On 10/12/11 10:49 PM, Ron W8RJL wrote:
> If you take a direct lightning hit on your tower the size wire between the
> tower legs and the ground rods does matter.
> In the book "Compehesive Grounding and Protection of Communications Sites
> they say #2 AWG minimum.
> I believe the Motorola R56 Grounding document also shows #2 AWG for
> grounding towers.
> The wire should be as short as possible and have as few bends as possible,
> any bends should be gradual. If possible have the wire cadwelded to the
> ground rods. Do it right the first time and it will last a long long time
> with little maintenance.
> 73, Ron W8RJL.
Before depending on those kinds of references, you might want to see if
you can find out *why* they recommend AWG 2. For commercial
installations, there's a fair amount of "the wire cost is a tiny
fraction of the job cost, we've used AWG X for the last 40 years, so why
analyze it to death, just use the same as we've always used".
Any awful lot of lightning protection around the world uses wire much
smaller than AWG 2 for the downlead. From a physics standpoint, looking
at the largest lightning strokes and fusing current for wires, etc., AWG
2 is overkill.
However, from a mechanical ruggedness against things like weed whackers,
people with shovels, etc., you might want something bigger. (the
electrical code requires a bigger wire if it's not attached to the side
of the building, or in a conduit, for instance) A service call to a
remote mountain top site in a storm to restore an essential service is a
lot more expensive than upgrading from AWG6 to AWG2 or AWG 2/0.
It's that maintenance thing, as you pointed out.
SHort is good (lower inductance = lower transient voltages), lack of
bends is good (electromagnetic forces is why.. not inductance), no tight
bends for same reason (a complete loop will probably break itself apart
with the current transient, especially if it's small in diameter)
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