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#48157 02/05/05 01:52 AM
Joined: Feb 2005
Posts: 19
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buck Offline OP
Member
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
#48158 02/05/05 10:02 AM
Joined: Jan 2001
Posts: 1,044
Tom Offline
Member
I don't believe that your utility company is going to provide you with an isolated ground back to Xo on their transformer.

As far as I can tell, you have met the NEC code requirements. I'm glad they didn't ask you to run this isolated ground to an individual ground rod as so many end users do.

Tom


Few things are harder to put up with than the annoyance of a good example.
#48159 02/05/05 10:08 AM
Joined: Jan 2003
Posts: 4,391
I
Moderator
It should not run back to XO in this case.

In this case running it to the location of the Main bonding jumper at the service disconnect is the correct way to do it.


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#48160 02/05/05 11:46 AM
Joined: Oct 2003
Posts: 156
D
Member
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...

#48161 02/05/05 04:32 PM
Joined: Feb 2005
Posts: 19
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buck Offline OP
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Thank you all for your replies. I'll have to try and educate my end user who took a two day seminar on grounding. Again, thank you.
Rick Buck


Buck
#48162 02/05/05 06:15 PM
Joined: Oct 2003
Posts: 156
D
Member
Buck, now it makes sense. Your client took a ground class and heard the IG should originate from the Xo which is 100% true, except in this case.

#48163 02/06/05 12:22 AM
Joined: Oct 2003
Posts: 156
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Member
Buck, FWIW I assume your client is going to use the IG circuit to feed a UPS, and the UPS is supplying the telephone equipment. If that is the case an IG is not nessecary to feed a UPS.

#48164 02/06/05 12:25 AM
Joined: Feb 2005
Posts: 19
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buck Offline OP
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yes, you are absolutely right. Thanks for the second opinion. Buck


Buck
#48165 02/08/05 01:19 AM
Joined: Jul 2004
Posts: 9,696
Likes: 11
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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).


Greg Fretwell
#48166 02/08/05 02:00 AM
Joined: Feb 2005
Posts: 19
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buck Offline OP
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Thank you for the info. - Buck


Buck

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