Equipment Groundig Requirements

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joanie

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Tyndall Air Force Base, Florida (Panama City)
On an Air Force Base, communications equipment for about 80 facilities are being upgraded/installed. The sub panels are about 40 years old, but met the code at installation and through 2005 (I believe). The facility has not been upgraded nor has any "significant" work been done on the electrical system.

The communications people say that they need an equipment ground all the way back to the main distribution panel. Existing installation uses rigid conduit between panels, continuous, and bonded either to the enclosure or to the ground bus,with the enclosure bonded properly. Code now shows PVC instead of rigid - there is nothing that I saw in the Handbook that shows the setup with RSC.

Is the equipment ground, or what we have been using as an equipment ground, sufficient for the facility? If so, then the additional requirement falls to the contractor for the comm equipment to upgrade as required. Civil Engineers are responsible for facilities and to provide an electrical connection (panel), but special requirements to support equipment are the responsibility of the user (communications people).

In your opinion, does the subpanel circuit, with a properly sized equipment ground, have to be replaced all the way to the MDP?

Thanks very much, Joanie
 
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petersonra

Senior Member
Location
Northern illinois
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engineer
The customer is always right. If the customer wants something that is beyond the bare minimum the code requires and is willing to pay for it, that is their right.

In any case, there may be more stringent requirements in their specs than in the code.
 
augie47

augie47

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Tennessee
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State Electrical Inspector (Retired)
If the original system was installed with GRS conduit it was likely to Codes then. If it were installed with GRS conduit today it would meet the requirements of the NEC (not speaking of local codes or specs).

If the GRS conduit is as old as the panels (40 years) then the integrity of the conduit/grounding means might well be in question.
 
charlie b

charlie b

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Location
Lockport, IL
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Retired Electrical Engineer
It sounds like they are asking for an "isolated ground." See 250.146(D).

I think it works something like this: On the sub-panel you will have a neutral bar and an equipment grounding bar that are not connected to each other, but the ground bar is connected to the enclosure. You also have another isolated ground bar that is not connected to the enclosure or to either of the other two bars. A wire will need to be run from the isolated ground bar all the way back to the main panel's equipment ground bar, without landing on any other location along the way. At the communication equipment being served, the ground pin of the receptacle outlet is not bonded (internally) to the yoke. So you have four wires to deal with: The phase conductor and the neutral conductor are connected to the receptacle in the usual way. But the EGC is bonded to the receptacle yoke and not to the green ground screw, and at the other end it lands on the ground bar in the sub panel. The isolated ground wire is attached to the green ground screw, and at the other end it lands on the isolated ground bar.

It would be a good thing if someone were to check my wiring description. I don't do wiring for a living, and I might have described it incorrectly.
 
qcroanoke

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Sorta retired........
charlie b said:
It sounds like they are asking for an "isolated ground." See 250.146(D).

I think it works something like this: On the sub-panel you will have a neutral bar and an equipment grounding bar that are not connected to each other, but the ground bar is connected to the enclosure. You also have another isolated ground bar that is not connected to the enclosure or to either of the other two bars. A wire will need to be run from the isolated ground bar all the way back to the main panel's equipment ground bar, without landing on any other location along the way. At the communication equipment being served, the ground pin of the receptacle outlet is not bonded (internally) to the yoke. So you have four wires to deal with: The phase conductor and the neutral conductor are connected to the receptacle in the usual way. But the EGC is bonded to the receptacle yoke and not to the green ground screw, and at the other end it lands on the ground bar in the sub panel. The isolated ground wire is attached to the green ground screw, and at the other end it lands on the isolated ground bar.

It would be a good thing if someone were to check my wiring description. I don't do wiring for a living, and I might have described it incorrectly.
Click to expand...

That is a very good explanation. But I did not know that you needed 2 grounds. Of course I also have never had the fun of doing an isolated ground system. Sounds like I haven't missed anything.........
There are also a lot on here that will tell you isolated ground receptacles are useless as........ on a bull.
 
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