Human Electrocution: How is the resistance not ridiculously high?

Several people have tried to explain that a human body is not very much like a typical carbon resistor. I suggest that you start by trying to wrap your mind around that concept. Think of a cap for a second. Frequency is one factor that impacts the impedance. In a somewhat similar way there are many factors that affect the resistance of a human body. Your trusty multimeter is incapable of giving you meaningful information about a capacitor. Likewise it's incapable of giving you any real info about the human body. Life is more complicated than we like sometimes.

I'd be very surprised if your feet don't sweat at least a tiny bit. I'm sure you know that salt waater conducts pretty well.

Would depend a lot on the shoe. For a street shoe and some boots which use nails or tacks to help hold the sole on I think you might be way off.

On a good day it won't. The problem is that other day when a shoe becomes compromised. Even when the shoe isn't the path all it takes is an entry and an exit. Techs are taught habits that reduce the chances you will get into a bad situation. Removing watches and rings, keeping one hand in a pocket when reaching into a live circuit, and others. At really high power at rf frequencies even providing a sharp point can be a bad thing. Techs are taught not to point at anything while in the transmitter room at LORAN stations. Pointing with a single finger can privide an opportunity for an arc that won't be any fun.

Anyway, the resistance of a human body is very complex and bears almost no relation to the common and much simpler carbon resistor. You're going to have to shift your thinking here.

Being retired military we always had a lot of meggars around. Guys would play around with them once in awhile. It was always amazing how heavy those test leads got so fast. I was also surprised that apparently meggars seem to turn people into geneologists. Soon after dropping the heavy leads they would start questioning ancestry almost without exception. I can't explain the brain chemistry behind it, but but it was clearly a statistically significant observation. :)

No body has said much about AC and Ground.

Most everything that is not a live wire is connected to GROUND in one way or another and so the whole environment you are in is one side of a capacitor and so if you touch a live wire then you are the other side of this capacitor and some current will flow because the power system is Alternating Current.

No measurements with your punny little multimeter will mean anything in this case and depending on a myriad of facts already discussed you could be dead.

John G.

There is some more information at

where it states that the external human body reistance is about 1k to 100k Ohms, and the internal resistance is 300 to 1000 ohms. Only a thin layer of dry skin separates the internal resistance from an external object.

The human body capacitance to a far ground is 100-200 pF, which is really a minimum value. This correlates to an impedance of about 13 MegOhms at 60 Hz, which corresponds to a minimum of 9 uA at 120 VAC to ground. This is enough to be sensed and used for capacitively operated light dimmers.

Here is a way to measure your body capacitance:

The inside of your body can be considered a conductor, and thus if you place your hand flat on a metal plate, you will form a capacitor with an area of perhaps 15 square inches, with a thin (maybe 0.005") insulating layer of dry skin, which will form a capacitor much higher in value than the 200 pF stated above. According to a formula in

this would be C =

0.2249 * k * A / d = 1350 pF, (assuming k for skin is 2, about like dry paper). This will be an impedance of about 2 MegOhms, and current of 60 uA. This is still below the normal threshold of sensation, and still far below the usual safe current levels of 1 to 5 mA.

The actual thickness of the epidermis (per

varies from 0.05 mm (0,002") for eyelids to 1.5 mm (0.06") for palms and soles, but the actual outer layer of the epidermis that is a good insulator is composed of flat, dead cells, which is much thinner. So the capacitance could be much higher than the quick estimate above.

Probably the main reason for electrical current to reach levels high enough for electrocution to occur (6 to 200 mA for 3 seconds, according to

is when skin becomes sweaty or otherwise loses its dry protective layer, which quickly exposes the underlying 1000 ohms or less, which will conduct 120 mA at 120 VAC.

There are safe ways to measure the body's resistance and capacitance using realistic higher voltages, skin conditions, and contact surfaces, but I'm not going to suggest anyone try it. Suffice it to say that ohmmeter readings are misleading, and any carelessness around any kind of voltage source can be dangerous.

For very high voltages, there are standard minimum distances that must be maintained between a worker and an energized line:

I found this on a search for rubber glove testing. My previous company manufactured oil and glove insulation breakdown testers.

The field intensity near high voltage lines is so great that it might be fatal to touch them even if you were suspended in free air. You may notice that birds can sit on lower voltage transmission lines which are 5kV to 50 kV or so, but not on 200kV+ lines.

Paul

I do find readings like this for this situation to be common.

Now, for some factors to complicate this:

1. Skin being just a little on the moist side - due to body chemistry, mood, recent past activity, body response to ambient temperature and humidity - it's a little common for this to be a few hundred K-ohms rather than a couple megohms. Occaisionally this kind of reading can get down to

50K ohms or so.

1. Current in the roughly-1-milliamp range or more can do a few things to make the resistance decrease:

a) Stimulate sweat glands - especially if the current is AC or pulsating DC of frequency probably anywhere in/near the lower half of the audio range, especially 50/60/100/120 Hz

b) Cause electrolysis that results in a decrease in contact resistance over time.

Try holding tightly two bare wires coming from a DC power supply of voltage of whatever voltage is low enough for you to assume is safe and not have yourself or next of kin sue me over if things go wrong in any way, with a milliammeter or microammeter in series with the current path. If that voltage is around/above 12 volts, see if that current stays steadily low or starts increasing. Imagine what could happen at 120 volts.

c) At/near 120 volts or more, localized heating could occur at skin contact points. Skin and body fluids generally have negative temperature coefficients for their resistance, especially skin.

===========================

Other things to consider:

1. You may get accidentally shocked or shocked by malfunctioning equipment with skin contact area larger than that typical with handling of ohmmeter leads.

1. You could get such a shock if sweaty or otherwise wet.

2. The most-widely-mentioned "fatal range" of current, for causing ventricular fibrillation, is 100 mA to 1 amp for an arm-to-arm or arm-to-leg shock with 50-60 Hz AC. (Increase of current past 1 amp has fatality rate less than that of .1-1 amp, in case of arm-to-arm shock with "power line frequency AC", but there is still some fatality rate from outright cardiac arrest - plus risk of vital organs getting outright cooked.)

This is merely a "most deadly range", with the "deadliness" not dropping to zero at 99 or 90 mA. Some sources say 50 mA is the lower end of the range of having a fairly significant chance of causing ventricular fibrillation from an arm-to-arm shock, and a small number of sources say that 30 milliamp neon sign transformers (which have current-limiting means, unlike most transformers that are not "lamp ballasts") have a bit of a body count!

For that matter, I have seen one bit saying that there is some chance of fatality at currents as low as around 5 mA - from someone being paralyzed by the shock, with paralysis including paralysis of breathing muscles.

Keep in mind that shock causing someone to involuntarily maintaining a position that maintains exposure to the shock is widely said to be worse with DC, but is actually worse with AC (or pulsating DC as opposed to steady DC). Steady DC is "less-shocking", since most effects of electric shock result from variation of current. The horror stories from people receiving severe electrical burns on (and also inside) their bodies mostly involve those zapped with either DC or radio frequencies - so that they survive to tell the horrors!

======================

Keep in mind that electrocution can get unreliable. The "Electric Chair" appears to me designed to rely on the jolt either cooking vital organs, and/or paralyzing breathing muscles (and preferably also the heart) long enough to have the brain deprived of oxygen severely enough to be unable to restart breathing when the jolt stops. Sometimes the condemned is subjected to more than one jolt.

As unreliable as electrocution is, lack of fatality from electric shock is similarly unreliable.

======================

The human body is a 470K-ohm 1/4 watt resistor with tolerance of

+5000/-98 % and a negative temperature coefficient!

(I don't know who started this, and I could easily be "off" with the numbers somewhat for that one)

- Don Klipstein ( snipped-for-privacy@misty.com)

AC is worse. Pulsating DC is worse than steady DC.

DC gets some worse reputation due to higher survival rate of those getting zapped with current that causes horrific burns, especially to internal tissue that heals slowly or never heals right. Being jolted into maintaining body position that has you getting zapped is often said to be worse with DC, but that is actually worse with AC (or pulsating DC). The horror stories come more from survivors than from those who don't live to tell horror stories.

- Don Klipstein ( snipped-for-privacy@misty.com)

On Apr 4, 10:47=A0pm, snipped-for-privacy@manx.misty.com (Don Klipstein) wrote: > =A0 AC is worse. =A0Pulsating DC is worse than steady DC. >

Has Tom=E1s =D3 h=C9ilidhe stopped responding because he offed himself ?

GG