Isolated Ground

[ken44]

I am working on a commercial remodel job which requires several isolated ground panels. The job requires working with a State Agency and they have their own electrical supervisor/inspector who I am at odds with on the following code issue. The diagram on the blueprints shows a 3 wire system with the conduit(EMT) being the equipment ground. The supervisor/inspector says the system must have a fourth wire because the EMT is not sufficient for an isolated ground receptacle, he cites article 250-30 which makes mention of seperately derived systems, I dont see where he is coming from, we are using EMT with steel, set screw connectors and steel, set screw couplings, I am of the understanding from my past mentors and the NEC that this is sufficient for an equipment ground. For those of you who are knowledgeable in these isolated ground systems, I would appreciate your insight and sharing experience which lends itself to this subject.

 

[don_resqcapt19]

Re: Isolated Ground

The conduit can be used as an EGC, but not as an isolated equipment grounding conductor...its not isolated. An isolated equipment grounding conductor is insulated and kept separate from the raceway all the way back to the service or the source of the SDS. Look at 250.146(D) and its FPN.
Don

[ August 14, 2004, 03:46 AM: Message edited by: don_resqcapt19 ]

 

[ron]

Re: Isolated Ground

Here's some visuals:
http://electrical-contractor.net/ubb/Forum15/HTML/000049.html

 

[dereckbc]

Re: Isolated Ground

There are two ways to install the IGR when conduit is being used.

1. Use 3-wires (L-N-IG), and the conduit as the EGC.

2. Use 4-wires (L-N-EGC-IG), the conduit then becomes a redundant EGC and a shield.

To make IGR effective you should use a isolation transformer which is SDS, maybe your inspector is confusing terms.

 

[ken44]

Re: Isolated Ground

Gentlemen, Thank you for your insight. By coincidence, I had those very drawings to the link Don showed in my possession, I had not seen them for a while and when I pulled them out and showed them to the inspector on Friday, the first one of those pages has a receptacle demonstrating a green wire going to the box and that led me to believe that the inspector was correct until I took a closer look at the pages which followed that clearly demonstrate the use of a EMT conduit for the EGC. This just happened Friday, so I have not gotten back to the inspector.

 

[ken44]

Re: Isolated Ground

derickbc
Can you give me a little more insight about an isolation transformer and how it would differ from a standard dry-type transformer. Once again the inspector said that I need to go back to the building electrode(I will have to pipe down two floors to access that electrode)then he wants that electrode taken directly to XO and from there a wire to the transformer case and then to the panel neutral which will be bonded to the panel can. This all sounds correct to me and appears to be what the link Ron posted is demonstrating and is in accordance with the NEC, however, If you know of anything that is contrary to this, then please let me know.

 

[ron]

Re: Isolated Ground

An isolation transformer allows you the benefits given by a standard dry transformer (separately derived source) without transforming the voltage level.

 

[dereckbc]

Re: Isolated Ground

Ken44, sounds like you got a handle on the situation. Question for you.

How is the building constructed? Could it be steel framed? The reason I ask is you would be required to use it rather than pipe down two floors. 250.30 requires the closest electrode availible in specific order.

Comment: As far as treating a isolation transformer differently over a dry type, there is no difference. Both can achieve the same thing. Isolation transformers usually have some sort sheilding to further de-couple capacitive coupling between input and output. This de-coupling increases the common mode rejection ratio needed for noise reduction. In addition there may also be some type of voltage regulation like ferroresonant cores.

Hope that helps... Dereck

 

[ken44]

Re: Isolated Ground

Ron, Don and Dereck
Thank you all for your help!
PS: Dereck, the building is all concrete and no steel beams are available to my knowledge.
Ken