Small Contact Voltages - Setting Appropriate Limits

[meby]

When the general public receives a shock of some sort, it can quickly grab national headlines. But setting acceptable voltage and current limits isn't as easy as one might think. Here's what one electrical professional has to say on the subject. Read the thought piece.

Anyone out there like to share their thoughts on how limits should be selected and enforced?

[jayguy]

in the 1st paragragh of the article, the author states: "Excluding operators of DC electric welding equipment, no case of greater than or equal to 50V electrocution to a human is known to exist."

this seems wrong and counter-intuitive to the article.

[timebuilder]

I think it's a typo.

My thoughts:

while there may be a time function to exposure leading to death, the first order of importance is the amount of current to which the heart muscle is exposed, which of course backfeeds into the nerve ganglia of the Vagas nerve.

http://en.wikipedia.org/wiki/File:Gray791.png

While higher currents can permanently damage the tissues of the body, it takes only a small current to create a confusion of contractions known as ventricular fibrillation. I doubt that any experiments have been done on live volunteers to establish the minimum current required to initiate this condition, but I recall reading that the level is supposed to be about 6 Milliamps when applied to the chest. This eliminates the hand-to-hand and hand-to-foot resistance of the body, which varies due to several factors, but which would require a higher applied voltage to achieve that current in the chest.

So, the problem in addressing the standard issue is that the bodies of workers are not standard. Internal tissue and external surface resistances vary, which leads to a variance in the amount of voltage applied (and how it is applied) in order to determine the lethal amount of current that will be the result.

And, applied in the prescribed manner, I'm sure that you could force a heart into fibrillation without using a full 50 volts.

[jayguy]

I think it's a typo....

i think so too. just wasn't sure. i also think that there are too many factors to determine what is a safe level. i guess we need to start somewhere.

[Nuclrchiller]

The Navy uses 30 V AC as the "cutoff" point. Up to 30 is considered safe to work on live for troubleshooting, without extra precautions. Above 30 V AC, precautions such as tieing off (for another man to pull you back), and removing all metals from your person (watch, belt buckle, etc...) are required. This was in the late 70's - early 80's. May have changed since then. I seem to recall that this number was based on the Navy's studies and beliefs, along with their experience with accidents. And a worst case scenario of an individual's conductivity, including sweating.

[timebuilder]

The Navy uses 30 V AC as the "cutoff" point. Up to 30 is considered safe to work on live for troubleshooting, without extra precautions. Above 30 V AC, precautions such as tieing off (for another man to pull you back), and removing all metals from your person (watch, belt buckle, etc...) are required. This was in the late 70's - early 80's. May have changed since then. I seem to recall that this number was based on the Navy's studies and beliefs, along with their experience with accidents. And a worst case scenario of an individual's conductivity, including sweating.

I think I saw that in a rate training manual in the 70's.

Always better to be cautious!

[meby]

in the 1st paragragh of the article, the author states: "Excluding operators of DC electric welding equipment, no case of greater than or equal to 50V electrocution to a human is known to exist."

this seems wrong and counter-intuitive to the article.

Well, that was definitely an item missed in the final edits of this article. Good catch!

It was definitely a typo. I just went in and changed the word "greater" to "less".

[WhoIsThat?]

I've heard that the lowest voltage at which anyone was electrocuted in the whole history of the US Navy was 47 volts; dunno' if it was DC or AC (400 Hz?).