Grounding Separately Derived System (again sorry)

[shortckt07]

Ok so I have looked everywhere to find the actual verbiage on this. So the ground rod to the newly installed or even existing XFMR or separately derived system cannot really be used unless there is no water or steel electrode? Now I guess what I am asking is that the EI wanted me to bring the grounding electrode conductor back to the main grounding electrode system. The only thing I see is for the water or steel per 250.30 (A) (7) that requires either of the 2 however I do see 250.50 that requires these to all be bonded together. So is that where the EI gets that it has to go back to the main grounding electrode system? For example: Existing grocery store with new switchboard, and some new XFMRs installed. Would each one of these XFMRs be required to go back to the main grounding electrode system where the new switchboard is? There is no building steel. And do we really rely on water given that most services dont have 10' of copper in the ground? Im just having a hard time understanding the electrode for the separately derived system going all the way back to the main grounding system. Thanks

 

[Smart $]

You've essentially answered your own questions.

SDS's required to be grounded must be grounded to the nearest of water pipe or structural steel electrodes. If neither exist, you can use or install other electrodes per 250.30(A)(4) Exception No. 1... but the result must comply with 250.50.

 

[shortckt07]

How could I go to nearest water if the water piping doesnt comply as an electrode? I mean at the least I could comply with 250.104d. To bond the water pipe. So if I install a ground rod at the sds.....then I have to run back to the service where there is a ufer ground etc where the grounding electrode system is?

 

[Smart $]

How could I go to nearest water if the water piping doesnt comply as an electrode?

You can't. It has to qualify as an electrode. And if it did, it woul (should) be connected to the service GES.

I mean at the least I could comply with 250.104d. To bond the water pipe.

Yep. Its required.

So if I install a ground rod at the sds.....then I have to run back to the service where there is a ufer ground etc where the grounding electrode system is?

Yep, pretty much. 250.50's requirement is straight forward.

But your SDS-to-service GEC doesn't necessarily have to go back to the service itself. It can go to the nearest point on the pre-existing GES, provided all interposing GEC's are properly sized for the SDS.