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I've been reading up on electrical safety, and the commonly cited controversial quote "it's not the volts that kills you, it's the amps". What I have gotten out of these sources is that it is in fact the amperes that stop one's heart, but the voltage determines this current, and the body is a non-ohmic resistor. (I have read this which mentions a lot about the volts and amps).

However, I am confused as to how much time a current must be passed through your body in order to be fatal. This video states that if you were to model the human body as a 100 picofarad capacitor with a 1500 ohm resistor in series, a high voltage static shock would pass a very high current through the body, but for an extremely short time (less than a microsecond). How long would a potentially dangerous (>5mA) AC/DC current have to be passed through the body in order to harmful? I understand that this is maybe too general a question, but is it possible to give a figure accurate to a few orders of magnitude?

Also, if it is in fact the amperes delivered over time that are detrimental, would a unit such as the Coulomb be more suitable for determining how fatal an electric shock might be?

EDIT: I am assuming that the electric shock is delivered hand to hand where it is most likely to pass through the heart and thus heart fibrillation is of most concern. If there are other significant causes of death from hand to hand shocks that I have overlooked, please tell me.

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    \$\begingroup\$ I like where this question is coming from, but it seems like there are several modalities to causing a lethal injury from electricity, brain-seizure route, directed cardiac interruption, some of the water in your body boiling and or going through electrolysis route. \$\endgroup\$ – Grady Player Mar 8 '13 at 3:19
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    \$\begingroup\$ Yes, I probably should've explicitly mentioned a specific cause of death due to electricity. \$\endgroup\$ – hedgepig Mar 8 '13 at 3:20
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You need to bear in mind that there are many different modes of damage possible,

  • Large current passed though tissue may burn the tissue
  • Modest current passed through a vital muscle (like that of the heart) may cause that muscle to operate in a manner that it's not used to and strain it so that it is damaged from the strain.
  • Very tiny current may interfere with the normal nerve signals that drive the heart's operation, throwing off the heart rhythm leading to ineffective pumping for long enough to be fatal.

So it's not a matter of stating a particular voltage, current, charge, energy or power. It's a matter of stating a range of very particular and quite different circumstances that pertain to different body systems.

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  • \$\begingroup\$ From my understanding, current passing through the heart causes it to beat irregularly/stop beating. Are you trying to imply that a sufficiently large current passed over a very short amount of time would not interfere with nerve signals, but actually burn the tissue? \$\endgroup\$ – hedgepig Mar 8 '13 at 4:18
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    \$\begingroup\$ A sufficiently large current will of course cause burning. The tissues of the body have resistance -- current through a resistance causes heat. As time progresses, heat that doesn't dissipate immediately will cause escalating temperatures, and at various temperatures different chemical reactions occur, the early ones being familiar to us as "cooking". At higher temperatures burning occurs. And none of that precludes any of the other effects... an electrical encounter which results in burning almost certainly affects the nerves in the path of the current as well. \$\endgroup\$ – gwideman Mar 8 '13 at 4:48
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    \$\begingroup\$ I think it should be said that the complexity of the body, along with the complexity of the manner in which one may come in contact with electricity, are sufficiently great that very little can be said with the precision you seem to be asking for. What is laid out with considerable precision is the set of safety regulations pertaining to electrical utility wiring, equipment, (particularly medical equipment) etc in order to err consistently and considerably on the safe side. Violating those rules doesn't mean immediate fatal results though. \$\endgroup\$ – gwideman Mar 8 '13 at 4:52
  • \$\begingroup\$ Yes, but why is it that a static electricity shock is of extremely high voltage and high current but is not fatal? There must be some sort of time range x in which one can safely experience high current shocks of y amps. \$\endgroup\$ – hedgepig Mar 8 '13 at 4:54
  • \$\begingroup\$ Again no general answer can be given, because it all depends on the path (possibly diffuse) of the current through the body what skin/muscle/nerve or organ lies in that path, what effects that is susceptible to, and what current over what period of time might damage that (or disrupt its normal operation). If the mode of disruption is interference with nerve signals, then these are less than 100 mV, so a shock that results in voltages of that level at a crucial nerve location will interfere with normal operation. Even there, one can't say precisely how much disruption is necessarily a problem. \$\endgroup\$ – gwideman Mar 8 '13 at 5:22
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From Wikipedia: http://en.wikipedia.org/wiki/Electric_shock

The lethality of an electric shock is dependent on several variables:

  • Current. The higher the current, the more likely it is lethal. Since current is proportional to voltage when resistance is fixed (ohm's law), high voltage is an indirect risk for producing higher currents.
  • Duration. The longer the duration, the more likely it is lethal—safety switches may limit time of current flow
  • Pathway. If current flows through the heart muscle, it is more likely to be lethal.
  • High voltage (over about 600 volts). This is an additional risk over the simple ability of high voltage to cause high current at a fixed resistance; high voltage may cause dielectric breakdown at the skin, thus lowering skin resistance and allowing increased current flow.

Also:

...cardiac tissue has a chronaxie (response time) of about 3 milliseconds, so electricity at frequencies of higher than about 333 Hz requires more current to cause fibrillation than is required at lower frequencies...

For lethality due to cardiac fibrillation, the current must flow for long enough to affect the cardiac muscle/nerves.

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  • \$\begingroup\$ Does this mean that the current should pass though the heart for a few miliseconds in order to be dangerous? \$\endgroup\$ – hedgepig Mar 8 '13 at 3:39
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    \$\begingroup\$ It means that someone can survive a higher magnitude current across the cardiac muscle if the duration is very short. Very short duration currents can still be fatal. \$\endgroup\$ – HikeOnPast Mar 8 '13 at 3:43

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