Re: Electrical Wire Safety

[John Denker <jsd@AV8N.COM>]

Quoting Tony Wayne <wayne@PEN.K12.VA.US>:

> We've all seen and heard the warnings from our local power companies
> about being electrocuted by leaning a ladder against a wire going into
> your house. (I'm not going to test this one.) Here's the question, how
> does the electrocution occur through this, energized, insulated wire?
> Does the wire set up such a strong e-field, that when contacted,
> charges are drawn through you, along the ladder, to the wire? I've done
> some google searching and all the electric company sites that I hit on
> tell me is not to do it. None offer a science explanation.

There are alas many different scenarios.

About 99% of the cases, sensibly enough, are harmless.  But
the remaining cases can ruin your whole day.  So a logical
analysis of the "usual" case does not suffice.

Usually (but not always) the wires running into the house
(which are not the only wires in the world) are 120 volts
or thereabouts, two phases plus ground, insulated.  Usually
you can touch the insulated wires and nothing will happen.
Which is good, but don't take it for granted.

Bad scenarios are easy enough to construct:  Maybe the
insulation is old and cracked.  Maybe the aluminum ladder
has a sharp edge or point on it that penetrates the insulation.
Et cetera.  Now suppose the ladder leans against the wire.  If
the ladder is not grounded and you are, if you touch the ladder
you become part of the circuit.  This might (or might not) be
the last mistake you ever make.

An even worse scenario concerns the distribution wires, high
up on the poles.  Commonly those are something like 17,000 volts.
It would be prohibitively difficult to insulate those wires, so
they don't.  Now suppose you are a teenager hired to do swimming-
pool maintenance, and your pool-skimmer has a long, long aluminum
handle, which you swing against a distribution wire.  Kaboom!
(I didn't make this one up.  This is based on a recent news story,
but I haven't been able to dig up the exact reference.)

You don't even need aluminum ... I've seen trees catch fire when
a branch came in contact with a distribution wire.

Bottom line:  It's a complicated subject, with considerable
downside potential.  Many, many linemen are killed every year.
Analyzing the "typical" case or the "ideal" case is not
sufficient.  There are people who know how to handle the
stuff, but unless you have been thoroughly trained the best
rule is to give the stuff a wide berth.

HIOSH offers an 88-page manual
  http://www.cdc.gov/niosh/docs/2002-123/pdfs/02-123.pdf

It's also amusing to buy a copy of the National Electrical Code
and look through it.