Answer Posted / srinivasan rengarajan
it is used for sensitive earth fault protection , especially when the feeder CT ratio is very large & max EF current to be sensed is very low as much as 5A or 10A.. This kind of C.T namely ring type CT encompases all the phase cables ,the secondary side of it is
connected to a sensitive EF relay, whose contact will in turn be connected to trip coil of CIRCUIT BREAKER.Under no fault
conditions the vector sum of all the three phase currents
is zero; so no output to c.t secondary but when there is an
earth fault ,the sum would not remain zero (it would be 3 I0- namely 3 times Zero phase sequence currents) and the trip
coil would act and circuit breaker would trip.
The fact that it would not detect phase to
phase fault,is not a disadvantage, because the very purpose of CBCT is that it is specifically designed for sensitive earth faut protection or earth leakage protection.
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