Re: Human Electrocution: How is the resistance not ridiculously high?
- From: don@xxxxxxxxxxxxxx (Don Klipstein)
- Date: Sat, 5 Apr 2008 06:41:39 +0000 (UTC)
In <97de30be-74bd-456b-96ca-ba2ce8b96024@xxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Tomás Ó hÉilidhe wrote:
I've been doing electronics for three years now but I don't understand
how a person can be electrocuted by touching one part of the circuit
in a mains supply.
If I hold one lead of an ohmmeter in my left hand, and the other in my
right hand, it registers the resistance to be approximately 2
megaohms, which is ridiculously high.
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.
2. 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.
2. You could get such a shock if sweaty or otherwise wet.
3. 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 (don@xxxxxxxxx)
.
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- Human Electrocution: How is the resistance not ridiculously high?
- From: Tomás Ó hÉilidhe
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