RCB-Residual circuit breaker to detect earth fault ; how it works.

The RCB or residual circuit breaker is a protective device to sense the earthing fault in circuit. Basically RCB is used in circuit to protect human life from electrocution or electric shock.

If a person is some how electrified the RCB will sense it and disconnect the circuit.If some how an electrical equipment like motor or utility like panel is electrified, hence is grounded or earthed – RCB will detect it and disconnect the circuit.

Why we need to use RCB- residual circuit breaker

As we know that only 40 mili amp current is enough to kill a strong man, so saving human life from lethal electric shock is an obvious thing to consider.

Generally circuit breaker are of minimum 5 amp rating, so as usual circuit breaker is assigned to protect the circuit from overload or short circuit other than human from electric shock.

RCB or residual circuit breaker is a special purpose circuit breaker to protect human being from electric shock.

The theory behind RCB – residual circuit breaker.

A healthy circuit has a unique characteristics- the phase current and the return path neutral current are same. Now if a human (who is earthed) touch a live circuit or a part of live circuit is some how connected to a metal part or panel which is earthed, there will be another path to normal current.

In this case the phase current and neutral current will not be same. The theory of RCB lies here- to detect if the phase current and neutral current has any difference, if difference is found of a certain level – it will disconnect the circuit. Its detection sensitivity starts from 5 mili amp.

As for construction- the RCB is nothing but a differential current transformer for single phase graound fault detection and for three phase to watch neutral (discussed below) it is a simple ring type current transformer. It is set around neutral and phase wires but the earthing wire is kept free.

The main principle of RCB- residual circuit breaker / a ground fault relay is to detect the imbalanced current where general circuit breaker detects excessive current.

RCB connection for single phase and three phase

As discussed above at normal condition as the phase current and neutral current both are same but of opposite direction- their difference will be zero- taking in to consideration of some fluctuations. If some how a fault occurs there will be some difference detected by this differential current transformer.

For a single phase circuit both phase and neutral wire are led in to through RCB but in case of three phase circuit there are two options -firstly to watch the three phase wire and secondly to watch the neutral wire. As the three phases are of 120 electrical difference, the sum of there current will be zero under normal condition but in case of a fault- this difference will break.


The neutral alone can be watched in case of transformer or alternator. Under normal condition the neutral has zero current, but under faulty condition like a phase is grounded by human or by other means the three phase will be imbalanced and there will be current in the neutral. This can be detected by RCB.

RCB for single phase

Residual Circuit breaker or ground fault relay  for single phase.
Residual Circuit breaker or ground fault relay for single phase.

RCB connected for three phase

RCB connected around three phase wires
RCB connected around three phase wires

RCB connected for three phase neutral wire

RCB connected to neutral point
RCB connected to neutral point

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