Grounding In Two Locations

[mikeybmdb32]

So here's the situation: I have 5 generators paralleled to a paralleling switchgear. The generators are installed outdoors and the switchgear is indoors. The installer grounded the neutral of each generator and also grounded the neutral of the paralleling gear. When I discussed this with the installer, I told him that he needs to break the ground connection at either the generators or paralleling gear so that both are not grounded at the same time. His reply was that the generators, being outdoors are similar to a transformer being installed outdoors which, per NEC 250.24(A)(2) would require grounding at both the source and service panel. Does anyone have an opinion or code reference for this argument either way?

 

[erickench]

If there is a connecting metal raceway between the switchgear and generators you will have objectionable current. Point out to him that it may be a code violation and refer him to NEC 250.30(A)(1)

 

[Smart $]

... His reply was that the generators, being outdoors are similar to a transformer being installed outdoors which, per NEC 250.24(A)(2) would require grounding at both the source and service panel.

Being similar doesn't matter. What the Code states does. First stop is 250.35. From there it depends on whether the generators function as separately derived or non-separately derived system. If non-separately derived, that's an automatic no bonding of grounding to grounded. If separately derived, the requirements of 250.30 apply...

Also see 250.6.

 

[mikeybmdb32]

My confusion is around the fact that 250.30(A)(1) calls for grounding the neutral at only one point, but then goes on to say that, if the source is outside, that the neutral should be grounded at the source. The handbook comments immediately following 250.30(C) state that 250.30(C) is similar to 250.24(A)(2) (which calls for grounding the neutral in 2 locations when the source is a service), but a major difference is that I find between the two is that 250.24(A)(2) specifically calls for one additional ground-neutral connection at the source whereas 250.30(C) just calls for a ground-neutral connection at the source. In the case of outdoor sources, is the NEC stating that there is to be only one point where the neutral is grounded and that this point should be at the source, OR is the NEC stating that it is ok to ground at both the source and first disconnecting means?

 

[templdl]

Do you agree that an EGC should not carry current unless there is a ground fault?

This is the way I reason this out:
When your neutral is a grounded conductor it is grounded at only one end. As such the EGC never carries neutral current under normal conditions. If you also ground that neutral at the other end then you open up Pandora?s Box and have now permitted neutral current to be shared in some proportion with the EGC. When the EGC carries current any device that is bonded to the EGC now can carry current which presents the possibility of a shock hazard. The neutral, even though grounded, it is part of the electrical current carrying circuit. When you think about grounding the neutral at more than place the results could be dangerous. Grounding the EGC in more than one place would be allowed as far as I can tell as long as the EGC only grounds the neutral in only one place.

 

[Smart $]

My confusion is around the fact that 250.30(A)(1) calls for grounding the neutral at only one point, but then goes on to say that, if the source is outside, that the neutral should be grounded at the source. The handbook comments immediately following 250.30(C) state that 250.30(C) is similar to 250.24(A)(2) (which calls for grounding the neutral in 2 locations when the source is a service), but a major difference is that I find between the two is that 250.24(A)(2) specifically calls for one additional ground-neutral connection at the source whereas 250.30(C) just calls for a ground-neutral connection at the source. In the case of outdoor sources, is the NEC stating that there is to be only one point where the neutral is grounded and that this point should be at the source, OR is the NEC stating that it is ok to ground at both the source and first disconnecting means?

The main difference is the type of grounding, earth vs. equipment. Note 250.24(A)(2) calls for additional grounding connection from the grounded service conductor to a grounding electrode. This is earth grounding, not equipment grounding... but since the grounded service conductor is permitted to be and typically used for equipment grounding, the point is moot regarding service transformers.

When the same is implied toward separately derived systems outside, you would bond an electrode to the grounded conductor at the source, but if this is not where the system bonding jumper is located, the grounded conductor is not to be bonded [directly] to equipment's non-current-carrying metal (NCCM). A supply side bonding jumper is run from the system bonding jumper location to ground the NCCM.

 

[Smart $]

PS: If your gensets have integral ocp, I believe the system bonding jumpers are required to be located there... so the grounded-to-grounding bonding jumper(s) would be removed at the switchgear.

 

[iceworm]

So here's the situation: I have 5 generators paralleled to a paralleling switchgear. The generators are installed outdoors and the switchgear is indoors. The installer grounded the neutral of each generator and also grounded the neutral of the paralleling gear. When I discussed this with the installer, I told him that he needs to break the ground connection at either the generators or paralleling gear so that both are not grounded at the same time. His reply was that the generators, being outdoors are similar to a transformer being installed outdoors which, per NEC 250.24(A)(2) would require grounding at both the source and service panel. Does anyone have an opinion or code reference for this argument either way? ..

Asumption: The system is grounded 480Y (seems reasonable from some of the comments) But if 208Y, my response still stands. if 13.8KV, well, solidly grounding 13.8 is pretty nuts.

Yes I have an opinion. I deal with this alot - multiple, paralleled, generators, both on-grid and off-grid.

The code, bless their souls, just don't deal with this. Their one rule covers all - does not fit. This is not a cookie cutter installation. If you looking for a code reference that you can stamp on the drawing and say "make it like this.", you're beat.

The idea of bonding multiple paralleled gens at the sources is a really bad design choice - ground grid or not - OCP at the gen or not - outside or not. This is not something that can be designed using a code section. One must actually apply engineering. Any decent solution will be far past the safety, reliability, and performance mandated by code.

Just curious, how come you are dealing with the installer? I've never seen a multiple gen project that did not have engineered drawings. Isn't there an engineering group that is "engineer of record" - the ones that came up with the design and drawings? I would think these are the ones that should be defending the installation.

I can tell you this about bonding each gen neutral to the equipment grounding conductor at each gen:
1. Consider most multiple gen sites are industrial, and have ground grids, metalic piping connections, and probably equipment grounding between all of the structures. Any neutral current will eventually find it's way back to a generator neutral - and some of it might even travel down a neutral conductor.

2. This sounds like small generation (each <800KW - probably < 500KW) One issue often assocoated with paralleled, small generators is if one gen winding faults, the others (in parallel) can deliver sufficient fault current that the faulted gen disappears into flying molten slag. Yes, this is with the gen mains set up correctly.

Generally speaking, it's a really good idea to arrange the N-G bond to limit the maximum ground fault current and keep the neutral current on the neutral conductors. Regardless of the code, N-G bonds at each gen won't do that.

ice