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Lightning protection...

Discussion in 'Electronic Basics' started by Tom MacIntyre, Jul 5, 2005.

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  1. 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
     
  2. 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.
    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.
    What counts is the voltage across two points on your body. That is
    what drives current through it.
     
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