# Description of RCDs

The essential features are shown schematically in Figure F46.

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 vectorial 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 ${\displaystyle {\overrightarrow {I1}}+{\overrightarrow {I2}}=0}$ 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.

Fig. F46 – ﻿Principle of RCD

In the Voltage Independent (VI) technology (Figure F47a), 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. The energy necessary to trip mechanism comes directly from residual current, independent of line voltage.

With this technology, whatever the line voltage is (i.e. even in case of neutral breakdown or voltage drop < 50 V), the RCD is able to trip in case the residual current exceeds the value required to operate.

With the Voltage Dependent (VD) technology (Figure F47b), the summation current transformer measures residual current, an electronic circuit detects the tripping level and then sends an order to tripping unit to open the protected circuit. In this case, the power supply of electronic circuit and energy for tripping unit come from the line voltage.

With this technology, the RCD will be able to detect but not to trip if the line voltage is too low, because the electronic circuit and tripping units needs to be powered.

The minimum supply line voltage is 50 V, in order to allow tripping.