MCB may or may not operate on earth fault here is the explanation:
i) MCB is protective device that is made to break the circuit in case of overload or short-circuit. For Overload protection, it has bi-metallic strip which heats up during overload. The heating causes uneven expansion of bi-metals leading to bending of the strip. This bending movement is used to release the spring to trip or open the circuit. For short-circuit protection, an electromagnet is used, which produces strong magnetic field during short-circuit condition and attracts the magnetic plunger. The movement of plunger is used to release the spring to trip or open the circuit.
ii) MCB will work only in case when the current through it exceeds its rating. i.e a 5A MCB will trip the circuit when current greater than 5A passes through it.
Consider the above circuit in which the current flowing through MCB is sum of load current and fault current. If fault path resistance is less, the current flowing through the MCB may exceeds the limit i.e rated value of 5 A and the MCB will trip.
Consider a second case where the earth fault is through highly resistive path. So the fault current may be very less and total current flowing through MCB will be within the limit. Hence, even under earth fault condition, the MCB will not trip.
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