On 1/10/16 10:08 AM, Paul Christensen wrote:
"So two 8 ft rods, 8ft apart is essentially the same as a single 16 ft
rod."
That assumes the soil content is the same along the depth of the rod. After
driving 50 ground rods in sandy soil conditions last spring, there was no
substitute for rod depth - at least in our soil condition. We achieved
nearly all the low resistance earth readings with four 26 ft. rods out of
the 50-rod system design. As a rod is driven deeper in some areas, it may
strike a more conductive region where one longer rod results in
substantially less earth resistance than well-spaced shorter rods.
So, I accept the 2x rod length spacing distance as a general guideline (as
required now by many AHJs), but don't always agree with the quote above if
we're discussing earth resistance.
AN excellent point..
All these calculations are totally bogus if the "uniform soil
properties" assumption isn't met. The IEEE Stds (there's a couple of
them, at least) do go into all the details of layered soil, etc.
For substation grounding, where there's significant amount of site
preparation before they install the equipment, and where they usually
use a grid of buried grounding cables, that's a significant factor.
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