents cause overheating 
of the primary of a delta-wye distribution 
transformer.  Therefore, some means must be 
provided to prevent such overheating.

There are several mechanisms for preventing such
overheating:

1.  Use a distribution transformer with a k-
factor rating.

2.  Use a trap (zig-zag transformer) between the
transformer and the load.

3.  Require linear loads.

There may be other mechanisms.  There is no 
technical argument for any one of the several 
mechanisms that prevent distribution
transformer overheating.  Each works.  

Pick one.  

It is probably best to kill the problem at
its source.  

On the other hand, it is likewise probably 
best if the electricity supplier can supply
power to any load rather than restrict the
loads to which he is willing to supply power.

Because all work, the choice is subject to
other criteria.  One major criterion is that 
of cost:  

   If you are an electricity supplier, you 
   would not be in favor of choices 1 and 2.

   If you are a product manufacturer, you
   would not be in favor of choice 3.

   If you are a consumer, you will pay for
   choices 1 and 2 through higher electric
   bills, and you will pay for choice 3
   through higher product cost.  No matter
   the choice, you pay forever, either 
   through higher electric bills or for 
   higher product costs.  If you buy lots
   of products on a continuing basis, your 
   cost may be higher than your long-term
   electric bills.  

Of course, no one has shown that unacceptable
overheating will actually occur.  Hence, 
Hunter uses the phrase theoretical harmonics.


Best regards,
Rich






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Harmonics -- WSJ opinion.

[Rich Nute]

With thanks to Ed Jones...

On Thusday, February 22, The Wall Street Journal Europe
published an interesting opinion on the harmonic current
emissions standard.

The opinion is by Rob Hunter, a lawyer and Chairman of
the Centre for the New Europe, a Brussels-based think 
tank.

Mr. Hunter is quite critical of the EU New Approach
process.  He says:

In this procedure, the EU sets vague safety and
technical rules for everything from toys to super-
computers -- for example, toys shall be 'safe.'  The
EU then delegates to private standardization bodies
the drafting of detailed requirements explaining 
what the delphic rules mean.

The supposed advantage of this New Approach is 
twofold.  For industry, it gets to write the detailed
rules applying to it.  For the Commission, the New
Approach frees it from a burdenom task; it also 
allows the Commission to claim that it has nothing to 
do with writing the standards, and hence cannot be 
held responsible.

All this sounds quite above-board.  It isn't.

For one thing, the standards are not merelay a means
of proving compliance with the underlying legislation.
They actually determine the meaning of the law itself.

Mr. Hunter discusses ...the way these standard-setting
bodies can be gamed by industry insiders for advantage.

Mr. Hunter goes on to show how the New Approach process
allows the Commission to sidestep ...WTO laws prohibiting
'mandatory' product measures that create 'unnecessary 
obstacles' to international trade.

Mr. Hunter's opinion goes on to show that the only ones
who benefit from the harmonic current emission standard
are the European electricity distributors.  They avoid
investments in bolstering their networks against the
theoretical harmonics risk at the cost of manufacturers
and consumers.


Best regards,
Rich







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Re: FLAME RATING OF STANDOFFS

[Rich Nute]

Hi Kaz:


Answering the question, what's the safety concern,
is a reverse-engineering process with respect to the
various requirements contained in the standard.

Applying the requirements of the standard to the 
specific construction may involve or even require 
interpretetation of the standard.

These are two, separate activities.  

My point is, while a certification house engineer 
(and you) may enjoy a reverse-engineering discussion, 
he cannot judge a construction based on what he 
learns from reverse engineering.

However, he can judge a construction based on his 
interpretion of the requirements.

Since small part is not defined in the standard, 
application of the small part rule is an
interpretation of the standard, and is not an 
application of what is the safety concern. 

Likewise, whether the V-1 PWB can serve as a barrier
is an interpretation.

The exemption of certain small parts from the V-2
requirement may or may not have a valid rationale.
Reverse engineering suggests that the qualities of
small-ness, function, distance or a barrier,
(with respect to an electrical part which, under 
fault conditions, is likely to exceed the ignition 
temperature of the small part) will prevent ignition 
of the small part.  Or, that if ignited, will not 
contribute sufficient fuel to spread the fire within 
the equipment.

Unfortunately, the reverse engineering that I have
stated above, is also an interpretation.  And, it
is quite wrong from a thermodynamics point of view.  
Anyone who has lit a campfire knows that it is much
easier to ignite small units of fuel than large 
units of fuel, and that the burning small units of 
fuel ignite the large units of fuel.

Many of the requirements in our safety standards 
are the result of inverting a bad experience.  That
inversion of a specific bad experience is then 
generalized.  The generalization makes reverse-
engineering to the specific bad experience almost 
impossible.

Those of us who are manufacturers always object to
anything found by a certifier that causes us to 
change the product.  In this case, the certification
house has taken a position that the stand-off must
be V-2.  Establishing traceability of flame-rating 
of a stand-off may be quite difficult or impossible.
So, we seek a way out.  Most often, we either attack
the certification house or we seek a favorable
re-intepretation.  Attacking the certification house
never works (and THAT is the voice of experience).

Discussion can be very effective at arriving at a 
favorable re-interpretation.  But, while the
discussion may involve some degree of what's the 
safety concern, the decision must be one of
interpretation.

Where the interpretation remains unfavorable to the 
manufacturer, testing always proves (or disproves) 
the hypothesis.  


Best regards,
Rich





   Rich,
   
   As always, you've succinctly provided the full story on an approach that in
   this case, will work to get Terry's product approved.  I think your
   interpretation of my suggestion of asking what's the safety concern is a
   bit off however.  
   
   Personally, I'd rather have a 5 minute conversation first before taking the
   time to run the tests right away.  It's possible the agency rep. may not be
   all that familiar with the product and so is simply going by the book on
   default (or what he/she's advised to from more senior staff), thereby
   eliminating the application of engineering judgment in those areas open to
   interpretation...a discretionary thing.
   
   By asking the question, both parties will bring to light the exact concerns
   raised in the course of this particular product approval.  While it's true
   that a safety certification agency certifies a product to a given standard,
   it's the interpretation of that standard's requirements by the parties
   involved that have raised many an interesting discussion I'd wager.  In my
   own view, engineering judgment is not only to be used by a safety agency
   rep. when applying the requirements.  How else does one raise questions
   which might result in an interpretive dispute but also deal with such
   disputes?
   
   By the way, the exemption also applies to materials separated by a solid
   barrier of V-1 or better (60950 ed.2), which might also be the case
   heredepends on whether the stand-off falls into a small part criteria
   as agreed upon by the agency.
   
   My opinion and not that of my employer.
   
   Regards,
   Kaz Gawrzyjal
   kazimier_gawrzy...@dell.com
   

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Re: FLAME RATING OF STANDOFFS

[Rich Nute]

Hi Kazimier and Terry:


Kazimier suggests asking the question:

what's the safety concern

Unfortunately, safety certification houses
do not have the option of accepting products
based on the answer to this question.

A safety certification house certifies a 
product to a standard.  Supposedly, the 
requirements contained in the standard make 
the product safe.

In this case, IEC 60950, Sub-clause 4.4.3.2
requires all materials and components be
rated V-2 or better.  So, the certification
house is requiring that the construction
comply with the standard.  You can't fault
the certification house for imposing a 
requirement explicitly stated in the standard.

The small-part exemption cited by Terry only
applies to small parts separated from electrical
parts by at least 13 mm (1/2-inch) of air.  I
would guess, from Terry's description and the
action of the certification house, that this is 
not the case.  (If it is the case, then you can
invoke this sub-clause and the matter is closed.)

Fortunately, the standard provides an option of
testing.  

If you test the stand-off for flammability and
it is flame-retardant, then your construction 
is acceptable.  Now, instead of proving to the
inspector that the material is V-2, you need 
only prove to the inspector that the stand-off 
is the manufacturer and model number that was
tested.

If you can't do this, then there is still 
another test option.  You can test a non-flame-
retardant standoff.  If the resulting fire does
not spread within the equipment, then you have
proved that the standoff is indeed inconsequential
to any fire.  If you prove this, then there is
no need to control the material.


Best regards,
Rich


ps:  Being a long-time certification house 
 basher, I can't believe I've written a
 message defending a certification house!





   Hi Terry,
   
   Sounds like a discussion with your agency safety engineer might be in order.
   It's certain there's a line of reasoning behind the new approach taken by
   the agency, that you've described below. Question is, since the standard
   clauses you've called out make certain allowances, the real issue might
   easily be addressed by asking what's the safety concern?  If the agency
   rep. understands your product, your reasoning and it all falls into an area
   of interpretation without any blatant standard violations, a certain amount
   of engineering judgment might help resolve the situation.
   
   
   My opinion only and not that of my employer.
   
   Good Luck.
   Regards,
   Kaz Gawrzyjal
   kazimier_gawrzy...@dell.com
   
   
   -Original Message-
   From: Terry Meck [mailto:tjm...@accusort.com]
   Sent: Tuesday, February 20, 2001 9:44 AM
   To: emc-p...@ieee.org
   Subject: FLAME RATING OF STANDOFFS
   
   
   
   Hi group!
   
   I need a sanity check on a `new approach' our safety agency has recently
   taken.
   
   We have an open frame power supply ( has all the certs through the CB report
   etc. for EN 60950 UL 1950 )
   
   On of the conditions of acceptability is one mounting standoff shall be
   insulated.  We have this supply in no less then 4 listed products without
   any reference to the flame rating of the standoff having to be checked when
   the inspector comes in.  
   I consider that to be reasonable. section 4.4.3.3  UL 1950 has exception:
   gears, cams, belts, bearings and other small parts which would contribute
   negligible fuel to a fire;
   
   Recently new products have been reviewed and the new procedures require
   `traceable 94V-2' standoffs!?!?  Which manufacturing engineering says is
   difficult to procure a traceable recognized plastic standoff.
   
   Questions:
   Has my fever and pneumonia the past weeks clouded my reasoning?  What am I
   missing?  You place a .5 inch #6 standoff between a V-0 board and a medal
   chassis what requires a recognized part except maybe `straining out the
   gnats so we can swallow the camel' somewhere else.
   
   Sick and Tired
   Terry J. Meck
   Senior Compliance / Test Engineer
   Accu-Sort Systems
   
   
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Virus and what we're doing.

[Rich Nute]

It appears that one of our subscribers has been subjected
to a virus.  The IEEE listserver stripped the virus from
the posted messages, so the virus was not passed on to
you and our other subscribers.

It appears that the worst that has happened is that 
duplicate messages were posted to the listserver.

I have temporarily unsubscribed that address.  This will
prevent further multiple postings.  When the virus is 
fixed, I'll re-subscribe him.

If you have any questions, please contact me or Jim or
Michael.


Best regards,
Rich

Richard Nute   ri...@ieee.org
Jim Bacher jim_bac...@mail.monarch.com
Michael Garretson  pstc_ad...@garretson.org







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Re: Unreliability of earthing/groiunding

[Rich Nute]

Hi Bob, Pete, Gert:


   A common equipotential environment is the goal. It's unfortunate we ever 
   started with the terms earthing or grounding.

Absolutely!

Furthermore, the equipotential environment can
be, and in some cases is, largely independent 
of whether the protective conductor is connected
to earth with a reasonably low resistance.  

Indeed (and this can be seen as heresy), for all 
installations, TN, TT, and IT, the higher the 
resistance between the ground rod and the earth, 
the better the equipotential environment!  The 
better the equipotential environment, the better 
the protection against electric shock.

I have prepared a paper on this subject.  Actually,
it is just a part of the objective of the paper:
derivation of the grounding resistance of the
equipment.  If you are interested in a copy, 
please send e-mail.  The paper is intended for 
presentation, so, for reading it is a bit terse -- 
possibly too much so for good understanding of why 
the PE connection to ground is not critical to 
safety.


Best regards,
Rich






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Would you help run the IEEE listserver?

[Rich Nute]

Many subscribers to emc-pstc have expressed that this
listserver provides valuable information and answers
to questions.

While the IEEE provides the computer, the 
administrative work of running this listserver is 
provided by volunteers.

We currently have three administrative volunteers:

Jim Bacher
Michael Garretson
Richard Nute

Jim has been doing the admin work for several years,
and now wants to retire.  So, I'm asking for help
from our subscribers -- for one of our subscribers
to volunteer to do the admin job for about 2 years.

There are two main job functions:

1.  Maintain an accurate list of e-mail addresses
for our subscribers.

This requires:

Processing subscribe and unsubscribe 
requests, including verifying subscriber
addresses.

Correcting or unsubscribing addresses of 
undelivered mail.  (Majordomo returns 
undelivered mail to our admins for action.)
Some undelivered mail is due to servers 
being down; the admin must learn by 
experience how to detect when a server is
down versus when an address is no longer
valid.

2.  Dealing with non-subscriber posting attempts.

There are two kinds of non-subscriber posting
attempts:

The first is where the subscriber address
has changed, but his old address has become
an alias.  This means the admin must contact
the subscriber and suggest a change of
address to the new address.

The second is a true non-subscriber attempt
to post.  Some such attempts are spam, and
some are from folks who would like to 
subscribe.  Spammers sometimes will make
several attempts, and sometimes will attempt
to subscribe solely for the purpose of
spamming our subscribers.  Our admins have
been very good at detecting such folks and
have successfully prevented spam.

After learning to recognize the various messages sent
by Majordomo, the admin job is about 20 minutes per
day.  (The learning period may take as much as an 
hour per day for a few weeks, and can be a bit over-
whelming.)  The admin job is split between two people.  
This split also means that job travel and vacations 
can be covered by the other admin.  And, the second 
admin can provide help in deciphering various problems 
that come up.

If you find this listserver to be of value to you, and
you can spare some time each day to help, and you have
a high-speed connection to the internet, please 
consider volunteering for 2 years to help keep this
service running.

Please contact me or Jim Bacher.


Best regards,
Rich Nute
ri...@ieee.org

Jim Bacher
jim_bac...@mail.monarch.com


ps:  The last time I asked for help, I had 2 responses,
 both of which fizzled.  Considering that there are
 over 800 subscribers almost all of whom feel this
 is a valuable resource, that was a poor response.





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Re: Product Marking - new twist

[Rich Nute]

Hi Andy:


   Is it possible that a piece of equipment with an NRTL listing can be
   disconnected by a local electrical inspector/electrician enforcing the NEC
   because that paticular NRTL is not approved in their jurisdiction?

Yes.

Please recognize:

   NRTL is an OSHA program governing electrical
   products used by employees in the workplace.

   Requirements for NRTL are imposed on employers,
   and enforced by OSHA inspectors.

   NRTL is determined solely by OSHA for OSHA
   purposes.

   Approved is an NEC requirement for equipment
   and appliances used in electrical installations.

   Requirements for approved electrical devices
   are imposed by local electrical inspectors 
   enforcing the electrical code.  Licensed
   electricians are expected to only install 
   approved electrical devices.

   Approved is determined by the local
   jurisdiction enforcing the code (i.e. a
   branch of the local building code 
   administration).

   Doesn't the NRTL approval by OSHA take precedence over whether or not the
   local authorities accept the NRTL's listing?

No.

These are separate and independent requirements.  We
cannot make a generalization that all NRTLs are also
approved in every jurisdiction.  Some NRTLs (e.g.,
UL, CSA, ITS) are indeed approved in every 
jurisdiction (because they make it their business to
be so approved).

Likewise, we cannot make a generalization that all
approved electrical devices are also NRTL-certified.

Determining an NRTL is easy by going to the OSHA-NRTL
web page.

Determining an approved product is not easy as each
jurisdiction makes its own determination.  However, 
each safety certification house knows which jurisdictions
have approved its certifications.  So, determining 
whether a product is approved in a particular 
jurisdiction is a simple matter of asking the 
certification house.

Having said all this... products installed after the
electrical construction is complete and signed off
by the local electrical inspector are rarely inspected.
Thus, manufacturers of such products rarely get any
feedback regarding approval of their products.  So,
it appears that NRTL certification also satisfies 
local approval whereas in fact such products are
never inspected.  

Anecdote:  Some years ago, our equipment was installed
in the finished basement of a multi-story building 
that was still under construction.  The electrical 
inspector, during his normal inspection round, returned
to the basement and saw all the various products that
had been installed (plugged in) after he had signed
off the floor.  He inspected the products, and found 
a number of them without any safety certification.
These were disconnected and red-tagged.  The basement
operations were shut down.  The customer was irate.

I hope this explains the difference between NRTL and
approved.


Best regards,
Rich


ps:  For our colleagues not familiar with the USA...

 NRTL  =  Nationally Recognised Testing Laboratory
 OSHA  =  Occupational Safety and Health Administration
 NEC   =  National Electrical Code




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Re: Product Marking - new twist

[Rich Nute]

Hi Chris:


   For instance, one of the messages says that the NEC gives an electrician the
   right to unplug an un-NRTL-marked piece of equipment.  Another message
   states that the NRTL's mark is the proof of product safety.  

NRTL is a designation issued by OSHA.  

Approved is a designation used by the NEC.

An NRTL may or may not be approved, depending
on the jurisdiction, i.e., the authority 
enforcing the NEC.

The authority enforcing the NEC can disconnect
any equipment that is not approved.  The
electrician may be delegated (through licensing)
to enforce the NEC, including disconnecting 
equipment that is not approved.

   1. Are other certifications from other labs, such as A2LA and/or NVLAP
   allowed as long as there is a test report?

Under the NEC, equipment must be approved.
Approved is defined as acceptable to the
jurisdiction enforcing the NEC.  The jurisdiction
decides approved on a lab-by-lab basis, and
sometimes by standards or equipment type covered 
by that lab.

Ultimately, it boils down to a certification
mark from one of the labs accepted by the
local jurisdiction.  The test report is nothing
more than a record maintained by the certification
house for its own purposes of granting the right
to use the mark on the equipment.

Under the NEC (and OSHA), it is possible to 
install a non-certified product provided it
is tested in place.  In such a situation, the
test report may be highly useful.

(In Europe, the test report is essential, as
Europe relies on the manufacturer proving the
safety of the product.)

   2.  Does it matter what the voltage rating of the product is?  

No.

Safety certification process almost always requires
the product to be safe (and therefore certified)
in accordance with the product's ratings, including
its input voltage rating.

Although rare, it is possible to certify a multi-
voltage product for one voltage by one lab and 
another voltage by another lab.  This is done by 
agreement between the submittor and the lab.  In
such a case, the voltage for which the certification
applies is specifically related to the certification
mark.

   3.  For products with external AC power supplies, would the NRTL mark need
   to be on the supply and the product?  Or the supply only?

The external ac power supply must be approved.

The product may or may not need to be approved.  If
the rated input voltage exceeds 30 V rms or 42.4 V
dc, then it must be approved (per the NEC).  If the
rated input voltage is less than 30 V rms or 42.4 V
dc, then the NEC does not require it to be approved;
it is a manufacturer's option whether to seek third-
party certification.

   4.  Does it matter where the product is used? (home, farm, factory ...)

The NEC applies to almost every location (except 
electric utility locations).



Best regards,
Rich




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Re: Product Marking

[Rich Nute]

Hi Courtland:


   I have a question concerning labeling a product. If we go to a NRTL and get
   Safety testing performed, we typically put the Safety logo (UL for example)
   on the product label. Our marketing people have a problem with having
   different logo's. They would like to standarize on a single logo such as UL.
   This kind of thinking hinders the process of getting the best price
   possible. I would like to get the testing performed at a lab which doesn't
   use UL. Would it be possible to just put Conforms to UL 1950 and CAN/CSA
   1950 on the label and forget the logo? Or is there a requirement to have a
   logo?

The NRTL does not *require* you to use the mark.  For most 
NRTLs, if the specified mark is not on the product, then 
the product is not certified -- even though it meets all of 
the requirements.  

The problem is that almost all USA jurisdictions require
safety certification by a lab acceptable to the jurisdiction.  
Certification is demonstrated by the product bearing the mark 
of an accepted lab.

Recall that there are two jurisdictions that apply:  OSHA for
products used in the workplace, and the local version of the
NEC for products used in areas covered by the code.  Both 
OSHA and the NEC require third-party safety certification.

So, while the NRTL does not require use of the mark, OSHA 
and the NEC *do* require use of the mark -- at least in those
areas subject to OSHA or NEC regulations (which is almost
everywhere in the USA).

Since, according to one management consultant, the purpose of
a business is to create a customer, it would seem prudent to
satisfy your marketing people.

In practice, however, it is my belief that customers simply
presume that products comply with OSHA and NEC requirements.
The choice of NRTL or consistency of NRTL from product-to-
product is inconsequential to customers.  Diplomatically,
you could ask your marketing people to validate through a
customer survey the marketing position that your customers 
want a single NRTL for all of your products.  Of course,
such a survey is frought with the danger that one NRTL's
mark is well-known, while others are hardly known.

If marketing wants just one NRTL, and you are interested in
least cost, then you might want to consider entering into 
an annual or product-by-product contract with your cert house
that results in costs acceptable to you.


Good luck, and best regards,
Rich





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Re: Client Presence During Testing

[Rich Nute]

Hi Kate:


   a) Do they allow presence in lab (technical area) itself ?  If not, then
   where are clients who are at the lab normally placed?

Depends on whether the lab is testing a competitor's
products.  If not, then I can usually observe the
testing.

If I can't go into the lab to witness testing of
my product, then I won't go to the cert house at
all.  Or, I will re-schedule for a date when I can
witness the testing.

   b) Are engineering/design type tests handled differently than compliance in
   this respect?  

We do most engineering/design testing at our site.
There is one test we cannot do.  So we engage our
test house, and we observe the test.  If it fails,
we go home and fix it.  If it passes, then the
test results count towards our certification.

   c) What about formal witnessing of tests?

As mentioned, we do most tests at our site; when
we do certification tests, we invite an engineer
to our site to witness the tests.

   d) How you feel about the policies that are in use?  Do they influence your
   choice of labs?

I wouldn't work with a lab that wouldn't let me
witness the tests.  If the test fails, then you
may not understand the test process, and may not
be able to fix the problem.  There is nothing 
like witnessing a test, either pass or fail, to 
fully understand the intent of the requirement. 

   f)  Have any related polices recently changed in the labs you use?  How do
   you feel about this, and is it an influencer?

Not that I know of.

   e) Any other comments about this?

No.


Best regards,
Rich



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Re: Color or marking of buttons - safety question

[Rich Nute]

Hi Paul:


   However, to release the interlock, a PAUSE or STOP button is pressed which
   allows the instrument to stop in an orderly way and then release the cover.
   
   Best as I can tell, a red pushbutton, with PAUSE or STOP legend and the ! in
   a triangle (to refer the operator back to the manual) would do the trick.  I

To my knowledge, an interlock need not be classed as
an emergency stop device (which would require the
red color).

Interlocks are found everywhere, yet they are not 
usually identified by the color red.

For the sake of discussion, consider two controls.

The first control is as you describe, and provides
the orderly stop followed by release of the cover.
This control is just a normal operation control and
can be any color.

The second control disconnects all power from the
unit (i.e., a disorderly or dirty stop) and releases 
the cover.  It would be labeled emergency stop or
equivalent.  This second control would be red.

Neither or these controls would necessarily require 
the use of the ! in a triangle.

In the case you describe, you simply have an interlock.
The interlock itself is the device which provides the
safety, i.e., the protection against the moving parts.  
There are no further safety requirements regarding 
operation of the interlock.  De-actuation of the 
interlock by a manual control does not mean that the
manual control is a safety device.

I would not use red.  To me, you have described a
functional control, not a safety control.  The 
interlock is the safety control.


Best regards,
Rich




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Administrator's comments: F-Squared labs

[Rich Nute]

I'd like to clear up a few things about the 
situation regarding F-Squared Labs.

1.  F-Squared Labs did NOT post its message to
the IEEE listserver.  It appeared in our
distribution because one of our subscribers
inadvertantly included it as an attachment
to his posting.

As you know, we have a strictly-enforced
policy against advertising.  Any advertising
posting results in immediate removal from 
our mailing list.

The listserver is set up so that no non-
subscriber may post anything to our list.

2.  Our subscriber list is ONLY available to 
subscribers.

I maintain copies of our subscriber e-mail
addresses.  I collect these several times a
year.  I searched my past subscriber lists
for all combinations of 'mark,' 'nabar,' and
'labs.'  I did not find any matches back to 
September of 1998.  So, unless it was some 
other subscriber name, it appears that F-
Squared Labs did NOT get a copy of our 
subscriber list.

Jim Bacher, one of our admins, told me that
he received two copies of the F-Squared Labs
message, one using his subscribed address 
and the other using an address that has not
been subscribed for several years.  So, we
don't really know where F-Squared Labs has 
gotten our addresses.

So, somewhere, F-Squared Labs obtained a 
list of many of us who also subscribe to this
listserver.  (I received my own copy of the
message today!)  We don't think they got a
copy of our subscriber list unless it is from
several years back, but not older than 1996.

Or, they could have gotten a copy of the list
from one of our subscribers.  We have no 
means of preventing a subscriber from down-
loading the list and providing it to someone
else.

3.  Kate McLean pointed out that, by discussing 
their message, we have given F-Squared Labs 
better publicity than they could have expected!

When these sorts of things come up, please
don't post admin-type issues to the entire
list.  Instead, please get in touch with 
one of us who administers the listserver.
We're listed at the bottom of every message!

If you want to discuss this further, please direct
your e-mail to Jim, Michael, and me.


Best regards,
Rich



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Re: FW: power factor

[Rich Nute]

Hi John:


   Can you tell me the typical power consumption of a desktop
   VDU. For example a Sun workstation VDU rating plate indicates
   220-230 V 1.5A. Rather than assume or guess a power factor
   value, what would the power reading (Watts) be approximately
   at 220V for nornal operation ? What power factor value 
   would you attribute to a modern desktop VDU ?

I don't understand the question.  The third sentence 
doesn't want to guess a power factor, but the last 
sentence seems to want such a guess.

In order to guess the power, we need to guess at the
voltage, the current, and the power factor.

Current:  The rated current is pessimistic and will be 
110% or more of the actual current.  Furthermore,
the actual current will be a linear function of the 
voltage.  

Voltage:  If the nameplate ratings are quoted properly, 
then the rated input current will be at 220 V, not 
230 V.  

If the manufacturer accounted for 220 - 10%, then
the current is for 198 V.

(If the nameplate includes 120 V, then the current
*may* be the maximum current at 120 V, which will
drastically reduce the value of the power.)

Power factor:  Most products today use switching-mode
power supplies.  Power factor for SMPS will be in
the neighborhood of 0.6, maybe less.

Power:  Using the above assumptions, the power input
should be about:

220 x 1.5/1.10 x 0.6  =  180 watts

or:

198 x 1.5/1.10 x 0.6  =  162 watts

So, my guess would be that the actual power consumption
is between 160 and 180 watts.


Best regards,
Rich




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Re: EN61000-3-2 / EN61000-3-3 (Again)

[Rich Nute]

Hi Doug:


   There are four observation periods listed in para 6.2.4 of A14 (table Z1), I
   assume that I only have to pick the one that suitably qualifies my
   equipment's operation, or must I apply each one to determine the best period
   of observation?  

From my reading of prA14 (I don't have A14), you must 
decide which of the four behaviors best fits your
product.  I doubt that performing each measurement in
turn would be the best way to determine your equipment
behavior.  

I would connect the EUT to a current-measuring 'scope
(slowest sweep speed), turn on the EUT, and observe 
the input current as a function of time for a period 
of ten minutes or longer.  From this record, you can 
determine the EUT behavior as one of the 4 types.

The equipment in question is bench/portable test equipment
   with and internal switching power supply (approx 150W) whose represented
   load does not change after initial turn on.

From your description, load does not change after
initial turn on, your equipment would be quasi-
stationary.  This means that you simply make the
harmonic current measurements without regard to the
EUT function.

   Question #2, Repeatablity per para 6.2.3.1, can you verify repeatablity by
   evaluating the individual 1.5 second smoothed rms values over your test
   observation period or must we repeat the entire test at a later time to
   prove repeatablity?

I'm not sure I understand your question.  I would 
think that the concept of repeatability is that of
repeating the entire test.


Best regards,
Rich





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Re: EN61000-3-2 / EN61000-3-3 (Again)

[Rich Nute]

Hi Craig:


Be careful:

 EN61000-3-2 - Applies only to products with input power higher than
 75W.  Per paragraph 7.4 of the standard, no limits apply for equipment
 with an active input power up to and including 75W.

No.  The standard does NOT say that it applies only
to products with input power higher than 75 watts.

The standard says that no *limits* apply to products
with input power less than 75 watts.  The standard 
still applies!  Your equipment is NOT exempt from the
standard.

Read the standard carefully.  The scope statement 
tells what equipment the standard applies to:  All
equipment.  Then, the standard says there are no 
*limits* for products with input power less than 75
watts.  This means that you need not measure your
product since the results would be meaningless.

Your equipment *complies* with the standard.  Since
there are no limits, you need not make a measurement
for proof of compliance.

 If a product does not fall under the applicability of EN61000-3-2 or
 EN61000-3-3 per the above explanations, what is the consensus regarding
 referencing these standards on the DoC?  

*All* products fall under EN 61000-3-2.  If your 
product is rated less than 75 watts, then there 
are no limits, and a measurement is not required
for determining conformance.

Because the product is subject to EN 61000-3-2, 
you must reference the standard -- and indicate
compliance -- on your DoC.

 If a product does not fall under the applicability of EN61000-3-2 or
 EN61000-3-3 per the above explanations, what is the consensus regarding
 referencing these standards on the DoC?  

*All* products fall under EN 61000-3-2.  You *must* 
claim compliance on your DoC.  Your documentation
back-up to your claim need only say that the unit
is rated less than 75 watts, for which there are no
applicable limits.

 Recently I have been asked to sign a document from one of our
 distributors that states all product provided after 01/01/01 will comply
 with EN61000-3-2 and EN61000-3-3.  However, my products fall outside the
 scope of these standards (per above explanations), so what I am
 wondering is can I say I comply because I have evaluated the standards
 and found they are not applicable.  I face the same dilemma on the
 DoC's.  Is it reasonable to claim compliance via non-applicability?

No products are outside the scope of EN 61000-3-2.
Some products, e.g., those rated less than 75 watts,
have no limits applicable to them.  Therefore,
without measurement, such products *do* comply with 
the requirements of the standard! 

You cannot claim compliance by claiming the standard
is not applicable.  It *is* applicable.  Your product
(if less than 75 watts) complies with the standard
(without measurement since there are no limits).


Best regards,
Rich





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Re: Spacings Issue

[Rich Nute]

Hi Ed:






   First of all the floating secondary ground (if not connected
   directly to earth) should be treated as part of the secondary circuit
   which in many cases will require reinforced insulation to the primary
   circuit according to the working voltage measured.

There are two issues here:

(1)  Is the secondary circuit, pole-to-pole, a hazardous 
 voltage?

(2)  Is one pole of the secondary circuit accessible?

With respect to (1):  In this case, see my paper:

http://www.ewh.ieee.org/soc/emcs/pstc/tech-spk.htm

This paper describes how to achieve protection against
electric shock due to the voltage of the secondary 
circuit.

With respect to (2):  In this case, protection against
electric shock *from the mains* must be provided by 
reinforced insulation (or equivalent) between the primary 
and secondary.

   Next, in most applications, the secondary circuitry can be cap coupled
   to earth ground and does not require basic insulation spacings for a
   primary circuit.  

Agreed -- PROVIDED at least basic insulation is between
the mains and the secondary circuit.  If not, then the
capacitor from the secondary to ground must be a Y cap.


Best regards,
Rich



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Re: Spacings Issue

[Rich Nute]

Hi Ken:


See my paper on floating circuits in the Product Safety
Newsletter:

http://www.ewh.ieee.org/soc/emcs/pstc/tech-spk.htm

This should answer all your questions.  If not, please
get back to me, and I'll answer any additional questions
you may have.


Best regards,
Rich


ps:  Provide a copy to your certification houses.



   Hello Group,
   
   A circuit uses a floating ground in the secondary, and caps are used for
   protection from the secondary outputs to earth ground.  The question is,
   under spacings requirements, would the secondary have to meet the high
   voltage requirements for spacings for primaries due to this earth ground.
   A few agencies have expressed desires to short the primary to secondary and
   require the the secondary to meet primary voltage spacings to this earth
   ground trace.   
   
   Any help would be greatly appreciated.   And thanks again for all your
   opinions
   
   
   
   Thanks,
   
   Ken Matsuda
   

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Re: Risk assessment

[Rich Nute]

Hi Stig:


I believe you are interested in severity of injury,
not potential for injury, or risk of injury.

Here is a numerical assignment for severity of injury
that I found in my files.  I have no idea of its 
source.

10   Death
 9   Long-term or permanent coma
 8   Full body paralysis (permanent)
 7   Loss of more than one organ or limb
 6   Loss of one organ or limb
 5   Loss of a function (permanent)
 4   Broken bone or tendon
 3   Heals with scar
 2   Heals with no scare
 1   No injury

While I can't cite any references, I believe that 
there ought to be some literature that thoroughly
discusses both risk of injury and severity of injury.
This isn't a new topic.


Best regards,
Rich




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Re: Polarity Markings on AC Adapters

[Rich Nute]

Hi George:


   Thanks for the quick response!  Does this mean that the symbology is only
   a Japan requirement?  and does it apply to ITE as well as
   audio-visual
   equipment?

The referenced standard, EIAJ CP1104, is similar
to IEC 417.  Indeed, EIAJ CP1104 contains many
symbols from IEC 417 as well as from other sources.

While the symbols are primarily those relating to 
audio-visual equipment, many of the symbols have
much broader application, e.g., ground symbols,
ac and dc symbols, etc.

I cannot say whether or not these symbols are
requirements in Japan or whether or not the
standard applies to ITE.  I would guess, however, 
that EIAJ CP1104 is a document which is referenced 
by end-product standards just as IEC 417 is 
referenced by end-product standards.  (While I 
have a copy of the standard, it is in Japanese and
I cannot read it!)

As far as I know, EIAJ CP1104 is the only standard
worldwide that has the dc polarity symbol for 
barrel connectors.  (Likewise, as far as I know,
Japan has the only standard for barrel connectors.)


Best regards,
Rich




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Re: Polarity Markings on AC Adapters

[Rich Nute]

Hi George:


The dc polarity symbol is specified in:

Symbol 01060 (taken from JEIDA 11)
EIAJ CP-1104 (1998)
Terms and Grpahical Symbols for Audio and Audio-Visual Equipment
Technical Standardization Committee on Audio-Visual Equipment and Systems
Electronic Industries Association of Japan

This standard now specifies the mark using 
diamonds rather than circles to enclose the
+ and - signs.

I'll send you a copy of the page in a separate
message (because my UNIX mail system can't send
attachments).  If anyone else wants a copy, 
please send private mail to me at:

ri...@sdd.hp.com


Best regards,
Rich





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Re: component spacing question

[Rich Nute]

Hi Ken:


   I wanted to pose the group a question in regards to spacing requirements for
   the U.S.  Many UL standards allow for the exception of components (such as
   semi conductors, switches, etc) to not meet spacing requirements (as they
   usually have different standards they meet anyways)   The question is what
   about PCB trace spacing for those components, such as a capacitor in a
   switching power supply to ground.  High voltage, the components is an
   exception, but what about the pads under the component, are they required to
   meet the spacing requirements, or would that fall under the component
   exception?  

You've brought our attention to the anomaly that a
component must meet its spacing requirements, and
that the end-product (i.e. PWB) must meet its spacing
requirements, even when the component requirements
are less than that of the end-product.

From an engineering point of view, this is nonsense.

Nevertheless, our standards have such requirements,
and our certification houses must enforce such 
requirements.

There are several options:

1)  Design the PWB to meet the spacing requirements
by trimming edges off circular pads, or by bending
the component leads.

2)  Test by short-circuiting the spacing and observing
the results (i.e., no shock or fire or damage to 
basic insulation).  This is especially appropriate 
for semiconductors since, by definition, they 
alternate or vary between open and nearly short.

3)  Remember that spacings is a special case of 
insulation, either air insulation (clearance) or a 
surface insulation (creepage).  Only those insulations
that are required by the safety standard (i.e., basic,
supplementary, or reinforced) are subject to the
spacings requirements.  Typically, these insulations
only exist between primary and ground, and between
primary and secondary.  Typical products do not have
components between primary and ground and between
primary and secondary except those specifically 
rated for such use such as Y-caps, transformers, and
opto-isolators (and which therefore meet the spacing 
requirements of the end-product).

Some standards may require pole-to-pole spacings in
primary circuits.  In this case you must measure the
voltage and then determine the spacing from a table.  
A typical SMPS has lots of low-voltage control
circuits with respect to the negative rail.  So, you
can lump all those circuits together as not requiring
insulation from each other.  Then, they can be taken
as a whole and spaced from the positive rail.  That
will generally only leave the bulk capacitor and the
switching transistor(s) as requiring spacings.  And,
the snubber circuit, which can be considered a voltage
divider so that the spacings across any individual
snubber component need not be the full voltage across 
the snubber.  Etc.

It really makes no sense to require a PWB to have greater
spacings than the component itself.  It further makes no
sense to enforce spacings across capacitors (that are not
Y capacitors) and semiconductors and similar components.
Internally, these components do not have insulations that
are equivalent to their terminal spacings or to the PWB
spacings.  So, why require a higher level of insulation
than the device itself can provide?  Short-circuiting of
the component will tell the story of whether the circuit
is safe; if safe, then the spacing is inconsequential to
the safety of the product.


Best regards,
Rich





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Re: Got a beef with an NRTL ...

[Rich Nute]

Hi Doug:


Your message was not clear whether you submitted
your product to the same NRTL that certified the
power supply.

Basically, your cert engineer has put YOU in the
middle of a beef between cert engineers or between
cert houses.

Your cert engineer found a fault in the power 
supply cert.  Rather than go to the power supply
cert engineer or the other cert house, your cert
engineer makes you be the bad guy once removed.
You must go back to the power supply mfgr, who
will then go back to the power supply cert 
engineer with the bad news: the power supply cert
engineer messed up on the fuse.

This is typical cert house engineer behavior.  It
establishes who is the better cert engineer, and
belittles the other cert engineer.  Its a power
play by your cert engineer to put himself into a 
more advantageous position for future advancement
or promotion.  

The same can be said of differences between two 
cert houses.

If both engineers are in the same cert house, then
you will need to go fairly high in the organization
to resolve this.  Both immediate managers will back
their respective cert engineers.

There is no way you can win this one.  You are the
messenger.  Your product cert is being held hostage
until you fix the problem with any of four options:

1)  add the second fuse in your product (which your
cert house engineer knows is ridiculuous and 
that you are not likely to do);
2)  demand the PS manufacturer change the fuse and
re-certify (which will embarrass the PS cert
engineer -- which is the objective);
3)  go to another cert house (which your cert engr
doesn't believe you will do).
4)  assuming the Conditions of Acceptability do not
require a dc fuse, then climb the management 
chain to get the cert house to accept its own
certification (but this is not a good choice
because the fuse SHOULD be a dc-rated fuse).

For me, the best option is to yank your product and
go to another NRTL.  And let your cert engineer's
boss know why you are yanking the product.  Money
speaks.


Best regards,
Rich







   From owner-emc-p...@ieee.org Thu Oct 19 09:55:16 PDT 2000
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   Date: Thu, 19 Oct 2000 09:42:39 -0700
   From: Doug dmck...@gte.net
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   Subject: Got a beef with an NRTL ... 
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   I'm just about ready to escalate this issue. 
   
   Issue:  Major NRTL has recognized a DC-DC power supply. 
   Said ps is being used within the confines of 
   it's stated purpose, input power, output power, 
   temps, etc ... 
   
   Said product is submitted to NRTL for what appeared 
   to be a walk through.  Oh no, Mr. McKean.  You can't 
   use THAT power supply as intended.  Input fuse of 
   power supply (that is the fuse INSIDE the power that 
   is out of our hands) is an AC fuse.  It should be a 
   DC fuse.  (From the documentation from the ps mfr, 
   the approval was done with the aC rated fuse.) 
   
   You have to either: 
   
   1. have the ps mfr change the input fuse. 
   
   or 
   
   2. drop an in-line fuse between the power inlet 
  of the product and the input of the ps. 
   
   EXCUSE ME!?!  
   
   How the heck can a power supply mfr get NRTL approval on one 
   hand and, yet, when that power supply is used within it's 
   intended and stated purpose, get rejected? 
   
   Even bringing this to the attention of the test engineer 
   (who has approx over 10 years experience as a test eng) 
   it defaults to - well, that's just because the OTHER 
   test engineer interpreted it that way ...  
   
   I can understand and have been in those areas of 
   interpretation with NRTLs, but this one really ... 
   er ... surprises me. 
   
   Yours 

For UL watchers... a job opening...

[Rich Nute]

... check out:

http://www.ul.com/about/newsrel/nr101300.html


Rich



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Re: Laser Safety Calculations

[Rich Nute]

Hi George and Chris:


Better still, TC76 should provide a spreadsheet such
that, upon entering all the data, the spreadsheet does
the calculations and gives you the emission level and
the emission Class.


Regards,
Rich




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Re: Flame Testing of Plastics

[Rich Nute]

Hi Richard:


   When testing plastics, the flame is applied for a specified period of time,
   removed for specified period of time and the process is repeated for a
   specified number of cycles. Is the procedure attempting to correlate with
   observed fire patterns in equipment, or is it designed simply to provide
   repeatable results?

The UL 94 tests are to measure the burning 
characteristics of a plastic material AFTER
it has been ignited.  For flame-retardant
plastics, four parameters are measured:

1.  Duration of flaming.
2.  Duration of flaming and glowing.
3.  Dripping of flaming drops.
4.  Burning to the clamp.

Depending on the data, the plastic material will
be rated V-0, V-1, V-2, or will fail the vertical
burning test.

These flame-retardant parameters are highly
dependent on the temperature, thermal energy, 
and duration of the energy transferred to the 
material under test.  Consequently, the flame 
and duration parameters are specified by the 
standard:

1.  Type of gas.
2.  Type of burner.
3.  Pre-mixed flame parameters.
4.  Duration of flame application to sample.

These flame and duration parameters apply a 
standard temperature and a standard amount of 
thermal energy to the sample.

The procedure does not correlate to fire patterns
in equipment.  It is a measure of the degree of
flame-retardancy (i.e., V-0, V-1, or V-2) to a 
standardized flame source.

Yes, the procedure must be repeatable, but it is
not designed simply to provide repeatability.  
Rather, it is designed to apply a standard amount 
of thermal energy (determined by the preceding 4 
flame parameters) to the sample.

The flame-retardancy determined by the UL 94 tests 
applies only to flames not exceeding the test flame.  
If the applied thermal energy is significantly 
higher, i.e., a larger flame or a longer duration, 
the sample will ignite and burn continuously until 
the sample is fully consumed.  


Best regards,
Rich




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