[R-390] Grounds

[Tisha Hayes]

The next release of the National Electric Code will have additional details
on grounding antenna installations. The existing NEC requirements for
facility grounding only address the bare minimum requirement for the service
entrance.

You can conceivably have a ground resistance as high as 25 ohms at the
service entrance.

The ground downlead you see on utility poles usually go to a "butt-wrap" or
a "nailer". The butt wrap is just a few feet of bare wire wrapped around the
butt end of the pole. The nailer is a copper disk the size of a small dinner
plate that is nailed to the bottom of the pole. Sometimes they will drive a
galvanized steel ground rod next to the pole.

As mentioned by others, in a substation there are ground mats that are
buried under most of the yard. Ground resistances there are frequently less
than 1 ohm and absolutely everything is bonded together. There are even
special isolators that go on telephone lines that go into a substation.

With comm site design I specify less than 5 ohms to ground on non-critical
unmanned locations and less than 1 ohm to ground on repeaters, Access Points
or master stations.

When driving ground rods in an array they need to be distanced apart equal
to the length of the ground rod (area of influence). Using copper strap (2"
wide) is better than most conductors unless you have a bunch of 500 MCM
sitting around. Lightning is much more of a skin effect and the surface area
of a conductor is more important than the sheer mass or wire gauge.

In a substation where you may be trying to open a protective device on a 161
KV circuit at 250 amps you want the resistance to be very low (or dangerous,
lethal potentials exist) and the ground conductors need to carry the full
fault current.

The right way to test a ground is with a Megger type instrument capable of
testing a "three lead Wenner Array" for an existing site or to do a four
lead Wenner array test on a proposed site.

It gets really expensive to push for the 1 ohm ideal on a site with poor
soil or rock. I had that problem with my home the topsoil is six inches to a
foot deep, sitting on top of sandstone. When Alabama Power was setting the
utility pole they had to use multiple charges of dynamite to make a hole.
(they had more fun with that, it took an entire day to set one pole and
didn't cost me a dime)

The poured foundation walls are sitting directly on sandstone. I am 'ufer
bonded to the rebar and have 210 feet of 2" copper ground strap buried right
on top of the rock layer (even a run in the crawl-space from one side of the
house to the other). At the corners we core-drilled a few feet down into the
sandstone (and hit limestone a few feet down), then we drove copper coated
steel rods and backfilled with coke breeze to improve conductivity.

With all of this, and the ground radials at the antennas, bulkhead entrance
panel, and it all bonded together I still take lightning damage. Living on a
mountaintop I take 4-6 strikes a year just in my front yard. Unless I am
actively listening I leave all of the coax connectors disconnected. I should
own stock in Linksys for all of the Ethernet devices that have been toasted
over the years. I have also lost an SP-200 due to lightning when it
Chernobyl'ed the first RF stage coils and have given up on anything with a
pre-amp in it.

-- 
Ms. Tisha Hayes

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"I will not recant the truth. I am corn, not chaff; I will not be blown away
with the wind or burst by the flail. I will survive both."
-Walter Milne, 1558