Grounding Test Using Slope Method 1

[jerebear]

We are testing a large ground grid, approximately 400 feet by 200 feet using the fall of potential slope method with the C2 electrode distance of 500 feet from the test point of the grid. Attached is the test report for the grounding system with noted results. We have been told this is not a valid test and need a second opinion.

Thanks

 

[trosepe]

I could not open your test report; said file was damaged. From your desription of the placement of your test electrodes above: your electodes were too close to the grid under test. The distance of the P2 and C2 electrodes is determined by the diagonal dimension of your grid. Use the following link and go to page #24.

http://www.biddlemegger.com/biddle-ug/GettingDownToEarth-MC.pdf

 

[jghrist]

The slope method is a generally acceptable method to measure grid resistance where it is not practicable to place the current probe far enough away to be outside the zone of influence of the grid. Your calculation appears to be correct.

I would try including the slope calculations and a reference to an explanation of the slope method.

 

[jerebear]

Thanks for the response trosepe. Would this also be true if the slope method had been used to perform this test? That is the case in this instance.
I don't know why you could not open the attachment, I tried and it opened right up.

 

[Kiribanda]

jerebear,

Your test is accepted to me. But two notes.

1)In slope method, normally distance C2 to be taken as 2-3 times the largest dimension of the grid. The diagonal distance is 447 feet. That means if you have taken 2.5 times say 1118 ft (I donot know whether it is possible at the site)then the result would have been more accurate.

2)In slope method, normally the test should be repeated for a larger distance of C2.Therefore, is it possible to repeat the test for 1118 feet to check whether there is a considerable deviation from your first "True resistance value" of 1.072 Ohms?

Hope this helps

 

[alehman]

Great article, Trosepe.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[trosepe]

alehman,

The article, like some of us, old but still relevant.

 

[jerebear]

Thanks for the responses guys, these are the methods I have always used in the past, but just wanted a second or third opinion on the results. We have a local testing company promoting a new meter that they are the only ones in the area to possess, and they are saying this is the only way we can get a valid test.

 

[jghrist]

Another way that we have used is to make a Fall of Potential Plot with SES CDEGS software. This will provide a point at which the correct grid resistance can be measured. In one case, we used both the CDEGS method and the Slope Method with comparable results (2.18 ohms for Slope, 2.01 ohms for CDEGS).

I assume that the local testing company is using an EPRI Smart Ground MultiMeter developed by Sakis Meliopoulos and marketed by Advanced Grounding Concepts

http://www.ap-concepts.com/win_sgm.htm.

This method is not without controversy. See IEEE Transactions on Power Delivery, Vo. 8, No. 3, July 1993 pp 1095-1106, A PC Based Ground Impedance Measurement Instrument. A comment by F.P. Dawalibi (SES) was "Unfortunately, I have no alternative but to state that the authors have failed in their attempt" (to find an alternative to the fall-of-potential method).