[TowerTalk] copper or galvanized ground rods in red SC clay

[jimlux]

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.