# Naked Science Forum

## Life Sciences => Physiology & Medicine => Topic started by: Geezer on 27/07/2010 18:52:33

Title: How lethal is an electric shock relative to the voltage?
Post by: Geezer on 27/07/2010 18:52:33
Domestic line voltages in the US are about half the typical European (mains) supply voltage.

All other factors being equal, does this mean an electric shock is twice as likely to be lethal in Europe, or is it four times more likely?

In other words, does the power have anything to do with it, or is it simply the current?

EDIT:

As Sheepy is a great believer in the acquisition of empirical data, I did try to enlist his services as a subject to participate in a barbecue some experiments I'm conducting. Despite the offer of first class travel on Ramsome's aeroplanes, he has, so far, declined.
Title: How lethal is an electric shock relative to the voltage?
Post by: tommya300 on 28/07/2010 10:50:05
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"A common misconception is that larger voltages are more dangerous than smaller ones. However, this is not quite true. The danger to living things comes not from the potential difference, but rather the current flowing between two points. The reason that people may believe this can be explained by the equation V = IR. Since V is directly proportional to I, an increase in voltage can mean an increase in current, if resistance (R) is kept constant.

The amount of damage done by the electric shock depends not only on the magnitude of the current, but it also on which portions of the body that the electric current is flowing through. The reason for this is that different parts of the body have difference resistances, which can lead to an increase in current, evidenced by the formula V = IR.

An interesting fact to note is that it takes less alternating current (AC) to do the same damage as direct current (DC). AC will cause muscles to contract, and if the current were high enough, one would not be able to let go of whatever is causing the current coursing through the body. The cut-off value for this is known as the "let-go current". For women, it is typically 5 to 7 milliamperes, and for men, typically 7 to 9 milliamperes. This is dependent on the muscle mass of the individual.

In general, current that is fatal to humans ranges from 0.06 A to 0.07 A, depending on the person and the type of current."

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http://hypertextbook.com/facts/2000/JackHsu.shtml
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Title: How lethal is an electric shock relative to the voltage?
Post by: Pumblechook on 28/07/2010 14:12:33
I gather there are more electrocutions per capita in the USA than the UK.  It may come down to the quality and regulation of electrical installations.
Title: How lethal is an electric shock relative to the voltage?
Post by: Geezer on 28/07/2010 16:57:27
I gather there are more electrocutions per capita in the USA than the UK.  It may come down to the quality and regulation of electrical installations.

Ah, but in that case, all other factors may not be equal. The question was specifically about the voltage.
Title: How lethal is an electric shock relative to the voltage?
Post by: JP on 29/07/2010 01:38:25
Everything I've been told in physics classes and when being lectured about electrical safety for labs/work is that it's purely the current.  But I don't believe current isn't linearly tied to chance of death (even if the path through the body and resistance were held constant), so voltage shouldn't be either.

I found this interesting site when looking for information about what voltages are lethal:
http://www.physics.ohio-state.edu/~p616/safety/fatal_current.html

It suggests that the most lethal currents are between 0.1 and 0.2 Amps (across the heart).  Higher voltages cause the heart muscle to clamp down rather than twitch in response to the shock, which paradoxically leaves you a better chance of surviving at higher currents.  That means that the most lethal voltage is one that gets you into that window.
Title: How lethal is an electric shock relative to the voltage?
Post by: Geezer on 29/07/2010 02:51:35
Thanks for that link JP. Interesting, not to mention scary!

I seem to remember being told it was only the current too.

However, ultimately, we are just a resistor (but not necessarily constant), and, while the current through a resistor is proportional to the voltage, the power dissipated by a resistor is proportional to the square of the voltage. Also, each cell in our body is just a resistor, so the power is ultimately being dissipated in the cells that carry the current.

I suspect that the power dissipation only results in some additional heat that the cells are able to deal with if the shock is of short duration, whereas the current completely disrupts their normal operation, but I really have no idea if that's true or not.
Title: How lethal is an electric shock relative to the voltage?
Post by: JP on 29/07/2010 04:30:28
Good point about heat dissipation.  Electrical burns would probably quadruple if you double the voltage (keeping all else the same), since they're related to power dissipation.  But since current controls muscles, then current can also cause damage by stopping the heart (and causing muscles to tear as they contract forcefully).
Title: How lethal is an electric shock relative to the voltage?
Post by: Geezer on 29/07/2010 06:01:23
That makes sense JP.

I think any power dissipated in a cell is likely to increase its temperature. That might possibly reduce its resistance, which would increase the current flowing through it.

Maybe, maybe not!
Title: How lethal is an electric shock relative to the voltage?
Post by: MartinTheK on 07/08/2010 23:41:32
All of these consideration apply to dry normal skin.

For example, If a person has an external pacemaker in place with a direct pathway to the endocardium, it would take only a small electrical discharge to seriously settle their hash. Attention must be paid