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Residual Current Circuit Breaker falls under the category of a wide range of circuit breakers. A Residual Current Circuit Breaker is essentially a current sensing device used to protect a low voltage circuit in case of a fault. It contains a switch device that switches off whenever a fault occurs in the connected circuit.


Residual Current Circuit Breakers are aimed at protecting an individual from the risks of electrical shocks and fires that are occurred due to faulty wiring or earth faults.

RCCB is particularly useful in situations where there is a sudden earth fault occurring in the circuit.
For Example, A person accidentally comes in contact with an open live wire in the circuit.

In such situation, in absence of an RCCB in the circuit, an earth fault may occur and the person is at the risk of receiving an electrical shock.

Residual Current Circuit Breaker basically is installed to prevent a human from shocks or death caused by shocks. It prevents accidents by disconnecting the main circuit within a fraction of seconds.


It working based on Kirchhoff’s Current Law, the incoming current in a circuit must be equal to the outgoing current from that circuit. 

In case of an earth fault, the current finds a passage to earth through accidental means (such as accidental contact with an open wire etc.). As a result, the returning current from neutral is reduced. This differential in the current is also known as “Residual Current”.

RCCB is designed such a way that it continuously compares for difference (residual current value) in current values between the live and neutral wires. Any small change in the current value on account of such event would trigger the RCCB to trip off the circuit.


The basic operating principle lies in the Toroidal Transformer shown in the diagram containing three coils. There are two coils say Primary (containing line current) and Secondary (containing neutral current) which produces equal and opposite fluxes if both currents are equal.

Whenever in the case there is a fault and both the currents changes, it creates out of balance flux, which in-turn produces the differential current which flows through the third coil (sensing coil shown in the figure) which is connected to relay.

The Toroidal transformer, sensing coil and relay together is known as RCD – Residual Current Device.


Whenever the test button is pushed current starts flowing through the test circuit depending upon the resistance provided in this circuit. This current passes through the RCD line side coil along with load current. But as this circuit bypasses neutral side coil of RCD, there will be an unbalance between the line side and neutral side coil of the device and consequently, the RCCB trips to disconnect the supply even in normal condition. This is how the test circuit tests the reliability of RCCB.
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