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Author Topic: Rationale for definition of extra-low-voltage  (Read 6914 times)
EccentricAnomaly
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« on: December 14, 2010, 10:27:45 PM »

BS 7671 defines extra-low-voltage as not exceeding 50 V AC or 120 V DC.  This seems odd to me as, for a given voltage, DC is normally considered more dangerous from both the physiological (gripping vs throwing) and arc-drawing points of view.  Any idea why it's defined this way?
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stuartiannaylor
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« Reply #1 on: December 14, 2010, 11:36:47 PM »

I think it to do with stopping the old ticker. Its the pulses that can have more harm. I might be wrong but something echoes in that mush of mine that i fondly refer to as memory.
SELV is the same with 12vac and 24dc LV cant remember but I guess that will be the same.
« Last Edit: December 15, 2010, 06:14:19 AM by stuartiannaylor » Logged

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Ted
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« Reply #2 on: December 15, 2010, 10:19:45 AM »

The values are linked to average body electrical resistance, I believe. Don't forget that a.c. value is rms.

We need a copy of IEC 60479-1 Effects of current on human beings and livestock - Part 5: Touch voltage threshold values for physiological effects
and IEC 61201 Use of conventional touch voltage limits - Application guide to be certain.

There is some interesting detail in section 12.2.3 (p255) of this book: Forensic Neuropathology and Neurology, 2006, Oehmichen et al

(Probably best not to read if you are subject to any degree of hypochondria. Electric shock can cause many more problems than just heart failure.)

[edit]
A bit more googling turned up this: http://extranet.artc.com.au/nswdocs/nsw_engineering/engineering_standards/electrical/pds10_safe_limits_of_dc_voltages.pdf

which contains some of the information from the above IEC reference including a couple of charts at the end.

Specific to the original question it quotes:

“For shock durations longer than the cardiac cycle, the threshold of fibrillation for dc is several times higher than for ac. For shock durations shorter than 200 ms the threshold of fibrillation is approximately the same as for ac, measured in r.m.s. values.”

There is another reference to the contents of the IEC doc at section 4.1.2 of this http://ped.slac.stanford.edu:8080/pem/ewp/Linac/Linac_Sub_Booster_Modulator.pdf (can't copy and paste the text as it is a scan)

And this http://emedicine.medscape.com/article/433682-overview says:

Quote
AC is substantially more dangerous than DC. Contact with AC may cause tetanic muscle contraction, preventing the victim from releasing the electrical source and, thereby, increasing the duration of contact and current delivery. Thoracic muscle tetany involving the diaphragm and intercostal muscles can result in respiratory arrest. The repetitive nature of AC increases the likelihood of current delivery to the myocardium during the vulnerable recovery period of the cardiac cycle, which can precipitate ventricular fibrillation. In contrast, DC usually causes a single violent muscle contraction, often thrusting the victim away from the source.

Finally: http://www.marinerhosting.co.uk/bha/images/stories/publications/bhaelectricalsafetystandards.pdf

Chapter 2 - The Effects of Electricity on the Human Body covers a.c. and d.c. in different sections. I'll leave you to read it as I can't paste from that either.
« Last Edit: December 15, 2010, 04:01:50 PM by Ted » Logged

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EccentricAnomaly
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« Reply #3 on: December 15, 2010, 02:54:42 PM »

Yes, John Whitfield's Electrician's Guide to the 17th Edition... says "The reason for the difference is partly that direct voltage is not so likely to produce harmful shock effects in the human body as alternating current, and partly because the stated value of alternating voltage is r.m.s. and not maximum" but doesn't immediately talk about why the shock effects are different.

Looking for that quote this afternoon I also noticed further down: "It is interesting to note that a direct voltage with a superimposed ripple is more likely to cause heart fibrillation {...} than one which has a steady voltage" which indirectly but pretty solidly backs up stuartiannaylor's recollection.
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stuartiannaylor
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« Reply #4 on: December 15, 2010, 03:46:16 PM »

Just going at a tangent I am thinking to re-start an open source network LED lighting design again. I could never find out before what is considered a ripple and also there must be a minimum frequency when DC is not DC anymore. Anyone have the exact definitions for  24V dc
« Last Edit: December 15, 2010, 03:55:59 PM by stuartiannaylor » Logged

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Baz
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« Reply #5 on: December 15, 2010, 04:06:36 PM »

Wouldn't DC have to go 'negative' to become AC?
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stuartiannaylor
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« Reply #6 on: December 15, 2010, 04:15:34 PM »

No there are maximum ripple currents and I could never seem to find an exact specification. I wanted to superimpose a carrier signal and also provide a stop pulse in a similar way (ooof memory) the dc modulation scheme that trains use.
Doesn't matter that much but I just remember it annoyed me at the time.
« Last Edit: December 15, 2010, 04:20:58 PM by stuartiannaylor » Logged

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EccentricAnomaly
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« Reply #7 on: December 15, 2010, 04:58:10 PM »

I could never find out before what is considered a ripple and also there must be a minimum frequency when DC is not DC anymore.

I think you mean minimum frequency when AC is not AC any more.  E.g., nobody would suggest that a voltage which reverses about every 5.25 days (1 µHz) was AC.

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Anyone have the exact definitions for  24V dc

Sorry, no.  Nearest I have is the sentence preceding the "It is interesting to note..." on cited above: "The allowable ripple is such that a 120 V system must never rise above 140 V or a 60 V system above 70 V."  At least this confirms that there is a maximum ripple and gives an idea of the allowed magnitude (one sixth of nominal voltage?).
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stuartiannaylor
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« Reply #8 on: December 16, 2010, 11:48:51 PM »

Thanks for the post. High speed SMPS basically switch DC at high frequency so in essence it is just a ripple current. There most be a level that is acceptable.

AC doesn't really matter all that much that it goes negative, its ripple and pulses that cause the difference, thats why we have a DC 24 and AC at 12. Well thats what I presume. AC just has a zero crossing but its potential difference that gives us the shock. Otherwise AC would be twice the allowed DC value.

I know its confusing but with high speed switching DC converters there must be a minimum switch speed or it becomes as dangerous as a AC signal. I just could never find any details.

Stuart
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