Paul Chaffers, Technical Events Manager at NAPIT, looks at arc fault detection device (AFDD) requirements following the publication of BS 7671:2018+A2:2022.
The subject of AFDDs rapidly became a hot topic in 2018, following the introduction of the 18th Edition of the Wiring Regulations (BS 7671:2018). In spite of being a relatively new technology for the UK, it soon became apparent that other countries had been using such devices for years.
In fact, similar devices were actually mentioned in the 17th Edition of BS 7671 back in 2008 when a new Section 532 ‘devices for the protection against the risk of fire’ was introduced. At that time, Note 3 of Regulation 532.1 provided additional methods that could be used and listed devices intended to provide protection from the effects of arc faults.
2018 recommendation
The use of AFDDs introduced in 2018 was listed only as a recommendation but left many designers responsible for complex installations feeling uncomfortable. After all, choosing to disregard a recommendation contained in a National Standard is not a decision to be taken lightly, especially when this recommendation concerns fire prevention.
2020 Draft for Public Comment (DPC)
The draft text for Amendment 2 (BS 7671:2018+A2:2022) saw further strengthening of this subject with the mandating of AFDDs.
With the introduction of new requirements for AFDDs, for single-phase AC final circuits supplying socket-outlets and fixed current using equipment with a rated current not exceeding 32 A, there was considerable debate within the industry and there were many comments, as part of DPC, which were submitted to BSI ready for consideration by JPEL/64 – the national committee responsible for preparing BS 7671.
2022 AFDD requirements
Following the consideration of each and every comment submitted by DPC, the committee decided only to mandate AFDDs for specific higher risk areas in order to mitigate the risk of fire from the effects of arc fault currents.
BS 7671:2018+A2:2022 Regulation 421.1.7 now requires arc fault detection devices (AFDDs) conforming to BS EN 62606 to be provided for single-phase AC circuits supplying socket-outlets with a rating not exceeding 32 A in the following installations:
● Higher Risk Residential Buildings (HRRB)
● Houses in Multiple Occupation (HMO)
● Purpose-built student accommodation
● Care homes
HRRBs are assumed to be residential buildings over 18 m in height or in excess of six storeys, whichever is met first. See Note 1 of Regulation 421.1.7 for further guidance on HRRBs.
The installation types listed in Regulation 421.1.7 are deemed to be higher-risk installations. In other words, this does not mean that the electrical installation is more likely to develop an arc fault, but the type of installation is deemed to present a greater level of risk under fire conditions.
This is due to a number of reasons, such as:
● The complexity of the building and the level of difficulty of evacuation
● The type and capability of each user having been considered
It is recommended for all other premises that AFDDs are provided for single-phase AC circuits supplying socket-outlets with a rating not exceeding 32 A.
AFDD characteristics
AFDDs use microprocessors to identify arc faults by analysing an arc’s signature (waveform). When an arc fault is detected, the device will automatically trip the affected circuit. The device is activated by both series and parallel faults, as illustrated in Fig 1.
To avoid false tripping, electronic technology in the device enables it to differentiate between arc faults and the arcs that are present during normal switching activities, including those present during the operation of appliances.
AFDDs offer protection against arc faults in both the fixed wiring and equipment plugged in, including extension leads.
A difference of opinion exists regarding the suitability of using the devices on ring final circuits, with some saying they do not work on ring circuits. However, this is not true since they offer protection against parallel arc faults in the fixed wiring, as well as both series and parallel arcs in the equipment that is plugged into the ring final circuit.
The only fault that cannot be detected by an AFDD is a series fault in a ring final circuit. This is because the current will flow around the other (connected) leg of the ring.
Although a break in continuity is not ideal (as it could lead to one leg of the ring being overloaded), it is unlikely that a dangerous arc will occur.
Correct selection
AFDDs can be installed in consumer units and distribution boards to protect final circuits; however, there will still be a need for overcurrent protection and, where necessary, RCDs. Electrical fires can be caused by a number of factors that require protection from more than one device:
● Short-circuits and overloads: Protection provided by CBs and Fuses
● Earth leakage currents: Protection by RCD, RCBOs, etc.
● Overvoltage: Protection by SPDs
● Dangerous electric arcs: Protection by AFDDs
It is a requirement that AFDDs shall be installed at the origin of the circuit they are protecting.
It should be noted that there is an exception to this for busbar systems conforming to BS EN 61439-6 and Powertrack systems to BS EN 61534. For further guidance, refer to Note 2 of Regulation 421.1.7.
An AFDD is intended to protect against the effects of arcing faults by disconnecting the circuit when arc faults are detected. Appropriate ways to minimise the occurrence of arc faults must still be applied, such as:
● Correct cable installation in prescribed zones
● Correct cable radius on bends
● Correct terminal tightness
● Suitable intervals of periodic inspection (EICR)
● Regular inspection of appliances and flexible leads used in the installation
Conclusion
Although not all installation types will be affected by the AFDD requirements, some will be. As a result, there will be more devices developed, which will likely see greater competition amongst manufacturers and potentially lower prices, making the installation of AFDDs more palatable. Just look at the journey of the RCBO and how they’re now widely accepted.
As part of the launch of BS 7671:2018+A2:2022, all of NAPIT’s comprehensive collection of technical guidance books have been updated to give you the latest information.
They’re available, as always, in both electronic and paper version from NAPIT Direct – www.napitdirect.co.uk
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