scott <sco### [at] scottcom> wrote:
> An electric fence has a very low value current limiter (of the order of a 
> few milliamps), whereas you can get 10's of amps from wall sockets.

> U=RI is never violated, because a current limiting circuit will control the 
> voltage to ensure the current limit is not exceeded.  If you connected a 
> voltmeter to the electric fence you would see the voltage drop significantly 
> when you touched it.  BTW this is exactly how an LED driver circuit works, 
> because they need a constant current supply, rather than voltage.

  I think that what your explanation lacks is a description of how the
voltage (and hence the current, as 'R' doesn't change, as in this case
it's the resistance of the human body which is connecting the wire to
the ground) changes over time when the connection is made.

  If the wire is at 40 kilovolts and the ground is at 0 volts, when a
person connects them through his body, the current 'I' will be quite high,
because it will be I = 40kV/R, which is much higher than with a wall socket,
where it's I = 230V/R.

  However, the relevant thing is what happens immediately after the
connection is made. What happens is that the voltage of the wire decreases
very rapidly until it approaches the voltage of the ground. Thus the
voltage difference between the two will be 40 kilovolts only for a really,
really small amount of time.

  Thus I think I wouldn't be really wrong when I say "it's not the current
that kills you, it's the electrical power transfer (over time)".

-- 
                                                          - Warp

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