Grounding Test Results

[Turk1957]

Received this message from the Electrical Contractor on site, doing grounding testing: The ground rod tested at 26.6 Ohms from two different test directions. The 120 foot ground wire connected to the service rack ground rods tested at 2.0 Ohms. These two will be mechanically tied together resulting in an overall resistance of 1.9 Ohms. Does this appear to make sense to anyone? It has me thoroughly confused...

 

[Shaneyj]

Resistance in parallel.
26.6*2/(26.6+2) = 1.9 (rounded)

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[petersonra]

Perhaps he is treating the two resistances as being in parallel. That would work out to about 1.86 ohms.

Not sure why anyone would care in any case.

 

[Dennis Alwon]

What kind of soil do you have that gets 26.6 ohms. 120 wire to a rod and he gets 2 ohms? The wire must be underground. I was thinking the 120' was his 3 point test lead wire

 

[Mr. Serious]

I haven't ever had a test done like that or interpreted results from one, so I am confused too. A diagram would help, I think.

 

[Fred B]

Not sure why a test would have been done, usually for code reason just drive 2 and be done. Code calls for ground resistance to be 25ohm or less (nec250.53(A)(2) Exception) only if you are trying to use only 1 grounding means, but if you have 2 rods it doesn't matter. OP would need to answer if there is some underlying issue that a ground resistance test was being done, or maybe only one rod was present and trying to avoid driving another. (Cost - benefit ratio doesn't add up for the latter.)

 

[petersonra]

What kind of soil do you have that gets 26.6 ohms. 120 wire to a rod and he gets 2 ohms? The wire must be underground. I was thinking the 120' was his 3 point test lead wire

I think the 120 foot wire is what is going to be used to connect the two ground rod to the other grounding system so they will be in parallel.