Chaz Andrews, Technical Manager at Doepke UK Ltd, considers the, often inconsistent, requirements of RCD protection in modern homes.
Type AC RCDs may not be appropriate for modern homes which use energy-saving white goods, LED lighting, induction hobs, heat pumps etc. Most appliances these days include some form of “power electronic converter system” (PECS) to convert and or chop the supply, resulting in non-linear waveforms, increased harmonic distortion (three-phase) and increased operational leakage currents. Failing to take proper account of the electrical loads connected to individual circuits, increases the risks associated with residual currents.
BS7671 guidance note GN1 makes comment about Type AC, A and B RCDs, but goes on to state “For the majority of applications, type AC devices are suitable, with type A or B being used where special circumstances exist”. Many domestic, public and commercial installations will be running PECS controlled equipment, which may not be suitable for connection to circuits containing Type AC RCDs. White goods manufacturers may not give any advice or warnings relating to the connection of their equipment to AC RCD protected circuits. See Fig. 1.
Revised recommendations in HD60364-5-53
HD60364-5-53 published in July 2015* included a number of significant revisions and recommendations directly linked to improving the security and safety of electrical installations.
*Chapter 53 of BS7671 Amd.3 was based on FprHD60364-5-53 2014.
531.3.2 Unwanted tripping
BS7671 just states “limit the risk of unwanted/unnecessary tripping of RCD protected circuits”, but gives no guidance. An RCD is designed to trip when you exceed 0.5 -1 x I∆n, a fact that must be considered when installing additional appliances or circuits. Clause 531.3.2 in HD60364-5-53 recommends that to reduce nuisance tripping, the total earth leakage current of downstream appliances should not exceed 0.3 x I∆n of the upstream RCD. This safety margin is important when considering the increased loading (maximum demand) placed on installations today and the practicality of allowing for higher leakage currents, associated with PECS based technology.
531.3.3 Types of RCD
Clause 531.3.3 in HD60364-5-53 includes information on the Types of RCD (AC, A, F & B) and the general limits of operation for these devices. Note 5 refers to additional information contained in Annex A*. This gives details of some common PECS topology used in modern equipment, the associated residual current and the type of RCD required (example see fig 2.) This information highlights the importance of checking installation information for new appliances and or contacting the appliance manufacture for RCD compatibility information.
*Annex A of HD60364-5-53 is replicated in several safety standards (EN62477-1 and IEC/TR62350)
The up and coming publication of the 18th Edition of the Wiring Regulations will clarify some of the inconsistencies relating to RCD selection by including clear guidance on the selection of appropriate “Types” of RCDs for general domestic, public and commercial buildings. This will result in more reliable and safe installations.
Further reading: RCDs Principles and Selection (www.doepke.co.uk/download/Techpub-16)
This article first appeared in Issue 3 of Smart Electrician Magazine.