Why 2-Ground rods???

[kwired]

In our locality we do not require a second rod. I won't talk you out of a second rod but one will pass. Our reasoning isn't due to any electrical theory but our position on what we can legally enforce. We cannot prove a single rod doesn't meet the requirement and do not expect the contractor to purchase necessary testing equipment. That being said, I'm glad the 2011 language was changed. We are currently under the 2008. May not be the most sound reasoning but the contractors like us....until the next Code change!

Like I said earlier, that is likely a big reason for the change. No inspectors had equipment to test the resistance, and had little ground to require the second rod, with the rewording it basically makes the installer prove he has low enough resistance or he drives a second rod, less to question if you don't want to prove resistance of the first rod.

 

[zog]

Using the "Fall of Potential Test." Here is one explanation, but you can Bing it yourself to get others.
http://www.psihq.com/iread/grndsyst.htm

Hmmm, some decent stuff there but a lot of wrong info and misconceptions. That is some very dated material and some contect is just plain wrong. Here is a much better reference. www.biddlemegger.com/biddle-ug/GettingDownToEarth-MC.pdf

 

[K8MHZ]

Hmmm, some decent stuff there but a lot of wrong info and misconceptions. That is some very dated material and some contect is just plain wrong. Here is a much better reference. www.biddlemegger.com/biddle-ug/GettingDownToEarth-MC.pdf

This is from Biddle (a good book, I have a copy)

From the diagram for FOP testing it appears it is done with two test electrodes and that's it. That's not the case if you read on. You have to do a few tests to find the 62 percent point first.

Potential-reference rod P is then driven in at a number of points roughly
on a straight line between the earth electrode and C. Resistance readings
are logged for each of the points. A curve of resistance vs. distance, like
Fig. 5b, is then drawn. Correct earth resistance is read from the curve for
the distance that is about 62 percent of the total distance from the earth
electrode to C.

Isn't that the same as what is in the link Charlie posted?

BTW, the Biddle book is hardly new. I got mine in the late 90's.

What in Charlie's info was incorrect. It's all dated.

Nothing since Biddle's book with more recent info?