the secondary current of CT1 is I1 and secondary current of CT2 is I2. It is also clear from the circuit that the current passing through the relay coil is nothing but I1-I2. As we said earlier, the current transformer’s ratio and polarity are so chosen, I1 = I2, hence there will be no current flowing through the relay coil. Now if any fault occurs external to the zone covered by the CTs, faulty current passes through primary of the both current transformers and thereby secondary currents of both current transformers remain same as in the case of normal operating conditions. Therefore at that situation the relay will not be operated