ABSTRACT
In the operating coil, the current sent by the upper CT is cancelled by the current sent by the lower CT and the relay does not operate. For an internal fault, the polarity of the secondary voltage of the upper CT is reversed, as shown in Figure 15.1b, in that particular phase aected by the fault. Now, the operating coil carries the sum of the current sent by the upper CT and the lower CT, it operates and trips the circuit breaker (CB). is protection does not respond to external faults and overloads.
