Lightning protection...

[Tom MacIntyre]

Hi folks...this was in a baseball newsgroup that I participate in
(rec.sport.baseball). Faraday cages have been mentioned, implying that
it could be safe. I don't know that I'd trust this against lightning
(maybe a Van de Graff generator). What do you think?

"Just wondering if sitting on a wood bench inside a metal cage (i.e.
metal on top and 4 sides) is a safe place to be during a lightning
storm. This is similar to the situation of being in a car (except for
the rubber tires!). I have seen younger kids wait out a passing storm
this way and am wondering if it IS in fact safe? What would the effect
of metal cleats be? I'd appreciate any thoughts from anybody but
especially from physics teachers/physicists. Thanks!"

Tom

 

[John Popelish]

Hi folks...this was in a baseball newsgroup that I participate in
(rec.sport.baseball). Faraday cages have been mentioned, implying that
it could be safe. I don't know that I'd trust this against lightning
(maybe a Van de Graff generator). What do you think?

The concept of a Faraday cage is that for a perfectly conducting
object, all the voltage is on the outside. The only way to get
voltage difference between two internal points of a real conductive
object is based on ohm's law that says that voltage is proportional to
the current times the resistance. So if the resistance between the
point on the cage that the arc lands and the point where it continues
on its way is, say, .001 ohm, and the arc carries say, 10,000 amperes,
the voltage drop between those two points is
..001 ohms * 10,000 amperes =10 volts. That is no more dangerous than
a car battery. Should the current reach 100,000 amperes (not the
record holder) the voltage drop would reach 100 volts and you would
risk a shock touching both those points on the inside of the cage.

"Just wondering if sitting on a wood bench inside a metal cage (i.e.
metal on top and 4 sides) is a safe place to be during a lightning
storm. This is similar to the situation of being in a car (except for
the rubber tires!).

The wooden bench doesn't help much, as long as you contact only one
point on the cage. The tires don't help at all, since the arc has
already crossed possibly miles of air, it will have no trouble going
around 6 inches of rubber. Cars that are made with a completely
welded unibody are safer than ones with bolted together sections, but
the doors are not connected to the rest of the body very well, so it
is a good idea not to lean on them. The arc might hit the door, have
trouble getting to ground by way of the hinges and latch, and a bit of
it may go through you on its way to the rest of the car body.

I have seen younger kids wait out a passing storm
this way and am wondering if it IS in fact safe? What would the effect
of metal cleats be? I'd appreciate any thoughts from anybody but
especially from physics teachers/physicists. Thanks!"

What counts is the voltage across two points on your body. That is
what drives current through it.