Leakage Current Summation

[natfuelbill]

Picture four prime power parallel generators feeding a paralleling switchboard, then that feeding building load. 480/277V. NG bond in switchgear only. Breakers are LSIG. These are not standby generators. There is no utility service present.

There is measured ground leakage current coming from the building load displayed on the load breaker.

Am I correct in thinking that this current will move from the load ground paths, through the system bonding jumper, over to the generator neutrals, and then return to the generators? Or asked another way. Should I see ground leakage current on the generator breakers?

 

[steve66]

There should be only one N-G bond, and you stated that is at the paralleling gear. But your leakage current indicates another N-G bond. It is probably at the generators, or somewhere in the load.

If the extra N-G bond is somewhere in the load, the leakage current will recombine onto the neutral at the paralleling gear. So there will be no measurable current on the generator grounds.

If the extra N-G bond is at one of the generators, then you would see the ground leakage current on the load side of the N-G bond.

Steve

 

[Natfuelbilll]

..... the leakage current will recombine onto the neutral at the paralleling gear. So there will be no measurable current on the generator grounds.....
Steve

That is how I would picture it.

Does anyone have examples or experience on this issue?

 

[LarryFine]

Should I see ground leakage current on the generator breakers?

I don't see how you could not. No matter what the pathway, any current in a system neutral/ground/whatever pathway must equal any current not phase-to-phase.

 

[Natfuelbilll]

The main breaker feeding the building shows residual ground current coming back from the building. This current is the mismatch of the neutral and phaase currents in the building due to either a NG bond or conductor insulation leaks. This current must be flowing over many many different metallic paths trying to get back to the generator.

Is ground current trying to return to the phase, or to the neutral?

If it trying to return to the neutral, then a conductive path from ground to neutral is the system bonding jumper.

If is trying to get back to the phase wouldn't it still flow on the system bonding jumper, back to the generator on the neutral and complete the circuit to phase through the grounded midpoint at the wye.?

In each of these scenarios the ground currents are flowing on the neutral and not on the ground.

Ohhh.....I think I am getting the picture....or my wife just clobbered me in the head.... the fog is lifting....

Yes, the currents must be flowing on the neutral to the generator, but this current is still the difference (residual) of the phase and normal neutral currents with respect to the generator, as measured by the generator residual ground fault current transformers.

So is that why I still see ground current on the generator breakers?

 

[steve66]

In each of these scenarios the ground currents are flowing on the neutral and not on the ground.

You lost me there. If the current is flowing on the neutral, and not the ground, that's not ground leakage current. That's just an inbalance of the loads on all the phases.

 

[natfuelbill]

Given a situation with zero ground current the current flowing on the neutral would be normal and expected imbalance current. Current transformers around the neutral and phases would sum out to zero.

In this situation, with ground current, the ground current needs to get back to the generator. If it is trying to get back to the generator neutral point then it would flow on the neutral. If it trying to get back to the generator phase it would still flow on the neutral. In either case the current transformers around the neutral and phases would now not sum out to zero.

That is how I see it, please help clarify how these currents flow if not as I have tried to describe.