Residual current protection or residual current

Jan 30, 2024

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

*Residual Current = Algebraic sum of the currents measured at a point in an electrical circuit.

RCD Standards

RCCBs and RCBOs can be used in installations operated by ordinary persons. CBRs and MRCDs are for operation by instructed / skilled persons – see BS7671 clause 531.3.4.1.

RCD sensitivity and tripping time requirements are given in BS7671 based on the required protection level namely, Additional, Fire or Fault protection and the system earthing. Annex A53 specifies the general product standards for RCDs see table 1.

RCM Standard

EN62020 allows RCM actuating time < 10 seconds at the rated residual current. Consequently, RCMs cannot provide protection in TN or TT installations. Specific rules apply to RCMs used in IT systems - under the permanent supervision of skilled or instructed persons.

RCMs are designed for monitoring, providing pre-alarm and alarm status. This enables remedial action in situations where the unexpected tripping of an RCD would create a hazard (hospital, public place...) or production stoppage (manufacturing, process industry…).

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

Circuits that are permanently monitored by an RCM do not require periodic insulation resistance testing, providing the RCM is regularly checked using the trip test feature – see BS 7671 651.2

Types of residual current

All AC loads produce leakage (PE) currents at mains frequency and harmonic frequencies. Take this into account to avoid unwanted tripping – see 531.3.2. Under fault conditions some loads produce pulsed or smooth DC residual current components. The RCD or RCM "Type"( see table 2) must be suitable for the PE and residual currents that it could be exposed to – see BS7671 clause 531.3.3.

For modern commercial and industrial sites, the nature of the loads connected immediately down stream of the RCM, will determine if a Type A or Type B device is required to ensure reliable operation and signaling of potentially dangerous residual currents.

For further information on RCD / RCM application and related subjects visit https://www.doepke.co.uk/Downloads-technical-publications.php

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