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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