I would add new rods with as much separation as practical to increase
the charge distribution area. If your soil is particularly corrosive,
better rods are available from ERICO, either with thicker Cu or in
stainless ($$$ I'd guess).
http://www.erico.com/public/library/fep/LT0359_1.pdf
ERICO makes Cadwelds, which would be the best way to splice to the
existing ground wire.
Another good reference I found for my "grounding library" is
http://www.timesmicrowave.com/documents/resources/protectbrochure.pdf
I found this document the most helpful in understanding the why of tower
and coax protection systems.
Grant KZ1W
On 8/16/2015 5:28 AM, Jim Lux wrote:
On 8/16/15 12:01 AM, Tony wrote:
*John:*
Does corrosion make them not effective? John KK9A
*That's a good question. **
**
**It's my understanding that the reason for copper plating a ground rod
is to prevent the deterioration of the rods steel core. Why bother with
protective coatings if a slowly deteriorating rod is just as effective
as a new rod?
In my case, I have no way of knowing if the pitting found on the first
foot of the rod isn't much worse down further so wouldn't it be best to
either dig up one of the rods or simply replace them?
Tony
**
*
I'd not bother digging or pulling (although using something like an
auto bumper jack to yank them out of the ground might be interesting).
If you're concerned, drive new rods. I shouldn't think it matters how
close: you're driving as many new rods as old and connecting them.
The question about "effective" is an interesting one:
if the rod totally dissolved, it would be ineffective.
But, if it corroded, but was still in one piece, I don't know that the
slightly smaller surface area in contact would make all that much
difference. Thinking out loud, it's also possible that the corrosion
products might be more conductive than the native soil around the rod,
(of course, they might also have leached out).
a 5/8" diameter rod with 8 feet in the soil is 200 square inches. If
the rod corroded down to 1/2" in diameter, you're down to about 150
square inches. In a 20kA stroke, the current density goes up from 100
A/square inch to 133 kA/square inch. The resistance (and voltage at
the top of the rod) would go up by the same factor. That increases the
heating by about 80%.
Does that make any real difference? I'd think not.
You could also just bury a ground ring:
NEC 250.52(A)(4) requires a ground ring encircling the building or
structure, in direct contact with the earth, consisting of at least 20
feet of bare copper conductor not smaller than 2 AWG. AWG 2 is 0.26"
in diameter (a bit over 1/4")
Interestingly, 20 feet of AWG 2 is about 200 square inches, the same
as a 5/8" rod, 8 feet long, but even though the surface area is
roughly the same, the current is spread over a larger volume of soil,
which is a "good thing".
Larger sizes are often used: AWG 2/0 is 0.35 inches in diameter, for
comparison. For mechanical or convenience reasons. If you've got a
spool of 2/0 on the truck, the fact that you've tripled the cost of
the copper probably isn't a big deal in the overall scheme of things
in a commercial installation, where labor and "big stuff" (motors,
lighting fixtures, conduit) costs a lot more. (journeyman electricians
bill out at around $50/hr, apprentices around $30)
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