25 ohms from buss bar to g-electrode

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Bob Anchorite

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Ok, I am in code classes and the instructor makes a statement that one must have 25 ohms resistance between the buss bar and the ground electrode... searching through article 250 in general I could not find a reference to it. He would not explain it further (he is rather anal retentive). Does anyone know what his statement refers to?

Bob
 
Never mind, I found it....

Never mind, I found it....

250.56 Electrodes need to be 25 ohms or less....unless there is something else I am missing...
 
Your instructor is mis-stating the requirement. The requirement in 250.56 is the resistance of rod, pipe, and plate electrodes to earth and not the resistance of the grounding electrode conductor as inidcated by your post.

Save for impedance grounding systems, it would be practically impossible to not obtain 25-ohms resistance or less between the grounding electrode termination and its electrode.
 
Bob Anchorite said:
250.56 Electrodes need to be 25 ohms or less....unless there is something else I am missing...
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You are missing something else, as that is not what it says.

It says that if an electrode does not have a resistance to ground of 25 ohms or less, then you must augmented by an additional electrode. But once having installed that additional electrode, you do not have to measure either the first electrode or the augmented electrode, to see if either, or both, or the parallel combination of the two gives you a final resistance to ground of 25 ohms or less.
 
Bob,

For your anal instructor ask him if that measurement is with the buss bar energized or not. Energized your analog will scrrreech. Deenergized with all the breakers off "open" with the breakers on (except of course the main) it should read "infinity" (you will be reading through a neutral somewhere). He may like that one though because it is less than 25 ohms.

Andrew
 
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