Re: Human Electrocution: How is the resistance not ridiculously high?


"John G" <green@xxxxxxxxxxxxxx> wrote in message
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"Tomás Ó hÉilidhe" <toe@xxxxxxxxxxx> wrote in message
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Michael A. Terrell:

Then stick your hands into a live AC power line, after making sure
your will is up to date, and your insurance is paid up. You know just
enough to be dangerous. The resistance also depends on contact area,
and what part of the body. Loose the attitude, or someone will find
you
dead from your ignorance.


If you'll read my original post, I explicitly express that I _don't_
deny that people get electrocuted, so your accusations of ignorance
and "attitude" are illformed. What I'm questioning is the science of
it, as we understand it today. I've never, ever, not once, heard a
single valid explanation of how someone can get electrocuted by
touching a 230 V mains terminal.

I'm no denying that it happens. In fact I'm acknowledge that it
happens, and I also acknowledge that I don't understand how it
happens, and so I'm inviting people here to discuss the science of it.

I myself know about DC, AC, resistivity, resistance, capacitance,
inductance, impedance... but none of these things explain how 230 V is
enough to kill me if I grab the positive terminal.

Here's the path of the current:
1) From my hand to my foot: About 2 megohms.
2) From my foot thru to the other side of my sock: Probably in the
megohms, if not gigaohms.
3) From my sock to the inside of my shoe, to the sole of my shoe:
Probably in the megohms, if not gigaohms.

I just can't understand how 230 V is enough to push anything other
than a negligible current thru that gargantuan resistance.

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
http://van.physics.uiuc.edu/qa/listing.php?id=6793, 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:
http://web.mit.edu/Edgerton/www/Capacitance.html

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
http://www.sayedsaad.com/fundmental/11_Capacitance.htm, 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
http://dermatology.about.com/cs/skinanatomy/a/anatomy.htm) 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
http://www.codecheck.com/ecution.htm), 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:
http://www.dir.ca.gov/oshsb/rubberglove.html. 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


.

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