Grounding requirements for multiple services

[cppoly]

In an article in EC&M written by Mike, he poses a question and answer but I don?t understand the answer even after reading the actual answer. Can someone explain:

Q. When a building has more than one service (as allowed by Sec. 230.2), is there a requirement to ground these multiple services to a common grounding electrode? (Answer based on 2011 NEC.)

A. Where an AC system is connected to a grounding electrode in or at a building or structure, the same grounding electrode must be used. If separate services, feeders, or branch circuits supply a building, the same grounding electrode must be used [250.58]. Two or more grounding electrodes that are bonded together will be considered as a single grounding electrode system in this sense.

Metal parts of the electrical installation are grounded (connected to the earth) to reduce induced voltage on the metal parts from lightning so as to prevent fires from a surface arc within the building/structure. Grounding electrical equipment doesn?t serve the purpose of providing a low-impedance fault current path to open the circuit overcurrent device in the event of a ground fault.

Caution: Potentially dangerous objectionable neutral current flows on the metal parts when multiple service disconnecting means are connected to the same electrode. This is because neutral current from each service can return to the electric utility via the common grounding electrode and its conductors. This is especially a problem if a service neutral conductor is opened.

So if a building has two services and each service enters their own switchgear, do both switchgears need to be listed as service equipment with both having connections to a common grounding electrode or no?

 

[Little Bill]

In an article in EC&M written by Mike, he poses a question and answer but I don?t understand the answer even after reading the actual answer. Can someone explain:

Q. When a building has more than one service (as allowed by Sec. 230.2), is there a requirement to ground these multiple services to a common grounding electrode? (Answer based on 2011 NEC.)

A. Where an AC system is connected to a grounding electrode in or at a building or structure, the same grounding electrode must be used. If separate services, feeders, or branch circuits supply a building, the same grounding electrode must be used [250.58]. Two or more grounding electrodes that are bonded together will be considered as a single grounding electrode system in this sense.

Metal parts of the electrical installation are grounded (connected to the earth) to reduce induced voltage on the metal parts from lightning so as to prevent fires from a surface arc within the building/structure. Grounding electrical equipment doesn?t serve the purpose of providing a low-impedance fault current path to open the circuit overcurrent device in the event of a ground fault.

Caution: Potentially dangerous objectionable neutral current flows on the metal parts when multiple service disconnecting means are connected to the same electrode. This is because neutral current from each service can return to the electric utility via the common grounding electrode and its conductors. This is especially a problem if a service neutral conductor is opened.

So if a building has two services and each service enters their own switchgear, do both switchgears need to be listed as service equipment with both having connections to a common grounding electrode or no?

I would have to say yes. It's pretty clear from this:

250.58 Common Grounding Electrode. Where an ac system
is connected to a grounding electrode in or at a building
or structure, the same electrode shall be used to ground
conductor enclosures and equipment in or on that building
or structure. Where separate services, feeders, or branch
circuits supply a building and are required to be connected
to a grounding electrode(s), the same grounding electrode(
s) shall be used.
Two or more grounding electrodes that are bonded together
shall be considered as a single grounding electrode
system in this sense.

 

[charlie b]

I think this is the part you are confused about:

Caution: Potentially dangerous objectionable neutral current flows on the metal parts when multiple service disconnecting means are connected to the same electrode. This is because neutral current from each service can return to the electric utility via the common grounding electrode and its conductors. This is especially a problem if a service neutral conductor is opened.

If so, I share your confusion. I tried sketching out a couple diagrams, to see how current would be flowing through the metal parts of the building. I can't see it happening, not even with a service neutral disconnected. Can anyone explain this statement?

 

[cppoly]

Yea charlie, that's what I don't understand. In the first part of the answer it says where multiple services are in a building, they must share the same grounding electrode system. But the last part says that multiple service disconnecting means can't be connected to the same grounding electrode. Isn't this contradictory?

If not, how are you supposed to ground two service switchgears? One is allowed to get a GEC and the other isn't?

 

[charlie b]

Let's say I have two services entering a building. Let's say the load is so high that the building needs two 2000 amp main boards, to be served from two separate utility transformers. Finally, let's say I put the two main boards in the same room. I will call for an N-G bond in each main board, and run a wire from the neutral bar in each main board to the ground bar on the wall within the electrical room. I then run a wire from this ground bar to the grounding electrode system that serves the whole building. I don't see an issue with objectionable currents in this scenario.

 

[jtinge]

Let's say I have two services entering a building. Let's say the load is so high that the building needs two 2000 amp main boards, to be served from two separate utility transformers. Finally, let's say I put the two main boards in the same room. I will call for an N-G bond in each main board, and run a wire from the neutral bar in each main board to the ground bar on the wall within the electrical room. I then run a wire from this ground bar to the grounding electrode system that serves the whole building. I don't see an issue with objectionable currents in this scenario.

I concur. Seems like you'd want to have both bonded and referenced to the same GES to prevent a difference of potential between the ground fault paths back to each service.

 

[wwhitney]

Let's say I have two services entering a building. Let's say the load is so high that the building needs two 2000 amp main boards, to be served from two separate utility transformers.

So how are things typically done at the utility transformers? Might the two transformers' secondary neutrals end up connected there?

Cheers, Wayne

Edit: spelling

 

[Little Bill]

Yea charlie, that's what I don't understand. In the first part of the answer it says where multiple services are in a building, they must share the same grounding electrode system. But the last part says that multiple service disconnecting means can't be connected to the same grounding electrode. Isn't this contradictory?

If not, how are you supposed to ground two service switchgears? One is allowed to get a GEC and the other isn't?

Is this what you are talking about as being contradictory?

Caution: Potentially dangerous objectionable neutral current flows on the metal parts when multiple service disconnecting means are connected to the same electrode. This is because neutral current from each service can return to the electric utility via the common grounding electrode and its conductors. This is especially a problem if a service neutral conductor is opened.

If so, it's not being contradictory. Neither does it say anything about multiple service disconnects not being allowed to use the same electrode. It's just a word of caution from the author. I believe he is talking about being cautious when working on it if you only are working on one of the services. He means that if you have one service disconnected and think it is safe to work on, to be careful as the neutral current from the other systems can return via the grounding electrode system.
It's really no different than when a service is grounded to a community metal water pipe system that other buildings are also connected to.

There really is no way to avoid parallel neutral paths the way the NEC and POCO requires that grounding be done. The POCO grounds at the pole or transformer and we have to ground again at the service disconnecting location.

 

[cppoly]

Ok that makes sense. Thanks