24 Feb
1999
24 Feb
'99
4:20 a.m.
I have worked on various appliances ,electrical
devices and/or wiring
most of my life, close to 50 years, and I have been "bitten" on occasion.
Of course, except for stoves and dryers, the voltage here is 110 v. I
can
understand the respect given to 220v.(3wire) and have
never worked on
anything at that voltage without shutting down the circuit. I have also
been
very cautious about working on anything in damp
conditions like in a
washroom or wet basement. I realize that it is the total amperage that
kills
when you become ground. Most household circuits here
are 15-20 amps.
Having been "bitten" to no ill effects other than by my reaction to the
shock,
what are the parameters which would kill you on these
circuits. e.g.
rubber
soled shoes vs leather, etc.
There is a common misconception that the amperage is what kills you. That
is simply not true. At low frequency (50-60HZ) and DC, the human body is a
simple resistive circuit and will obey OHMS law E=I*R. It is impossible to
have one without the other!
IIRC a current of approx 100MA through the trunk of the body is enough to
stop the heart. If you measure the resistance of your body and plug those
factors in the foumula, you'll get an idea of how much voltage it takes to
kill you.
The internal body parts actually have a fairly low electrical resistance.
This is because your body is filled with fluids that are somewhat
conductive. Most of the bodys resistance is due to the insulating
properties of your skin. Dry skin is a relatively poor conductor of
electricity but, when it is dampened the resistance becomes much lower.
That is why medical probes have a conductive jelly on them. The electrolyte
in the jelly breaks down the resistance of the skin and provides a better
path for current into the body.
A cars battery is capable of producing 100s of amperes of current however,
it won't fry you. When you touch the poles of the battery, the amount of
current (determined by the formula above), is very low and generally cannot
be felt. However, if your hands are damp, you could get a minor electrical
shock from it. NOTE: A shock from DC is a burning sensation rather than the
"vibrating" sensation of 60HZ curent.
There are a couple of ways to getting shocked when working on appliances.
Some more dangerous than others.
If you are working on a device and not well insulated from the ground,
there is a current path from your hand through your trunk (and heart)
directly to the ground. If you are foolish enough to stand in water while
doing this, the resistance through the soles of your feet is much lower and
the potential of a fatal shock is greater. Thick rubber soled shoes like
tennis shoes and isolation transformers are the best protection here.
If you are working with both hands, it is very possible to get zapped from
hand to hand. Once again the current path is directly through the trunk
(heart) of the body and is quite dangerous. That is why many electricians
work with in hand in their pocket. A shock that is isolated to one hand
could be painful but, isn't nearly as dangerous.
The worst shock that I have received was working on a antenna for a HF
transmitter. I was dumb enough to leave the equipment running while
adjusting the antenna and someone keyed the transmitter from a remote
location. A high frequency shock (like DC) is a burning sensation and I had
the blisters to prove it.
Steve Robertson - <steverob(a)hotoffice.com>