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

Chuck skrev:
On Thu, 3 Apr 2008 09:05:36 -0700 (PDT), Tomás Ó hÉilidhe
<toe@xxxxxxxxxxx> 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.
Now if I hold one lead in my hand, and dig the other into the grass,
it doesn't even register -- I may as well be holding the leads apart
in thin air.

Current = Voltage divided by Resistance

Current = 230 volts divided by 2 megaohms = 115 microamperes

115 microamperes is nowhere near enough to electrocute someone.

So lets say I stick a metal rod into the socket on the wall. The
current has to flow thru my hand, down to my foot, thru my cotton
sock, thru my shoe, thru the wooden floorboards, thru the concrete,
thru the clay down to the metal rod we call ground. Now excuse me, but
is that not a RIDICULOUS amount of resistance, up in the gigohms
somewhere?

It may sound like I'm denying that people get electrocuted -- I'm not,
I realise that people do get electrocuted. But I can't for the life of
me understand how enough current can flow, given the massive
resistances that are involved.

Can anyone enlighten me?

Probably not I.

But have you considered the difference in potential between your head
and your feet when you stand outside? A somewhat different situation
that may cause you to pose the same questions.

Another thing that occured to me while reading this thread is the question of what kind of ohmmeter the OP used to get 2 Megohms...
My interests apart from electronics have occationally caused me to experiment with skin galvanic response (the resistance of the skin and how it drops and rises under certain circumstances (wikipedia it)).

One thing I noticed during those experiments is that I get a much higher reading with Digital Multimeters than with old-school Analog ones. IIRC, the main difference between the two types of meter is that the digital ones tend to measure the voltage drop over the resistor when connected to a constant current source, while analog ones tend to to be a battery
connected in seriers with the meter (through a variable resistor to be able to null the meter).

I do not claim to know the cause of this, but my hunch is that the amplifier (OP or Transistor) picks up hum from the body, making the reading be a little off. I do not know if making the amplifier insensitive to AC voltages is a priortiy for the designers, but it wouldn't surprise me if they save money on not making it so.

Oh, BTW, the readings I've got with my analog meter have been 50k to 150k -ish, which puts it near the area of being dangerous with regards to current passing through your body. Factors I've noted that affect skin resistance is your emotional state, area of contact (2 leads when compared to, say, two washer with leads soldered to them), to name a few...

And the traditional "my worst zap story":
I've accindentally connected myself to a 235VAC/50Hz power grid, phase in on hand, neutral in the other (don't ask). I actually heard the 50Hz hum in my ears, and my ticker occationally still bitches about it, a whole 14 (sic!) years later... Yes, I do respect the national power grid a lot more these days :-)

Just my $.02 worth,

/Teo.

--
Teodor Väänänen | Don't meddle in the affairs of wizards,
<teostupiditydor@xxxxxxxxxx> | for you are good and crunchy with
http://www.algonet.se/~teodor/ | ketchup.
Remove stupidity to reply. |
.

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