[TowerTalk] Grounding question

[Jim Brown]

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