IG and Equipment Grounding Conductor Termination

[Lee Graves]

I'm working on a project that involves the use of excessive isolated grounds. The notes on the plans say "the equipment ground and the isolated ground must only be connected / bonded at the service entrance grounding electrode". I've recently been studying grounding through the use of your DVD's and website and came across this information on your site:

"The isolated grounding terminal and the equipment ground would terminate on the same ground bus in your panel. The NEC requires the isolated ground to terminate at the "applicable derived system or service" 250-146(d). Since the panel is fed from a step down transformer, it is a separately derived service, and the grounded conductor (neutral), grounding electrode conductor and equipment grounds are all terminated together, in accordance with Section 250-30."

I have a problem with this statement. According to the Article 100 definition of "service", a service is "the conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served". According to this definition, a step down transformer does not create a separately derived 'service', rather a separately derived system.

Question 1: This would lead me to belive that you can terminate the IG conductors at the transformer load side disconnecting means or at the service disconnect?

Question 2: Assuming you can terminate the IG conductors at the service disconnect which will normally be outside, can you also terminate them at the main distribution panel indoors?

Question 3: It seems to make sense to terminate the IG conductors at the transformer, because that would be the source the fault current is trying to return to, correct?

Question 4: If we terminate them at the service, would it essential create a longer fault current path back to the source (transformer)?

Question 5: If the purpose of the IG conductors are to reduce "noise", which location will accomplish this the best?

Question 6: If we run an IG conductor from the panel where the branch circuit originates, all the way back to the building grounding electrode like it specifies in the plan notes, would this conducter be sized according to 250-66 even though it won't be traveling with service coductors? Or do we size it #4 if we're going to a concrete encased electrode?

Question 7: Can we use the same grounding electrode conductor to "loop" between all the panels with an IG bar assuming they all orginate from the same separately derived sourcE?


Any comments will be appreciated. Thanks,

Lee Graves
Lighthouse Electrical Contractors, Inc.
Jacksonville, FL USA

 

[haskindm]

Basically an IG Ground allows you take the IG Ground conductor through any sub-panels, without terminating, all the way back to the main distribution panel, where it will be terminated on the ground/neutral bar. In the event there are any incorrect ground/neutral connections downstream of the main distribution panel this should help to eliminate "noise" on the grounding conductors. In a properly installed and maintained electrical system IG provides little benefit in my opinion.

 

[dereckbc]

A1. The IG can originate any where from the service disconnect N-G bond point, or anywhere along its path including the yoke it sets in.

A2. Yes, anywhere you want.

A3. Well if you mean the service transformer that may not be possible.

A4. No as it would be the same size and path as the EGC.

A5. It is not possible for an IG form of grounding to remove or reduce noise. Surprise! Only way it could be possible used with some effect is in conjunction with an isolation transformer with the IG originating from the Xo bond point. Results from IG use ranges from NO EFFECT, DESIRED EFFECT, or WORSE NOISE.

A6. It is the same size as the EGC which is based on 250.122 or the OCPD size. As I said the best place to originate the IG is the N-G bond point. Going all the way back to the electrode is likely to generate more noise.

A7. Yes.

You are going to receive a lot of negative comments about IG. However enjoy the extra money they will generate. You can laugh all the way to the bank knowing your customer paid big bucks for something that does nothing and is antiquated history.

 

[don_resqcapt19]

Lee,

Question 1: This would lead me to belive that you can terminate the IG conductors at the transformer load side disconnecting means or at the service disconnect?

If the source of the power is a transfomer on the load side of the buiding service, then the isolated grounding conductor cannot be run back to the serivce disconnet. It must stop at the point where the system bonding jumper is installed.
Don