Low-Ratio CT with Instantaneous Element? 1

[CuriousElectron]

Greetings,
Is it a common practice to use a lower ratio CT if the Instantaneous tap(setting) is enabled on a protective relay? In this scenario since it is expected for the relay to pick up on the fault current without any delay and trip the breaker within 5-6 cycles. If coordination and time delay is required, then CT saturation is taken into account and a higher ratio CT along with appropriate voltage class rating of the CT is selected.

Thoughts?

Thanks,
EE

 

[davidbeach]

It depends. The right combination of things and saturation could be bad enough that the instantaneous element takes several cycles to pick up. On the other hand, the saturation could only last several cycles and if the time OC is set for many cycles, the several cycles of saturation may not make much difference. You pretty much have to look at the specifics of the case to know for sure.