Bonding Separate Services feeding a Common building.

[chuck@apsinc.biz]

NEC 250.28- Each service-disconnect shall have a main bonding jumper.

NEC 250.58- Where separate services supply a building and are required to be connected to a grounding electrode, the same grounding electrode shall be used.

I am working on an industrial project, where 2-services provide power for one structure. The (2)transformers and (2)Main disconnects are approximatly 400LF from the distribution boards with-in the structure. Equipment grounding conductors have been installed with each feeder.

I am pretty sure I need to provide 1-grounding electrode system at the structure and bond only the equipment grounding bus from each distribution board to the electrode.(NO neutral-to-ground bonds.)

I know I have to provide main bonding jumpers in each main disconnect(neutral-to-ground connection), however I am totally lost as to weather to provide (2) separate grounding electrode systems at the main-disconnects(One for each system) or if I need to provide (1)grounding electode and bond both systems to it?

My gut says that if I combine the systems at the main disconnects I am creating "objectional current" paths with-in the system. Can anyone point me in the right direction?

 

[chuck@apsinc.biz]

Re: Bonding Separate Services feeding a Common building.

The more I look at it, "objectional" current flow is not an issue.

By providing one electrode at the service disconnect it appears multiple return ground fault paths are created. This allows a fault in one system to return through the other to get back to it's source. Is this a problem? Don't we want the fault to get back to the source by any means, so the appropriate power can be provided to open the overcurrent protective device?

 

[jro]

Re: Bonding Separate Services feeding a Common building.

Art. 250.58 Where seperate services supply one building and are required to to be connected to a grounding electrode, the same grounding electrode shall be used. Two or more grounding electrodes that are effectively bonded together shall be considered as a single grounding electrode system in this sense.