NEC Grounding Measurements and Code

[jay31558]

I have a fire alarm in a large, new apartment building that has been hit by lightning twice in the last month. We checked the building ground and it was ranging from 3.0-5.0 Ohms. I have always believed that they maximum resistance to ground was 2 Ohms. Can anyone point me to the code that states that? I see in 250 that it says 25 Ohms. Help!

 

[infinity]

The NEC does not specify 2 ohms. It does specify that if you have a rod electrode that it must be 25 or less or supplemented by an additional electrode. You could have 2 rods with a resistance of 1000 ohms and it would be code compliant.

 

[petersonra]

The NEC does not specify 2 ohms. It does specify that if you have a rod electrode that it must be 25 or less or supplemented by an additional electrode. You could have 2 rods with a resistance of 1000 ohms and it would be code compliant.

or you could have a CEE with 1000 ohms and it would be compliant.

 

[petersonra]

I have a fire alarm in a large, new apartment building that has been hit by lightning twice in the last month. We checked the building ground and it was ranging from 3.0-5.0 Ohms. I have always believed that they maximum resistance to ground was 2 Ohms. Can anyone point me to the code that states that? I see in 250 that it says 25 Ohms. Help!

People want to believe that they can protect electrical equipment by "better" grounding. It is just not so, at least for the most part. There are some cases where it may matter but they are special cases and not the general case.

You can't get rid of "noise" by "better" grounding either.

 

[infinity]

or you could have a CEE with 1000 ohms and it would be compliant.

True, same for the water pipe electrode or building steel electrode. I mentioned the rod electrode because that's one example of when you need to adhere to a minimum resistance measurement.

 

[zbang]

I have a fire alarm in a large, new apartment building that has been hit by lightning twice in the last month.

Is there lightning protection? Sounds like either there isn't any at all or that it might not be properly installed (placement of air terminals, grounds, etc).

 

[publicgood]

We assume this building does not have a listed lightning protection system? What area of the country is it located? Have you done an assessment (NFPA 780 Annex L)?

Free Access: http://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-and-standards

Does the building contain distributed surge protection?