On 5/6/13 5:27 PM, K8RI wrote:
On 5/6/2013 6:48 PM, Hans Hammarquist wrote:
Maybe you can count on a "nice" weld if/when lightning strikes. It,
sure, will jump that little distance.
Yup, sure will and in the process apply many thousands of volts to
equipment tied to that ground. Poor grounds are the cause for many
equipment losses. It doesn't have to be a gap, just a few ohms. Just
think if a few ohmd resistance in the ground connection can cause a
great deal of damage, how about a small gap?
For the moment, let's look at that gap. Say it only has 300 volts
breakdown and lets ignore even that. The lightning flashes across the
gap creating a momentary plasma of just 1 ohm. The lightning strike is
not a strong one, but develops 5000 amps for only a millisecond. During
that one thousandths of a second the wiring in the house is E=IR or 1 X
5000 = 5000 volts above ground
actually, the voltage rise due to ohmic loss is a tiny fraction of the
rise due to L*di/dt. Typically, a lightning stroke might have a di/dt
of 10kV/us for a smallish stroke. With 1 microhenry inductance (about a
meter of any conductor, strap, wire, any size) that's 10kV.
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