[RFI] ISOBAR

[David Robbins]

>Physics says that most of
>the lightning strike current will flow into/out of that tower ground
>because it is closest to the struck object and has least length; hence,
>lowest Z.

not necessarily... 

>Good point about the voltage differential across a grounding wire (or
>system). Hey, it's real wire and has real resistance. The developed
>voltages are directly proportional to the length of that wire and its Net Z
>(actual Z + DC resistance).

yes, but its not the resistance that matters in the case of a lightning stroke (or even when talking about 'rf' grounds)... the length has MUCH more of an effect than the resistance.


> In a real strike situation, if "just" 10,000
>amps flow in a coupled circuit, that's 10 kV per 1 ohm of Z in the path. 
>Nice "tingle".

below i'l ignore the slowdown from the velocity factor of the line above the ground, it makes the effects a bit slower, but this is close enough.

lets say that 10ka stroke has a risetime of 1usec and hits the tower end of an elevated feedline that is 100m long.  in a typical situation where you have a decent tower ground, that will probably drain off 90% of the current, but that still leaves 1ka on the feedline... 1ka heads down the feedline, but it takes 1usec for the front edge to get to the shack end... meanwhile the voltage at the tower base/feedline end is up to about 200kv, but the voltage at the shack end is zero, that is way more than resistance alone would predict.  so what happens to the feedline?  if there isn't lightning protection at the tower the voltage on the center conductor stays at the original ground voltage but the shield is now at 200kv, could be a problem there if the coax insulation can't handle that voltage.

try another case, a typical indoor case of what many would consider a well grounded shack... there is a perimeter ground, on one end of the house is the power entrance connected to it, on the other end is the radio shack where all the incoming feedlines connect to the perimeter ground.   lets even have a whole house lightning arrester on the power entrance.  lightning strkes a tower on the shack end of the house, why does the radio that is also grounded to the perimeter ground get fried?  shouldn't the voltage be the same between the radio and feedline?  well yes, but that is the problem, the voltage on the case of the radio that is connected to the perimeter ground goes up, but it takes a few tenths of a microsecond for the current to go around the perimeter ground, to the whole house arrester, then equalize the power line voltage to the ground, then propagate back through the house wiring to the shack in an attempt to protect the radio... unfortunately its too late, the stroke has backflashed to the power line through the radio already and meets it part way.