This residual current circuit breaker, provides valuable additional protection to almost any type of installation by detecting earth fault current below the level sensed by traditional overcurrent device such as fuses of miniature circuit breaker. The RCD employ the current balance principle which involve the supply conductor to the load (phase and neutral) being wound onto a common transformer core to form the primary winding. Under healthy circuit conditions, the current in the phase conductor is equal to the current in the neutral and the vector sum of the current is zero. In the event of an earth fault, an amount of current will flow to earth, creating an out of balance situation in the transformer assembly. This out of balance is detected by the secondary winding of the transformer and at a pre-determined level of out of balance will activate the trip mechanism.