FIRE & SECURITY FEATURE
Electrical fires in the home increase by 35 per cent in one year
A report in September 2016 identified a 35 per cent increase in fires due to electrical faults, representing 27 per cent of claims and the second most common cause of fires in houses. Chaz Andrews, technical manager at Doepke UK, discusses
F
igures from the Association of British Insurers (ABI) would indicate
that electrical faults are on the rise and cost the insurance industry over £100M for the domestic sector.
INCREASED LEAKAGE CURRENTS AND HARMONICS The early detection of increased leakage and residual currents is a basic tool to be used in the prevention of electrical fires, particularly in appliances and installations. Considering the complexity of modern installations, it is questionable if the existing regulations offer enough general guidance on the selection of residual current devices (RCDs), particularly when compared to other protection devices. BS7671 gives limited information in the general clauses relating to the types of RCD protection for modern equipment (see GN1). Many white goods manufacturers, for example, do not give any advice or warnings relating to the connection of their equipment to conventional AC RCD protected circuits. Designing, specifying, building and
kitting out modern homes, requires additional knowledge and skills to deal with the implications of installing new technology, such as energy saving washing machines, LED lighting, induction hobs and heat pumps, into existing and new builds. Many of the appliances we use these days will include some form of power electronics to convert and chop the supply, resulting in non-linear waveforms, increased harmonic distortion and increased operational leakage currents. Failing to take proper account of the load characteristics connected to the electrical installation increases the fire risk associated with earth leakage and residual currents. Property owners and architects rely on
the electrical contractor and installer to leave the installation safe and appropriate for use. Using the wrong type of RCD results in reduced or ineffective protection, including the problems stemming from unwanted
earth leakage current of downstream appliances should not exceed 0.3 x I∆n of the upstream RCD. This margin is particularly important when considering the increased loading (maximum demand) placed on installations today and the practicality of allowing for the higher leakage currents associated with modern technology.
tripping. Installing equipment onto unprotected RCD circuits creates a potential fire hazard, especially as the equipment ages. There is a risk of increased earth leakage currents igniting surrounding materials before the operation of the overload protection device. Incorporating better design and installation practices can improve the reliability and safety of the installation.
REVISED RECOMMENDATIONS IN HD60364-5-53 HD60364-5-53, published in July 2015, included a number of revisions and recommendations directly linked to improving the security and safety of electrical installations. Chapter 53 of BS7671 Amendment 3 was based on FprHD60364-5-53 2014.
531.3.2 UNWANTED TRIPPING The existing guidance in BS7671 is restricted to limiting the risk of unwanted and unnecessary tripping of RCD protected circuits, but gives no further advice. An RCD is designed to trip when is exceeds 0.5 -1 x I∆n, a fact that may be overlooked when installing additional appliances or circuits. The revised clause 531.3.2 includes the recommendation from IEC62350, which suggests that in order to avoid nuisance tripping, the total
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531.3.3 TYPES OF RCDS In the general text of BS7671 Amendment 3, there is no reference to the various types of RCD that may be required to achieve a safe level of protection. The guidance note GN1 Amendment 3 makes a comment about Type AC, A and B and 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.” These days many domestic, public and commercial installations will be running some form of equipment which is not suitable for connection to circuits containing Type AC RCDs. Clause 531.3.3 in the revised standard
includes information on the types of RCD (AC, A, F & B) including the general limits of operation for these devices. Note 5 refers to additional information contained in Annex A, which details some common power electronic circuits contained in modern equipment, the associated residual current and the type of RCD required. This information is required to raise the awareness within the electrical installation industry and also draws attention to the importance of OEMs, providing adequate and safe installation information with new appliances. Annex A of HD60364-5-53 is replicated in several safety standards (EN62477-1 and IEC/TR62350) Designing and costing the installation
correctly at the start of the process is a lot easier than trying to take corrective actions during the final inspection and testing phase. The up and coming publication of the 18th Edition of the Wiring Regulations will hopefully clarify the existing anomalies, with the addition of new information contained in HD 60364-5-53 (531.3, residual current devices and Annex A). For additional information on RCDs principles and selection, download Doepke Techpub-16 at
doepke.co.uk/download/Techpub-16
Doepke UK
www.doepke.co.uk
ELECTRICAL ENGINEERING | MAY 2017 25
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