RCM versus RCD: How does residual current monitoring work?

What do RCM, RCMS and RCD mean?

These abbreviations identify a device’s measurement, multichannel monitoring or protective function.

• RCM — Residual Current Monitor: A device that measures residual current and signals when response values are exceeded.
• RCMS — Residual Current Monitoring System: A multichannel residual current monitoring system. RCMS products monitor multiple outgoing circuits using compatible sensors; the number of channels depends on the model.
• RCD — Residual Current Device: A residual current protective device that disconnects the circuit under its protective operating conditions.

Why is the sum of currents measured?

All live conductors of the monitored outgoing circuit pass together through the same measuring current transformer. Protective conductor PE remains outside the loop. In the ideal case with no leakage or fault current, the sum of the currents in the live conductors is zero. If part of the current returns via PE or another path, the sensor detects the difference. The RCM displays measured residual current and generates information when the configured threshold is exceeded. Multichannel RCMS systems monitor different outgoing circuits separately.

Giving an alarm and tripping for protection are different tasks

An RCD disconnects the circuit as part of its intended protective function. An RCM supports continuous monitoring of electrical condition and early detection of deterioration. Depending on the model and wiring, an RCM's relay outputs may be used for signalling or switching; this does not give the RCM an RCD protective function. MRCD protection is assessed using the device, sensor and circuit-breaker combination verified by the manufacturer. Considering monitoring thresholds alongside protective device behaviour can give the maintenance team earlier, clearer information.

The waveform changes sensor selection

In loads containing power electronics, residual current may include more than the mains-frequency AC component. Pulsating or smooth DC components and different frequencies require suitable detection technology. Bender offers Type A and AC/DC-sensitive Type B solutions; the actual measuring range and frequency capability must be checked for the model. The current transformer, evaluator and communication arrangement are selected together. A single mA value cannot support conclusions about the entire installation without knowing which section is monitored.