Ground resistance testing

[skeshesh]

Good day guys. I have a contractor asking me if there's a need for resistance ground testing for a small project. I'm pretty sure the NEC doesn't state any requirements for testing only the ohmic requirements for the grounding rod and system. I haven't been able to find anything in the local code (though I may need to search deeper.) I also don't really have any information about when the last resistance testing was done for the existing system. Any advise would be much appreciated.

 

[mcclary's electrical]

250.56:roll::roll:

 

[cpal]

I agree with you I am not aware of any where in the NEC that requires a fall of potential or any other method of test to establish a base reading. (but that might be a proposal if presented properly).

250.56 does establish a minimum requirment. 25 ohms is not great and usually cell towere and head ends will require a lower value.

don't forget the NEC is not a design manual. see 90.1

 

[skeshesh]

Grounding

Grounding

Thanks for the responses. Maybe I'm not being clear: I'm aware of the ohmic requirements for grounding (25ohms for each rod, no less than 5ohm for the grounding system). My question is where does one usually find requirements or recommendations regarding when or how often to perform the grounding resistance testing for an existing system.

 

[Cold Fusion]

.... 25 ohms is not great and usually cell towere and head ends will require a lower value. ...

charlie -
What's a "head end"?

cf

 

[roger]

Thanks for the responses. Maybe I'm not being clear: I'm aware of the ohmic requirements for grounding (25ohms for each rod, no less than 5ohm for the grounding system).

That is not what the NEC requires, all that is required by the NEC is that a "single" rod be 25 ohms or less, if this can't be acheived you add one more rod and go home regardless of what the resistance is. There is no 5 ohm requirement.

My question is where does one usually find requirements or recommendations regarding when or how often to perform the grounding resistance testing for an existing system.

This would be an engineering and / or performance spec.


Roger

 

[zog]

Good day guys. I have a contractor asking me if there's a need for resistance ground testing for a small project. I'm pretty sure the NEC doesn't state any requirements for testing only the ohmic requirements for the grounding rod and system. I haven't been able to find anything in the local code (though I may need to search deeper.) I also don't really have any information about when the last resistance testing was done for the existing system. Any advise would be much appreciated.

ANSI/IEEE Std 142 requires large commercial or industrial systems to be less than 5 ohms and generating or transmission stations to be less than 1 Ohm.

ANSI/NETA MTS/ATS and NFPA 70B recommend test is done every 2 years in accordance with IEEE std 80.

 

[skeshesh]

That is not what the NEC requires, all that is required by the NEC is that a "single" rod be 25 ohms or less, if this can't be acheived you add one more rod and go home regardless of what the resistance is. There is no 5 ohm requirement.

I took a look at 250.56 again and you're absolutely right I was misreading "a single" to mean "each". I could swear I read the 5ohm total system resistance somewhere, but I could be wrong. It could've easily been some recommendation in an IEEE color book or something like that since I've made that mistake before. In any case, our specs don't call out for testing on this job that's why the contractor sent an RFI, so I'm trying to figure out if it's a good idea to test for resistance or just a waste of money/time. The job site is a public park but I don't have any information about their soil nor how often they've tested in the past.

 

[roger]

Unless there is some reason for performance grounding I'd say it's a waste of money and time. In a public park I can't think of a good reason for it.

Roger

 

[zog]

If they want it done, do the test and bill them for it. But from the sounds of it there is not a requirement.

 

[Cold Fusion]

...ANSI/NETA MTS/ATS and NFPA 70B recommend test is done every 2 years in accordance with IEEE std 80.

zog -
I just looked through NFPA 70B and did not find any reference for the time - Except a reference to electrical testing for substation grounds every three years in Annex I

Could you give me a reference where this is in NFPA 70B?

cf

 

[zog]

zog -
I just looked through NFPA 70B and did not find any reference for the time - Except a reference to electrical testing for substation grounds every three years in Annex I

Could you give me a reference where this is in NFPA 70B?

cf

I don't have access to mine rigth now, it was added in the 2006 version, in the new RCM section I think.