system grounding damaging trip unit?

[alehman]

My client has a dual breaker transfer system utilizing low-voltage air frame power breakers. The alternate (generator source) neutral is bonded at the transfer board. The commercial source feeder breaker in the same board (upstream from the transfer breakers) is equipped with residual ground fault trip. The grounding arrangement is such that ground fault current while operating on the generator source will return through the neutral sensor of the commercial feeder breaker (it could be closed or open). I think this is fairly typical of a non-separately-derived system. I recognize that this could result in unnecessary tripping of the commercial feeder.

The local inspector is insisting this may be harmful to the commercial feeder breaker trip unit and is therefore not allowing it.

I have inquired with the breaker manufacturer and am awaiting a reply. I would be interested to know any opinions.

 

[davidbeach]

Lousy design from a protection stand point, but I can't see how it could possibly harm the breaker trip unit.

 

[DanDel]

This may damage the trip unit because it will see GF current through the secondary return circuit which will not be stopped by a trip of the C/B. If the internal GF circuit in the trip unit is not rated for the continuous GF current, it may overheat.

 

[alehman]

Thanks. There is ground fault protection on the generator source, so there should not be any sustained fault current. This is an existing installation requiring replacement of the generator.

 

[jghrist]

If I'm understanding this correctly, the generator neutral is bonded at the service board, not on the generator side of the generator ground fault protection. With this arrangement, the generator ground fault protection will not see a ground fault. Current will flow from the generator to the fault, to ground, to the service board bond, to the neutral, and back to the generator. The phase and neutral currents to the generator will balance.

 

[alehman]

Typically I would agree. I this case however we are providing 3-wire ground fault sensing at the generator (there is no neutral load).

 

[7anoter4]

Hi alehman
I agree with you that without any doubt the neutral current cannot damage the commercial breaker as the protection works on residual current insensible what current flow through neutral. But if it would be a toroidal CT I think could be a problem. If it will be an outage and the Generator Grounding Protection will fail to operate the problem will be with the cable and not with the breaker. See Attachment
Best Regards

 

[7anoter4]

Hi alehman
I have to apologize! There is a 4 conductor distribution then you have to include the neutral too in the residual system. No difference between residual and toroidal. In the extreme case of generator homopolar protection fail the commercial supply cable may be damaged.
Regards

 

[mlhspike]

If you have a b auxiliary contact available you might use it to short out the neutral CT input when the feeder breaker is open. This should make the inspector happy but I don't know the complete protection circuit and you will have to look into that.