RCCB works on the principle that in electrical circuits the incoming current is same as the outgoing current as shown in the diagram. RCCB incorporates a core balance transformer (CBT) having primary and secondary windings with sensitive relay for instantaneous detection of fault signal. The primary winding lies in series with the supply mains and load. Secondary winding is connected to a very sensitive relay. In faultless condition, the magnetizing effects of current carrying conductors cancel each other. There is no residual magnetic field that can induce a voltage in the secondary. During flow of leakage current in the circuit an imbalance is created in the circuit which gives rise to leakage flux in core. This leakage flux generates an electrical signal that is sensed by the relay and it trips the mechanism thereby disconnecting supply.

The trip mechanism is operated at a Residual Current between 60-80% of its Rated Leakage Current.

When pressing the TEST button ‘T’ (during load conditions) a fault is simulated via the test resistance and RCCB trips

Direct Protection in the event of a direct contact (unearthed) live part, extremely sensitive RCCB with rated residual operating current of 30mA or less used instead of a more conventional RCCB with higher residual operating fault currents.

Protection is necessary if:
1. The insulation of totally insulated device or their loads is damaged
2. The earth wire is interrupted
3. Tne earth wire and live wire is transposed
4. A component which is live in normal operations is touched during repair work

Indirect current when a person makes contact with a metal part which accidentally has been powered up following an insulation fault.