Fused Contactor Rated for Ground Fault?

[nightfox1925]

We have a 13.8kV solidly grounded system where the short circuit is relatively high.

One of my colleagues are specifying a NEMA E2 class starter to feed a padmounted distribution transformer. The 51G at the secondary is proposed to be wired to to trip the upstream contactor.

My concern is that the contactor is not rated to interrupt the amount of GF current on this solidly grounded 13.8kV, so I am pushing for a VCB instead. His argument is that as long as the fuse is current limiting, the contactor will be able ti interrupt....there is something wrong with that argument. If the GF current is high enough, then the fuse will interrupt a lot faster than the contactor. The concern lies on the gap between the fuse curve and the contactor rated breaking current. Since this is feeding a transformer, the fuse curve will be so chosen that no point on its TCC curve will touch the inrush point, hence, a gap will be there.

Any comments?

 

[dpc]

The 51G relay curve must be selected and designed to give the fuse time to clear high magnitude faults that are beyond the range of the contactor, which will be nearly all of them in a solidly-grounded system.

The current-limiting argument is not accurate. The only way the fuse will limit current is when it melts. This will occur within the first 1/2 cycle for high magnitude faults. If the fuse curve shows the fuse taking longer than 1/2 cycle to clear, it it NOT current-limiting.

This is really terrible application for a fused contactor. I agree with your idea of using a VCB instead.

 

[ppedUK]

You do not say what vector group transformer you are using, but in any case, I agree with dpc that this is a poor application for a Fused Contactor.

If your 51G is fitted at the secondary as you said and you are using a Delta / star transformer, then the solid grounding of the 13.8kV system plays no part for a ground fault at the secondary as a ground fault on the secondary will be seen as a primary phase-phase overcurrent.

Your colleague's argument is definitely flawed, particularly for a primary overcurrent or earth fault. You would have to very carefully co-ordinate the characteristics of the primary fuse / contactor breaking capacity / relay setting combination for a primary fault, but it MAY be possible (depending on the impedance of the transformer etc.) that the contactor has adequate breaking capacity to interrupt a SECONDARY ground fault seen by your secondary 51G.

As you said though, it is better to push for a circuit breaker to feed the transformer.

 

[jghrist]

I'd check for vacuum interrupters as well for a cheaper alternative to a breaker. Try Cooper Power, Elastimold, G&W, Joslyn, ABB, probably others.