Residual current monitoring (RCM) versus RCD — continuous measurement instead of tripping

The RCD types & classification guide covers the RCD as a protective switching device: when the leakage current exceeds the set I∆n, the RCD automatically disconnects the supply. This article covers a different device with a superficially similar function, but a fundamentally different purpose: the Residual Current Monitor (RCM), standardised in IEC 62020.

The key difference: signalling versus switching

RCD (Residual Current Device): a protective switching device. When the set leakage-current threshold is exceeded, the RCD automatically and directly interrupts the supply to the circuit — protection of persons/fire safety comes first, at the expense of availability.
RCM (Residual Current Monitor, IEC 62020): a measuring instrument. The RCM continuously measures the leakage current to earth and, when a set threshold is exceeded, gives an alarm (visual, audible, or via a communication interface to a building management system) — but does not interrupt the supply itself. The decision to intervene remains with a person or a separate, purpose-designed switching mechanism.

Note: an RCM does not replace an RCD where an RCD is legally or normatively required (for example the 30mA shock-protection requirement of §531). RCM and RCD are complementary: the RCM gives early warning and trend information about a gradually deteriorating insulation, the RCD remains the final safety layer that actually disconnects in the event of an acute fault.

Applications where continuous measurement is preferred over immediate switching

Critical processes and data centres: an unplanned interruption caused by an RCD trip can itself cause significant damage (loss of a production process, a server cluster failing abruptly). An RCM signals a gradually rising leakage current (for example due to ageing cable insulation) well before an RCD's disconnection threshold is reached, so that planned maintenance is possible instead of an unplanned stop.
Alternative to periodic insulation resistance testing: under IEC 60364-6, the periodic insulation resistance measurement during the re-inspection of an installation may be replaced by permanent monitoring with an RCM in accordance with IEC 62020, provided it is continuously followed up by a competent person — see also the insulation resistance testing guide for the classic, periodic measurement method for which this can be an alternative.
Trend analysis: because an RCM measures continuously (instead of a snapshot during a periodic inspection), a gradual deterioration of the insulation can be tracked over time — an insulation fault that develops between two inspection points is missed by a periodic measurement, but detected by an RCM at the moment the trend exceeds the set alarm threshold.

Where an RCM is not suitable

An RCM does not replace the RCD's protective function against electric shock hazard or against tracking-current fire (see the fire-risk premises guide for the 300mA requirement there) — in both cases automatic, immediate disconnection is the goal, and that requires an RCD, not an RCM.

Practical relevance

When designing the monitoring strategy for a critical installation (data centre, continuous process), it must be established which function is needed at which point: where automatic disconnection on an acute fault is required or necessary (RCD), and where early signalling without interruption is preferred to prevent unplanned outages (RCM) — and whether, when using an RCM as an alternative to periodic insulation testing, continuous follow-up by a competent person is actually guaranteed.

Common mistakes

Installing an RCM at a point where an RCD is normatively required (for example shock protection under §531) — an RCM only alarms, it does not disconnect, and therefore does not satisfy an RCD requirement.
Using an RCM as an alternative to periodic insulation testing without ensuring continuous follow-up by a competent person — the IEC 60364-6 exception precisely assumes that continuous follow-up, not just the presence of the measuring instrument.
Treating RCM and RCD as competing, mutually exclusive options — in practice they are often combined: the RCM for early trend signalling, the RCD as the final safety layer.