Page 34 - Electrical Safety of Low Voltage Systems

Fundamentals of Electrical Safety 17

FIGURE 2.7 Single-phase RCD.

If a fault puts in contact the phase conductor with the enclosure, a
8
current I G will flow through the protective conductor, causing phase
and the neutral currents to differ. If we consider the point of contact
with the enclosure as a “generalized” node, we can apply the first
Kirchoff’s principle:
I = I + I ⇒ I − I = I = 0 (2.2)
Ph N G Ph N G
As a consequence, the resulting magnetic flux along the RCD’s
toroid, which is proportional to the net current I G flowing through the
windings A and B, is no longer zero. Thus, an electromotive force is
generated within the dedicated coil C, which will quickly activate the
circuit breaker if |I | > I dn and disconnect the supply.
G
The same protective residual logic can be applied to three-phase
systems (Fig. 2.8).

FIGURE 2.8
A residual current
device in a
three-phase
circuit.