Residual current monitors (RCMs) or modular residual current devices (MRCDs)?

Sep 24, 2023 | Applications, Technology

Residual current monitors (RCMs) and modular residual current devices (MRCDs) are both low-voltage electrical products that are designed to detect and respond to electrical faults but do not directly break the faulty circuits. In this piece of article, our guest blog writer introduces the following four key points that are important to know, when RCMs or MRCDs have to be used together with circuit breakers for the purpose of residual current protection. 


1. Fault Information Output

RCMs, compliant with IEC62020-1, are primarily focused on providing fault alarm signalling rather than directly actuating other protective devices to break the circuit in the event of a fault. Therefore, RCMs have specific and strict requirements for their own audible and visual alarm outputs, ensuring that the alarm persists for 48 hours without causing any damage to the devices or related components. However, no explicit requirements are specified for the reliability of the mechanical outputs of RCMs.

On the other hand, MRCDs, compliant with IEC60947-2 Annex M, serve as accessories to circuit breakers and are intended to cooperate with them in interrupting faulty circuits. Unlike RCMs, MRCDs do not have requirements for their own alarm display. Instead, they have detailed requirements for the electromechanical performance of the auxiliary contact outputs, which should comply with the relay contact requirements specified in IEC 60947-5.


2. Application Environment

RCMs are designed for domestic and similar applications and are suitable for use in pollution degree 2 environments. They are intended for distribution systems with voltages of 230/400V and overvoltage category III. The relevant tests for the product are also conducted based on these specifications. If RCMs are installed in environments that exceed the standard requirements, additional protection measures and testing verification are necessary.

MRCDs, on the other hand, are primarily intended for use in conjunction with circuit breakers. Therefore, their application environment, overvoltage category, and system voltage requirements align with those specified in IEC 60947-2 for circuit breakers. They can be used in AC distribution systems below 1000V and offer greater adaptability in various electrical installations.


3. Residual Current Type

RCMs complying with the IEC62020-1 standard and MRCDs complying with IEC60947-2 Appendix M can be designed to meet different residual types such as AC, A, F, and B.


4. Supplementary protection against personal electric shock

According to the relevant requirements of IEC60364-4-41 and IEC60755, residual current protection products with a rated residual current of less than 30mA and without any delay can be used for supplementary protection against personal electric shock in power distribution systems. Therefore, when RCM or mRCD is selected or designed for personal electric shock protection, the inherent operating time of the circuit breaking component should be considered and necessary verification should be carried out.


To sum up, in order to better provide reliable residual current protection in the power distribution system, when selecting or designing the residual current detection and control components to cooperate with the circuit breaker, it is more reliable to refer to Appendix M of IEC60947-2.

Related Products

BMRCD01 (Type B mRCD)



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