RCCB works on the principle of Kirchhoff's law, which states that the incoming current must be equal to the outgoing current in a circuit. RCCB thus compares the difference in current values between live and neutral wires. Ideally, the current flowing to the circuit from the live wire should be the same as that flowing through the neutral wire. In case of a fault, the current from the neutral wire is reduced, the difference between the two is known as Residual Current. On spotting a Residual Current, the RCCB is triggered to trip off the circuit.
A test circuit included with the Residual Current device ensures that the reliability of RCCB is tested. When the test button is
pushed, the current starts to flow through the test circuit. As it creates an imbalance on the neutral coil of the device, the RCCB
trips and the supply is disconnected thereby checking RCCB's reliability.