I have a requirement to provide an isolated ground. The customer has asked for the Isolated ground wire to originate at the secondary side of the transformer, XO, and be brought into the panel and landed on an isolated ground bar. I have an existing 1200 amp 120/208 three phase service, exterior meter pack system. The utility transformer is about thirty feet away and comes right into two 600 amp disconnects. The service is grounded/bonded to the water and ground rods per NEC. From this point on the neutral is floating to individual retail locations. I have pulled into the same conduit an isolated ground wire and landed it on an isolated ground bar. Even though it may be isolated it is still electrically the same wire because it is terminated at the same point as the equipment ground. Is this not a permissible installation of an isolated ground?
The equipment being served is a 30kVA UPS and Telephone Switching gear.
Buck, I think your customer has recieved some information from an engineer or technician who is stating IEEE Emerald specifications, and has confused him/herself.
Emerald specs do call for the IG to originate on the Xo of a customer owned isolation transformer, not a utility owned transformer. As stated already what your customer is asking for will not be permitted by the utility. It would be nice if the utility would supply EGC circuits, but it is not going to happen.
For your application the IG has to originate from the in the disconnect that contains the N-G bond point.
If your customer wants a IEEE spec IG ground, they would need to install a shielded isolation transformer to supply the Telco equipment. This is a common practice, but it is not supplied by the utility. Hope this helps...
Bob is right. An IG for computer stuff should land on the same bus as the main bonding jumper and ground electrode conductor. You want a "star" originating at the MBJ/GEC junction. Grounding should then branch from there in a tree structure so there is no parallel path. Back in the day we never brought a neutral to a computer room panel (all L/L loads) and we used the isolated neutral bus for the IG. Equipment and raceway grounding was on the bonded supplimental bus. (In a prior life I was an IPR for IBM designing computer rooms).