How does an RCM work?
All conductors of the outgoing circuit to be monitored (except the PE conductor) are routed through a measuring current transformer. In a fault-free system, the sum of all currents is equal to zero so that no voltage is induced in the measuring current transformer. If a fault current (IΔ) flows via PE or other paths, the difference in current in the measuring current transformer generates a current flow which is detected by the RCM. This measurement method applies to RCMs for pure AC current and pulsating DC fault currents (type A in accordance with IEC 60755).
AC/DC sensitive RCMAs and RCMBs of type B require special measuring current transformers and a special measurement method to detect both DC and AC currents of different frequencies.