Robert Macy wrote:
Assuming most of lightning
energy is significant between 1MHz to 100MHz, calculate each conductor's
characteristics at 10MHz and at 100MHz using 2D finite element analysis.
This is at variance with real lightning and the simulated lightning generators
for equipment test.
Lightning Physics and Effects (Rakov &Uman) states that electromagnetic signal
levels from lightning peak in the 5 to 10 kHz region, then fall inversely with
frequency up to 10 MHz and then fall inversely with the square root of
frequency to 10 GHz.
Actual simulated lightning generators, such as the 1.2/50 and 10/1000
obviously have an even more restricted spectrum. Strandler (Protection of
Electronic circuits from overvoltages) shows the spectrum of several lightning
test pulses. The –6 decibel spectrum fall-off points are variously 300 Hz
10/1000, 5 kHz 1.2/50, 30 kHz 8/20 and 200 kHz for the 100 kHz ring wave.
Non-lightning waveshapes like EFT and HEMP have a much broader frequency
spectrum.
Also please be careful when talking about lightning energy - this can lead to
dreadful statements like "The lightning protection system diverts the
lightning energy to earth". It doesn't, current gets diverted to ground and
the amount of energy in the earth depends on the earth potential rise -
something the lightning doesn't have control over.
Mick
UK
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