Re: lightning dangers

[William Beaty <billb@ESKIMO.COM>]

On Wed, 28 Apr 1999, Carl E. Mungan wrote:

> > From time to time, someone will mention that it's dangerous to take a
> shower during a lightning storm. I'm assuming the idea is lightning could
> strike a tree or the ground nearby just as I put my hand on the tap. This
> doesn't seem like a terribly high risk to me, as I tend to think of
> plumbing pipes as generally being rather well grounded. But let's hear what
> you think. Is this an urban myth? Has anyone heard of any actual cases of
> electrocution this way? Carl

I think the danger comes about when the house itself suffers a strike
during your shower.  If we imagine the insulating materials to be
invisible to the e-fields of a lightning strike, then a wooden house is a
set of water pipes and wiring which juts up into the air like an antenna.

I suspect that a shower is not safe even if the main lightning path is
through some other conductor (say, through the electrical wiring.)  The
lightning is surrounded by fast-changing e-fields, and these fields cause
redistribution of charge in all surrounding conductors, and so electric
currents are induced in conductive objects near the main lightning path.
While this might not be fatal to a human who was suspended near the
lightning path, if that human is made electrically "larger" by touching
the plumbing and the sewer lines, their body might become part of the
antenna which is receiving a non-trivial impulse of current.  (Note that a
closed-loop conductive path is not required; this is electrostatic
induction.)

Here's a demonstration to try: stand on a chair, touch the sphere of a
vandegraaff generator, run the generator, then have an assistant hold a
large, grounded sphere up to the VDG sphere so that short, "hot" sparks
are created.  *You* will feel a shock!  But you are not part of the
circuit!  (Then ask yourself, "what circuit?")  The sudden changes in the
e-field surrounding your body will induce a current in your arm, making
your muscles twitch.  In this case you yourself were not an obvious part
of the spark-path, yet your body participated in the capacitive discharge
event.

Hey, here's how to say it: the person in the VDG demonstration was feeling
the displacement current, and a person in the shower could be ELECTROCUTED
BY DISPLACEMENT CURRENT!  Well, not exactly, since the "displacement
current" in the space around the lightning path resulted in a flow of
actual charges in the plumbing and in the victim.


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