Isolated Ground Circuit

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fozmo

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The installation of an IG requires you to ground back to the switch panel before the transformer. In the case of a 480-208/120VAC transformer supplied by a Distribution Panel I would need to ground back to the DP or all the way to the Switch Gear at the entrance of the plant?

Thanks Fozmo
 
Re: Isolated Ground Circuit

In your example you would take the IG back to the 120/208 panel on the low side of the transformer. See 250.146(D). Because your transformer establishes a separately derived system on the 208 side, you would stop your IG there.
Jim T
 
Re: Isolated Ground Circuit

I don't believe there is anything in the NEC that requires where the IG must originate. You can start the IG at the neutral terminal of the SDS or it can go to the panelboard that the the branch circuit originates. You are not required to take it all the way back to the service or through panelboards, however you are permitted to.
 
Re: Isolated Ground Circuit

Bryan
I believe you are right, the NEC doesn't mandate where to terminate the IG conductor. Let me try the to explain why I believe it should be terminated at the separately derived bond in this case.
One feature of an IG is to limit neutral-ground noise. If the IG is tied to the location where neutral and ground are bonded, the n-g noise introduced at that point will be 0. If you land your IG conductor anywhere else, there is the likelihood of n-g noice being introduced.
Secondly, by running your ground past the source of the separately derived system in this case, and landing it on an upstream panel or service, you are creating the possibility of a larger than necessary n-g voltage at the IG receptacle.
For those reasons, I always require that the IG conductors be terminated at the location where neutral and ground are bonded for the system that the IG's are being installed
Jim T
 
Re: Isolated Ground Circuit

250.146(D) ... so as to terminate within the same building or structure directly at an equipment grounding conductor terminal of the applicable derived system or service.
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In my opinion the isolated EGC cannot be landed upstream of the power source. In other words it can't go upstream from the secondary side of the transformer.
Don
 
Re: Isolated Ground Circuit

I think the words "... so as to terminate within the same building or structure directly at an equipment grounding conductor terminal of the applicable derived system or service." require the IG to go back to the source.

The code spells out specifically where to terminate.

Without going back to the source all you have is a parallel equipment ground path to a point (if you used EMT or equivalent).

In the real world (as has been discussed before) it really doesn't matter as most spec's for IG are not needed.
 
Re: Isolated Ground Circuit

Larry I do not see it that way at all.

Any grounding conductor terminal of that particular system meets the letter of that section.
 
Re: Isolated Ground Circuit

I see what you mean. I think the operative word is "of".
I can't argue the grammar used (like an English teacher) , but I look at the the word "of" in the text to mean "belonging to the service." If we look up service in Art 100, it has the service "ending" after the electric energy is delivered. So to terminate on an equipment ground terminal of a service, that terminal must be in the service.

If a seperately derived system, is involved then I think I might have to concede to you.
 
Re: Isolated Ground Circuit

I was taught to run a IG with service conductors from main service uffer or groung rod.Then take it to a IG buss in panelboard.I also believe you would size it according to the branch circuits you are feeding.Please correct me if I am wrong.
Thanks Rick
 
Re: Isolated Ground Circuit

Originally posted by harborrick:
I was taught to run a IG with service conductors from main service uffer or ground rod.Then take it to a IG buss in panelboard.
Click to expand...
That is not compliant or a good design practice. The code permits the IG to originate any where from the N-G bond point to anywhere downstream from the N-G bond point. If you are working with a transformer, the same applies, the Xo bond point. From a design point of view to minimize common mode noise, the IG should originate at the N-G bond point, but is not required to do so.
 
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