Jim Brown wrote:
> On Sun, 9 May 2010 22:56:33 -0400, Jim Miller wrote:
>
>> If grounding should be done in a low impedance fashion why do I always see
>> the service entrance ground on homes to be nothing more than a AWG#4 wire
>> about 5ft long from the service box to the ground rod embedded in the
>> basement slab?
>
> First, because electricians are mostly pipe benders and wire pullers, and
> rarely have technical education. Heck -- even most hams still think of
> lightning as a DC event, not the RF event that it acually is.
While the electricians might be benders and pullers (and a experienced
journeyman is substantially better than that), they don't write the
code. The folks on the code review panels DO know about lightning, RF,
and line frequency faults.
They also are finely calibrated on economic realities. The odds of a
line to case fault in a piece of equipment, or a short from external
line to internal wiring, or a antenna:power line contact are a LOT
higher than a lightning strike. And, protecting against ALL lightning is
economically not worthwhile in most cases. If you *did* require it, then
you'd see a lot of waiver/variance requests, or, just plain old cheating.
The main focus of the NEC is safety of people and not burning the house
down, and that AWG 6 jumper will work just fine for that.
>
> But also because a 5 ft piece of #4 wire to the earth electrode IS a fairly
> low inductance path to that electrode, and a big piece of concrete can be a
> fairly low impedance path to earth because it has a large surface area.
> Remember that inductance is primarily a function of LENGTH. Conductor
> diameter (or width) makes a much smaller contribution. That is, a fat or
> wide conductor DOES reduce the inductance, but shortening the conductor by
> 25% will usually make as much difference.
It's even more striking than that. The inductance is a very, very weak
function of diameter (off the top of my head it's proportional to
log(1/diameter)+constant.. and the constant is big, compared to the
magnitude of the log term)
>
> Jim Lux wrote material in the 2010 ARRL Handbook on grounding for lightning
> safety, and I wrote that material for K7LXC's recent "Up the Tower" book.
> Both are good places to learn how to do it right.
>
> There's also a discussion of Grounding and Lightning Protection in Appendix
> 5 of http://audiosystemsgroup.com/RFI-Ham.pdf
>
> 73,
>
> Jim K9YC
>
>
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