Isolated Grounds

[Jim_BE]

I am currently doing a job for a internet marketing company that is calling for isolated ground receptacles and sub panel. I have spec'd the receptacles, and all the computer circuits are in emt and steel boxes. To make it truly "isolated ground" I would have to add a insulated ground bar to the panel, pull an additional ground wire to the receptacle with the second ground going to the box or ground bushings and locknuts on each pipe run. In addition I need an additional ground wire going back to the distribution panel. Should this "isolated ground' be grounded in the distribution panel or be carried to a separate ground "grid" ...rod(s) or otherwise? I realize that the purpose is to help eliminate "noise" Am I all wet here?

Thanks
Jim

 

[don_resqcapt19]

Jim,

Should this "isolated ground' be grounded in the distribution panel or be carried to a separate ground "grid" ...rod(s) or otherwise?

It can never land on a separate grounding electrode. It must connect to some point on the electrical grounding system that will provide a fault clearing path. In many cases the specs would require that the insulated grounding conductor be run to the point where the main bonding jumper is installed or if the circuit is supplied by a SDS to the point where the system bonding jumper is installed.

To make it truly "isolated ground" I would have to add a insulated ground bar to the panel, pull an additional ground wire to the receptacle with the second ground going to the box or ground bushings and locknuts on each pipe run.

Nothing changes with the conduit system. You add the insulated ground bar at the sub panel and run the insulated grounding conductor to the receptacles from this bar. The bar must be connected to some point on the grounding system of the panel supply circuit.
Don

 

[dereckbc]

Ditto to what Don said. However the code does not dictate where the IG has to originate. You can simply short the IG to EGC at each of the receptacles, or run an insulated IG conductor back to the sub-panel and terminate it to the EGC bus in the sub-panel, or all the way back to the service or SDS N-G bond point.

FWIW from a design POV the origination point should be the N-G bond point rather that be the service or SDS. But there is nothing in the NEC that requires that, the code only permits that.

 

[nakulak]

Ditto to what Don said. However the code does not dictate where the IG has to originate. You can simply short the IG to EGC at each of the receptacles, or run an insulated IG conductor back to the sub-panel and terminate it to the EGC bus in the sub-panel, or all the way back to the service or SDS N-G bond point.

FWIW from a design POV the origination point should be the N-G bond point rather that be the service or SDS. But there is nothing in the NEC that requires that, the code only permits that.

I have to disagree with one thing: shorting to the EGC would not provide an isolated ground by definition (even though I can't seem to find where NEC defines it ), and in my opinion violates the intent of the code relative to an isolated ground. otherwise this is entirely corrrect.
250.96 B
250.146D

as a side note: in my jurisiction, the inspectors will always want to see a green/yellow wire for the isolated ground. if its not there, you are sol.

 

[don_resqcapt19]

Dereck,
It is my opinion that the wording of 250.146(D) would require that you run the insulated grounding conductor back to the the branch circuit source, but as with many sections of the NEC, it can be read in more than one way.

(D) Isolated Receptacles Where required for the reduction of electrical noise (electromagnetic interference) on the grounding circuit, a receptacle in which the grounding terminal is purposely insulated from the receptacle mounting means shall be permitted. The receptacle grounding terminal shall be grounded by an insulated equipment grounding conductor run with the circuit conductors. This grounding conductor shall be permitted to pass through one or more panelboards without connection to the panelboard grounding terminal as permitted in 408.40, Exception, so as to terminate within the same building or structure directly at an equipment grounding conductor terminal of the applicable derived system or service.

It is optional to run it back to the main or system bonding jumper.
Don

 

[dereckbc]

I agree Don, I was just trying to make a point, the NEC doesn't have anything to say about the origination point of the IG