Hello from San Diego:
IEC 950 contains the following definition:
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1.2.8.7 HAZARDOUS ENERGY LEVEL: A stored energy level of
20 J or more, or an available continuous power level of 240
VA or more, at a potential of 2 V or more.
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If you recall, I asked the question: At what point in time
after start of the measurement is the available VA
considered "continuous"?
I've received a number of answers, ranging from 0 to 1 minute.
0 seconds: Manning Rose
(based on the requirement being
a continuum)
83 milliseconds: Egon Varju
(based on 20J/240VA)
100 milliseconds: Egon Varju
(based on Table 9, Note 2)
Manning Rose
(based on GFCI operating time)
200 milliseconds: Chris Dupres
(based on cardiac fibrillation)
100-500 milliseconds: Egon Varju
(based on human reaction time)
1 second: Name withheld -- private correspondence
(based on company interpretation)
1 minute: Gabriel Roy,
Kendall Wilcox
Richard Schumacher
UL's Application Guideline
(all of the above based on
NEC Class 2 limits)
The application contemplates using a fuse to limit the VA.
Fuse operating time is proportional to the current through the fuse.
Here are the UL 198G and CSA 59 current-time operating points at room
temperature:
Percent of current rating Operating time
------------------------- --------------
110 4 hours minimum
135 1 hour maximum
200 2 minutes maximum
So, if we were to rely on a fuse, and since, as Egon points out,
operating times less than 2 minutes are not calibrated, then to use a
fuse requires "continuous" to be more than 2 minutes.
I'd like to suggest two approaches to this question.
1. The first approach is to check the application of the HAZARDOUS
ENEGRY LEVEL requirements in IEC 950.
The requirement is found in Sub-clause 2.1.5:
In an OPERATOR ACCESS AREA, "it shall not be possible to
bridge... two bare parts... between which a HAZARDOUS ENERGY
LEVEL exists" with the test finger.
So, since the requirement is for an operator-access area, we know
that the HAZARDOUS ENERGY LEVEL must also be SELV.
Since we are dealing with SELV, we are not concerning ourselves with
electric shock, since, by definition, SELV provides protection
against electric shock.
If we look at some common SELV voltages, we have:
2.0 volts at 120.0 amps is 240 VA. (minimum voltage requirement)
3.3 volts at 72.7 amps is 240 VA.
5.0 volts at 48.0 amps is 240 VA.
10.0 volts at 24.0 amps is 240 VA.
12.0 volts at 20.0 amps is 240 VA.
15.0 volts at 16.0 amps is 240 VA.
20.0 volts at 12.0 amps is 240 VA.
25.0 volts at 9.6 amps is 240 VA.
30.0 volts at 8.0 amps is 240 VA. (maximum ac rms SELV)
60.0 volts at 4.0 amps is 240 VA. (maximum dc SELV)
The requirement is, if 240 VA is exceeded, then the conductors
cannot be accessible if the test finger can bridge the conductors.
If the test finger cannot bridge the conductors (if they are too far
apart), then it is okay for the conductors to be accessible.
This means that 240 VA CAN be accessible, for example, hand-to-hand!
Or finger-to-finger! Continuously!
The requirement implies that the two conductors across which is 240
VA cannot be accessible to ONE FINGER!
If so, then what is the meaning of my question as to the DURATION of
the 240 VA? We need to consider some more facts.
2. The second approach is to understand what injury is prevented by
limiting the energy between two bridgable conductors to no more than
240 VA.
Turn to the "Principles of Safety" at the beginning of the standard
and read the part about "energy hazard." It says that "short-
circuiting between adjacent poles of high current supplies... may
cause arcing or ejection of molten metal resulting in burns."
So, the injury intended to be prevented by the 240 VA requirement
is a burn.
Clearly, a finger does not short-circuit two SELV conductors. How
does a burn occur from a finger bridging two conductors? It occurs
when a ring worn on a finger bridges the two conductors!
Now, my question is: Can a fuse be used to prevent a ring (or the
conductors which it touches) from arcing or ejecting molten metal?
Arcing and molten metal ejection both require time to heat the metal.
Is 2 minutes too long at 30 volts and 8 amps?
Is 2 minutes too long at 5 volts and 48 amps?
We don't know. Electrical heating in this case is a function of the
cross-sectional areas of the two conductors and the ring, and a
function of the contact resistance of the two ring contacts.
Is arcing and molten metal ejection a function of volt-amperes?
That is, is volt-amperes the proper parameter which governs whether
arcing and ejection of molten metal will occur when a ring bridges
the two conductors?
Best regards,
Rich
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Richard Nute Quality Department
Hewlett-Packard Company Product Regulations Group
San Diego Division (SDD) Tel : 619 655 3329
16399 West Bernardo Drive FAX : 619 655 4979
San Diego, California 92127 e-mail: [email protected]
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