Residual current protection or residual current monitoring ?
By Chaz Andrews – Technical Manager , Doepke UK Ltd
RCDs ( RCCB , RCBO , CBR , MRCD ) 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 4.11 .
* Residual Current = Algebraic sum of the currents measured at a point in a circuit .
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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 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 , summarized in the 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 .
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 further information on RCD / RCM application and related subjects visit www . doepke . co . uk / Downloadstechnical-publications . php