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 is that, if
> it can be proven that this signal will switch at a frequency no higher
> than 0.2 Hz under all normal and single fault conditions; then you have
> a non-hazardous "DC" signal. (Note that I'm not saying 0.2 Hz is the
> cutoff, it's probably higher.)
>
> OK... I've hung it out there. Either agree with it or refute it.
> Either way, we'll all learn something.
>
> Chris Maxwell | Design Engineer - Optical Division
> email chris.maxw...@nettest.com | dir +1 315 266 5128 | fax +1 315 797
> 8024
>
> NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA
> web www.nettest.com | tel +1 315 797 4449 |
>
>
>
>
> > -----Original Message-----
> > From: Ken Javor [SMTP:ken.ja...@emccompliance.com]
> > Sent: Friday, November 09, 2001 1:22 PM
> > To: Tania Grant; Doug McKean; EMC-PSTC Discussion Group
> > Subject: Re: Define Continuous DC Voltage
> >
> > I'm probably not providing any assistance, but what is the purpose of
> > the categorization of continuous dc vs. that rate of change where it
> > is not considered continuous anymore? I don't think it should be tied
> > to ability to deliver power to a load, which is in agreement with what
> > Ms. Grant is saying below. Pure dc provides power and no information.
> > A signal uses power to transmit information. If I have a security
> > alarm sensor on a window which always sends a low-level dc until the
> > window is broken then if I look at the physical parameters I could say
> > that low-level signal was dc because it could be on for years, but its
> > PURPOSE is to transmit information which makes it a signal. Actually
> > any single-sided digital transmission (meaning between 0 Volts and
> > some Vcc) is dc in the classical sense because dc means direct
> > current, as opposed to alternating current which changes direction.
> > In the sense which people in this exchange have been using the
> > terminology it refers to how much time rate of change is allowed. But
> > this is where the question as to purpose comes in. If the issue is
> > crosstalk, a low-level audio or video signal with lots of rate of
> > change is a much more benign source than a 48 Vdc source from which
> > lots of switched CURRENT is drawn. A dc POTENTIAL does NOT imply
> > direct CURRENT unless CE limits have been applied to loads. So the
> > question that has to be answered first is what is the purpose of the
> > discrimination implied by the term "continuous."
> >
> > ----------
> > From: "Tania Grant" <taniagr...@msn.com>
> > To: "Doug McKean" <dmck...@corp.auspex.com>, "EMC-PSTC Discussion
> > Group" <emc-p...@majordomo.ieee.org>
> > Subject: Re: Define Continuous DC Voltage
> > Date: Thu, Nov 8, 2001, 9:47 PM
> >
> >
> >
> >
> > Well now, Doug, how about the vast gray area in between?
> >
> >
> >
> > What if it is 1/50th of an amp?
> >
> >
> >
> > taniagr...@msn.com <mailto:taniagr...@msn.com>
> >
> >
> >
> >
> >
> >
> > ----- Original Message -----
> >
> > From: Doug McKean
> >
> > Sent: Thursday, November 08, 2001 7:39 PM
> >
> > To: EMC-PSTC Discussion Group
> >
> > Subject: Re: Define Continuous DC Voltage
> >
> >
> >
> > eric.lif...@ni.com wrote:
> > >
> > > So friends, how continuous must DC be to qualify as
> > continuous DC?
> >
> > Personally, I'd say when the source can deliver some
> > sufficient level of real power (rms level of power).
> > That is basically the definition of rms anyway.
> >
> > To make the point with two ridiculous examples,
> > (1) if by shorting a 5 volt digital signal to ground
> > you measure 1/100th amps of rms current, then I
> > wouldn't call it DC.
> > (2) if by shorting a 5 volt digital signal to ground
> > you measure 10 amps or rms current, then I'd
> > most definitely call it DC.
> >
> > But that's just me and my 2 cents worth.
> >
> > - Doug McKean
> >
> >
> >
> >
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