Ground Rods in series

[Electricjunky]

Ok, I'm on a spec job with primarily clean rock. 1st ground registered 944 ohms. I then ran a 2nd over to the fence line, so I could try to catch a little dirt. It lowered my ohms to 48. I then moved 20' further along the fence line, and drove another rod. That lowered my ohms to 14.85. Out of curiosity, I removed the 1st ground rod from the series, since it registered 944 ohms. My ohms on the remaining two then dropped slightly to 14.05 ohms. Why is this? Is there a threshold where ground rods in high impedence soil give no benefit to a series of ground rods? Same thing happened on a separate site across town. Removed 2 ground rods that registered 600 ohms independently, from a series of 6 ground rods. The remaining 4 had the the ohms, as it was with 6 ground rods. Is their a resource that I can refer to to better understand why that is. Thanks.

 

[iceworm]

Soares book on Grounding and Bonding is good. Also the IEEE-142 Grounding of Industrial and Commercial Power Systems.

Those would likely be my favorites.

ice

 

[GoldDigger]

I would ask what method you used to get the remarkably precise 14.05 ohm figure. If you are trying to use three point fall of potential, it can be difficult to repeat measurements and the presence of other rods in the area, even if not connected to the rod under test can affect the measurement.
If you are using a clamp meter, again this measurements taken at different times can vary significantly.

A difference of .8 ohms out of 14 is not significant, even when using a digital ground resistance meter.





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