Ground rod resistance

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David Rogers

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Lake County, California
Our company does ground testing for a gravel plant. The plant is in a high concentration of lava rock. We can only get 50 ohms at best.

There is a second rod driven the required 6' away from the original rod and tied together. Section 250-56 of the NEC indicates that one extra rod shall be driven.

When we send our report that 50 ohms resistance was read with the added rod, will MSHAW accept that as a safe and operational system?
 
David Rogers said:
There is a second rod driven the required 6' away from the original rod and tied together. Section 250-56 of the NEC indicates that one extra rod shall be driven.

When we send our report that 50 ohms resistance was read with the added rod, will MSHAW accept that as a safe and operational system?
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Couldn't tell you if MSHAW will accept it or not as I am not familiar with their standards. The actual impedance is not an NEC issue nor do they care. All the NEC has to say about it as you know if one rod does not produce 25-ohms or lower, drive another one and go home. So for example if you drive one rod and read a 1000-ohms, then you drive a second and you read 500 is good enough to satisfy the NEC and the girls they go out with.

Now what I will tell you is 6=foot spacing was a poor choice. It does meet NEC 250.33, but it does not conform to any recognized industry standard. Minimum spacing should be at least twice the rod depth. So if you used the NEC minimum rod length of 8-feet, the minimum spacing distance should be 16-feet. However this approach or method is not enforceable per NEC.

FWIW if you find out MSHAW requires something like 10-ohms or less, you would be well advised consulting with a company like Lyncole XIT Grounding or some other chemical ground rod manufacture, send them in soil samples, and spend the money. I kind of suspect something like a Gravel Crusher uses high voltage and will require a low impedance ground to protect the workers.

Moving to appropiate forum
 
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