Residual Current Monitoring: The Complete RCM Guide for UK Electricians

Residual current monitoring (RCM) is a method of continuously measuring the earth leakage current (residual current) flowing in a circuit and generating an alarm when that current exceeds a pre-set threshold. Unlike a residual current device (RCD), which automatically disconnects the supply when a fault is detected, an RCM simply monitors and alerts — leaving the circuit energised and allowing the operator to investigate and plan maintenance.

The technology works on the same principle as an RCD: a current transformer (toroid) surrounds the live and neutral conductors. In a healthy circuit, the current flowing out through the live conductor and returning through the neutral conductor is equal, and the net magnetic flux in the toroid is zero. If current is leaking to earth (through damaged insulation, a fault, or a person touching a live part), the outgoing and return currents are no longer equal, and the difference is detected by the toroid. The RCM processes this signal and triggers an alarm if it exceeds the threshold.

RCM is used in situations where automatic disconnection would cause greater problems than the fault itself. Examples include: hospital critical care areas where power interruption could affect life-support equipment; data centres where server power must be maintained; industrial process lines where sudden shutdown could cause product damage, environmental contamination, or safety hazards; and installations where high availability is paramount.

For UK electricians, the most common encounter with RCM technology is in EV charger installations, where an integrated RCMU (Residual Current Monitoring Unit) detects DC fault currents as an alternative to an external Type B RCD.

Understanding the distinction between RCM and RCD is essential for correct specification and installation:

In practice, RCM and RCD are often used together. The RCD provides automatic disconnection for fault protection, while the RCM provides early warning of degrading insulation or developing faults — allowing planned maintenance before the RCD trips and causes an unplanned outage.

RCM is used across a range of critical installations where continuity of supply is essential:

The most common encounter with RCM for UK electricians is in EV charger installations. During AC charging, the charger converts AC to DC within the vehicle's onboard charger. If a fault develops in this conversion process, a DC component can appear on the AC circuit. A standard Type A RCD detects AC and pulsating DC faults but can be blinded by smooth DC — meaning a DC fault could prevent the Type A RCD from detecting a subsequent AC fault.

The solution to this problem is either a Type B RCD (which detects all types of residual current including smooth DC) or an integrated RCMU within the charger that specifically monitors for DC fault currents above 6 mA. When the RCMU detects a DC fault, it disconnects the charger internally.

Most reputable EV charger manufacturers (Zappi, Easee, Pod Point, Ohme, Wallbox, and others) now include an RCMU as standard. Their installation manuals specify that only a Type A RCD is required upstream. Always check the manufacturer documentation before specifying the protection devices — if the charger does not have integrated DC fault detection, a Type B RCD is mandatory.

Document RCM devices on EV charger certificates

Elec-Mate's EV charger certificate template includes fields for RCMU type, manufacturer, and verification status.

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BS 7671:2018+A4:2026 addresses residual current monitoring in several regulations:

The key principle in BS 7671 is that automatic disconnection of supply is the primary method of fault protection (Regulation 411.3). RCM is not a substitute for this — it is an additional measure used in specific circumstances where automatic disconnection is not appropriate, or where early warning of developing faults is beneficial.

Residual current devices and monitoring systems are classified by the type of fault current they can detect:

The significant cost difference between Type A and Type B devices is a key driver for the adoption of integrated RCMUs in EV chargers. By building DC fault detection into the charger itself, the manufacturer eliminates the need for an external Type B device — saving the installer and customer £150 to £300 per installation.

There is also a Type F RCD, which detects the same fault types as Type A plus certain high-frequency residual currents generated by variable speed drives. Type F sits between Type A and Type B in capability and cost. It is specified for some inverter-driven equipment but is less commonly encountered than Type A and Type B.

DC fault currents are becoming increasingly common in modern electrical installations due to the proliferation of power electronic devices: EV chargers, solar PV inverters, battery storage systems, variable speed drives, and switch-mode power supplies. These devices convert between AC and DC, and faults can inject a DC component onto the AC circuit.

The problem with DC fault current is that it can saturate the core of a Type AC or Type A RCD. When the core is saturated, the device may fail to detect a subsequent AC fault current — effectively rendering the RCD blind. This is a genuine safety concern: the circuit appears to be protected, but the RCD may not operate when needed.

The 6 mA DC threshold used by EV charger RCMUs comes from the IEC 62955 standard, which specifies the requirements for residual direct current detecting devices (RDC-DD) for EV charging. When the RCMU detects DC leakage above 6 mA, it disconnects the charger — preventing core saturation of the upstream Type A RCD.

RCM devices should be installed and maintained following the manufacturer instructions and the relevant standards. Key considerations include:

Proper documentation of RCM devices is essential for inspection readiness and scheme compliance. Elec-Mate makes it easy:

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Elec-Mate's certificate templates include dedicated fields for RCM devices, RCMU verification, and RCD type selection.

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