Arc fault detection device (AFDD)
Plain English. Practical detail. UK context.
An AFDD is an arc fault detection device. It analyses electrical behaviour associated with arcing faults and disconnects the protected circuit when its detection criteria are met. Arcing can produce heat without looking like the overload or earth leakage detected by other protective devices, so an AFDD has a distinct protective role.
Safety: use a registered electrician, for example one registered with a competent person scheme such as NICEIC or NAPIT, for electrical installation work. Do not remove covers or open electrical equipment yourself.
Last reviewed: (editorial review against the public sources listed below).
The problem it is designed to address
Damaged insulation, loose connections and damaged flexible cables can create conditions for arcing. Conventional protective devices and an AFDD do not all respond to the same signals. The device uses electronic analysis to distinguish fault signatures from normal operating behaviour. It does not inspect every connection or establish that workmanship is sound. A suspect accessory still needs investigation and repair; adding protection is not permission to leave a known damaged cable in service.
Where requirements need careful checking
BS 7671 contains both requirements and recommendations for AFDDs, depending on the circuits and premises. The 2026 amendment also changes wording, including the reference to high-rise residential buildings. Do not apply an old consumer summary saying they are always optional, or a sales statement saying every circuit in every home requires one. The electrician must check the current standard against the building use and circuit scope. Rental and shared-occupation descriptions need particular care because an everyday description may not settle the relevant classification.
Combined devices and customer operation
Some products incorporate other protective functions, such as RCBO protection. Check the actual product: AFDD is not a promise that every necessary function is included. Consumer unit compatibility, installation space and the manufacturer's instructions remain important. At handover, homeowners should be shown the user test and indication functions for the installed model. If it disconnects a circuit repeatedly, record the indications and seek competent investigation rather than bypassing it or assuming that all arcing protection is unreliable.
Evidence rather than a fire-proof claim
The IET's current edition page establishes BS 7671:2018+A4:2026 as the applicable current installation text. Its introductory AFDD guide describes how detection works, but its older statements about optional fitting need reading in that edition context. A designer should record the selected arrangement and provide appropriate verification. No device guarantees a fire-free property, and electrical protection does not replace smoke alarms or a broader fire safety assessment. This entry does not assign an inspection code automatically from the mere absence of an AFDD.
Frequently asked questions
Does an AFDD replace an MCB and RCD?
Only if the actual product incorporates the relevant functions and is correctly specified. The AFDD name alone does not establish that combination.
Are AFDDs always optional?
No. Requirements depend on the current standard, premises and circuits. Ask the designer to distinguish mandatory provision from a recommendation for the job.
Sources and further reading
Public sources and further reading. Sources checked: 8 November 2026. Consult the original for details and later changes.