Lets complete the example of 100 meter wire generating
30,000 volts. How much current in that 30,000 volts?
Remember, current (not voltage) causes the damage. Voltage is
only required to make current flow possible. But if the
electricity has near zero current, then the voltage becomes
irrelevant.
A long wire antenna might also suffer those 30,000 volts
(actually will be less due to other reasons). So we put an
NE-2 neon glow lamp on that lead. The milliamps or microamps
conducted by the NE-2 glow lamp is sufficient to reduce that
30,000 volts to near zero. Why? There is very little current
in that 30,000 volts. Same principle makes electronics inputs
resistant to such damage.
Properly noted because it cannot be said enough is the need
for single point earth ground. Principles demonstrated in
this technical note:
http://www.erico.com/public/library/fep/technotes/tncr002.pdf
Notice that even buried wires can be an incoming source of
destructive transients and therefore must connect to the
single point ground before entering the building.
A figure from the NIST is used to demonstrate the protection
of a fax machine:
http://www.epri-peac.com/tutorials/sol01tut.html.
Again the single point ground and how it is connected to is so
important.
How to create 300 volts per meter? Put electronics only 1
foot away from a cathode ray tube Television. 300 volts per
meter sounds impressive until we include other important
information. Current does the damage.
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