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Re: Weather/coil performance

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 08:30 PM 1/19/2007, you wrote:
Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>

Hi Dr, all,

Actually, all of the copper-clad ground rods that I have
bought in home Depot or Lowes are 8 feet long. Their
diameter does vary from 1/2" to 5/8", with the 5/8" dia
ones obviously being a bit more costly. The addition of
salt while driving the rod into the ground is a different
idea that I would have never thought of. The only thing that
I wonder about in this scenario is the salt corroding the cop-
per cladding and ultimately reducing the concuctivity. I think
the surface of copper is going to eventually oxidize in the
presence of persistant moisture anyway, though, so the salt
thing is probably a good idea. I drove (3) 8 ft rods only 4 ft
apart and interconnected them with heavy braided welding
cable (1-0 AWG, I think) for the grounding system of my
Green Monster coil.

Corrosion, hard soil, and the like is why the recommended practice for grounding of electrical systems these days is the "concrete encased grounding electrode" or Ufer ground (Herbert Ufer invented it in the 40s). From an electrical code standpoint, 20 ft of something conductive (rebar or AWG 4 bare copper wire) encased in at least 2" of concrete on all sides, but for coiling you might want a different configuration. In any case, in numerous tests, the Ufer ground provides a ground impedance <5 ohms (which is a lot better thana single rod), and is more durable than any sort of chemically enhanced rod type ground (the chemicals leach away). (Ufer's original grounds, all with resistance <2 ohms, after 18 years had risen to an average of 3.8 ohms)

google for concrete encased grounding electrode for more info.. there's quite a bit out there.
As far as the freezing moisture is concerned, I think an
8 ft. ground rod should reach well below the frost line
level for most of us, unless you're coiling on the North
pole or in Antarctica ;^) To get an idea of the frost line
of your area, check your local plumber's depth level code
for burying water lines and that will give you a good idea
of depth level that the ground would never be expected to
freeze to in your locale. I'm in the Memphis, TN area and
in my area, the water lines must be buried 18". I'm suspect
that depth requirement would be considerably greater in say
Minneapolis or Calgary, but still not near the near 8 ft depth
that can be reached with an 8 ft. long ground rod. I would
suspect the loamy, alluvial soil of the eastern US and Canada
would conduct better than the more arid rocky and sandy soil
of the western US and Canada, too.

Depending on the frequency, most of the coupling to "earth" might be capacitive, so the soil resistance (from freezing) isn't as important.