Answer / greenhat25

When a fault occurs, a current flows in th secondary winding
of th current transformer.
If th current is greater than th setting of th relay, th
electromagnet is energised and attracts th armature.
The action of th armature closes and opens th normally open
and close contacts respectively, thereby closing or opening
th trip circuit.
The shunt trip coil or UV trip coil is energised or
de-energised and th circuit breaker will trip.

Answer / avd

In continuation to above answer, there are different
schemes to detect earth fault in the system
1. By using residual connections of three phases CTs
2. By using CBCT
3. By using a neutral CT at transformer neutral point.

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