ELECTRICAL SAFETY | ARC FAULT DETECTION DEVICES Right: HV-conditioned sample of single-insulated conductor Arc fault detection devices have been recommended in wiring regulations since 2019. Based on his own experiments, Hoare Leas Kenelm Hoare MCIBSE looks at the realities of arc risk and discusses how effective the devices are in preventing electrical fires FAULT-FINDING MISSION A fter the tragic events at Grenfell, there is an increased emphasis on improving fire safety and, understandably, engineers are looking for technology that reliably reduces fire risk. Through 2018 to 2019, arc fault detection devices (AFDDs) entered the awareness of electrical engineers and became a recommendation of The IET Wiring Regulations 18th Edition (BS 7671:2018), effective January 2019. Building on that, the Draft for Public Comment (DPC) for the IET Wiring Regulations 18th Edition Amendment 2 (now closed for comments) proposes a general mandate for the use of AFDDs for most 230V AC circuits. The decision about whether to follow this recommendation tends to turn on legal and commercial pressure because of AFDDs being recommended by a British Standard. But what are the technical facts and how effective are AFDDs in reducing the incidence of electrical fires. The following analysis is supported by research and experimentation by the author. What is an AFDD and how does it work? IEC 60050 International Electrotechnical Vocabulary defines an AFDD as a: device intended to mitigate the effects of arc faults by disconnecting the circuit when an arc fault is detected. The alternating current waveform is ordinarily sinusoidal, cycling at 50Hz (Figure 1). However, arcing distorts the usually smooth sinusoidal waveform, which becomes shouldered. The shouldering is down to the striking and extinguishing of individual arcs as the voltage wave moves through the zero crossing point (the point where the instantaneous voltage is zero). The arc voltage, the voltage across the spark gap, is also distorted according to the change in impedance (Figure 2). Such wave distortion and high-frequency noise caused by arcing are parameters that an AFDD uses to identify an arc fault. What is an arc fault? IEC 60050 defines an arc fault as a dangerous unintentional arc. This Development of a series arc fault within a cable sample 30 October 2021 www.cibsejournal.com CIBSE Oct21 pp30-32 Arc Fault Detection.indd 30 24/09/2021 14:19