Motor Differential Protection w/ Fused Contactor

[bdm5066]

If we have an application with a medium voltage motor (4.16kV) that requires differential protection, would it be a requirement to have that motor protected with a breaker (as opposed to a fused contactor)? My thought is that the purpose of the differential protection is to clear faults internal to the motor instantaneously, and a contactor is not designed to clear a fault (that's what the fuse is for).

FYI, this would be a core balance CT configuration for the motor differential protection.

Thanks in advance for the help!

 

[Jraef]

Typically, Differential Protection just acts upon the contactor. Yes, technically a vacuum contactor is coordinated with the fusing in terms of short circuit fault interruption rating, but the entire purpose of Differential Protection is to act faster than the fuse (or breaker) would act anyway, so not waiting for the current to get to SC values.

 

[GoldDigger]

Would a shunt trip breaker's shunt trip be considered fast enough? Since the Differential Protection is sensitive, it seems to me almost anything should be fast enough.

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[Jraef]

Would a shunt trip breaker's shunt trip be considered fast enough? Since the Differential Protection is sensitive, it seems to me almost anything should be fast enough.

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I interpreted what he was meaning to be that the "fault rating" of an E-2 fused vacuum contactor controller used in MV starters requires that it be coordinated with the fuse. So since it will not be the fuse that interrupts the fault when detected by a differential relay, the contactor may not be capacle of interrupting it, requiring one to use a Circuit Breaker instead of an E-2 fused vacuum contactor controller. But that is not the case.

 

[bdm5066]

Jraef, your interpretation of my question is correct. To summarize, the differential element will trip the contactor so quickly that the contactor will not experience true short circuit current. Makes sense to me.

Thanks for setting me straight.