Description of RCDs

Principle

The essential features are shown schematically in Figure F63 below.
A magnetic core encompasses all the current-carrying conductors of an electric circuit and the magnetic flux generated in the core will depend at every instant on the arithmetical sum of the currents; the currents passing in one direction being considered as positive (Ι1), while those passing in the opposite direction will be negative (Ι2).
In a normally healthy circuit Ι1 + Ι2 = 0 and there will be no flux in the magnetic core, and zero e.m.f. in its coil.
An earth-fault current Ιd will pass through the core to the fault, but will return to the source via the earth, or via protective conductors in a TN-earthed system.
The current balance in the conductors passing through the magnetic core therefore no longer exists, and the difference gives rise to a magnetic flux in the core.
The difference current is known as the “residual” current and the principle is referred to as the “residual current” principle.
The resultant alternating flux in the core induces an e.m.f. in its coil, so that a current I3 flows in the tripping-device operating coil. If the residual current exceeds the value required to operate the tripping device either directly or via an electronic relay, then the associated circuit-breaker will trip.

Fig. F63: The principle of RCD operation