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.
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.
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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