The RCDs (RCCB, RCBO, CBR, MRCD) defined in BS7671 and summarized in Table 1 below disconnect and isolate the circuit when the residual current* exceeds a predefined value. The sensitivity of the RCD (mA) and its time/current characteristics (milliseconds) determine the protection level. On the contrary, RCM is designed for monitoring purposes-see BS7671 clause 4.11.

* Residual current = the algebraic sum of the current measured at a certain point in the circuit.

RCCB and RCBO can be used in devices operated by ordinary people. CBR and MRCD are operated by instructed/technical personnel-see BS7671 clause 531.3.4.1.

RCD sensitivity and trip time requirements are given in BS7671, based on the required level of protection, namely additional, fire or fault protection, and system grounding. Annex A53 specifies the general product standards of RCD, see Table 1.

EN62020 allows RCM to operate for less than 10 seconds at rated residual current. Therefore, RCM cannot provide protection in TN or TT installations. Specific rules apply to the RCM used in IT systems-under the permanent supervision of a technician or a supervised person.

RCM is designed to monitor and provide pre-alarm and alarm status. This makes it possible to take remedial measures in the event that an accidental RCD trip would cause danger (hospitals, public places...) or production shutdowns (manufacturing, processing industries...).

Note: RCM is installed downstream of RCD; set RCM to <0.5 x RCD sensitivity to reduce the risk of RCM tripping before RCM-please refer to BS7671 clause 538.4.    

Circuits permanently monitored by RCM do not require periodic insulation resistance tests, provided that the RCM is checked regularly using the trip test function-see BS 7671 651.2

All AC loads will generate leakage (PE) current at the power frequency and harmonic frequencies. Take this into consideration to avoid accidental tripping-see 531.3.2. Under fault conditions, some loads will produce pulses or smooth DC residual current components. The RCD or RCM "type" (see Table 2) must be applicable to the PE and residual current that may be exposed-see BS7671 clause 531.3.3. 

For modern commercial and industrial sites, the nature of the load directly connected to the downstream of the RCM will determine whether Type A or Type B equipment is required to ensure reliable operation and potentially dangerous residual current signals. 

For more information on RCD/RCM applications and related topics, please visit https://www.doepke.co.uk/Downloads-technical-publications.php

sales@doepke.co.uk or www.doepke.co.uk

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