What is an AFDD (arc fault detection device)?

Glossary · Consumer units and protection

How an AFDD works

An arc is a visible discharge that happens when current jumps the gap between two conductive materials. The IET says the common causes are worn contacts in equipment, damaged insulation, a break in a cable and loose connections. It adds that the temperature of an arc can exceed 6000 degrees Celsius, enough to start a fire.^{[1]IET, Arc Fault Detection Devices (AFDD)}

An AFDD is a protective device installed in a consumer unit to guard against arc faults. It uses microprocessor technology to analyse the waveform of the electricity on the circuit and look for the unusual signature of an arc. If it finds one, it cuts power to the circuit, which could prevent a fire. The IET says AFDDs are far more sensitive to arcs than conventional circuit protective devices, and that they usually have a test button, like an RCCB or RCBO.^{[1]IET, Arc Fault Detection Devices (AFDD)}

An AFDD sits alongside the other devices. It does not replace the circuit breaker or the RCD. See RCD vs RCBO for those.

Is an AFDD required?

It depends on the building.

The first row comes from the IET.^{[1]IET, Arc Fault Detection Devices (AFDD)} The second comes from NICEIC’s Amendment 4 guidance. NICEIC says there is no requirement within BS 7671 for AFDDs on circuits supplying battery energy storage systems.^{[2]NICEIC, Understanding BS 7671:2018 + Amendment 4:2026}

The IET says AFDDs are worth considering where fire risk is higher. Its examples include premises with sleeping accommodation such as houses, locations with combustible building materials, timber-framed buildings, and places with irreplaceable items such as listed buildings.^{[1]IET, Arc Fault Detection Devices (AFDD)} When a consumer unit is replaced, Electrical Safety First recommends AFDDs in areas identified as higher risk, on socket circuits not exceeding 32 amps.^{[3]Electrical Safety First, Best Practice Guide 1: Replacing a consumer unit in residential and similar premises}

Fitting and insurance

The IET says AFDDs are typically twice the size of a circuit breaker and need a busbar with live and neutral connections. It is unlikely they can be added to an existing consumer unit.^{[1]IET, Arc Fault Detection Devices (AFDD)} It also suggests asking your insurer whether a payout would be affected if AFDDs were not installed as recommended.^{[1]IET, Arc Fault Detection Devices (AFDD)}

Example

Raj owns a timber-framed house in Otley. His electrician is replacing the consumer unit. She explains that the IET says AFDDs are worth considering where there is a risk from combustible construction, and that the choice is his.

Raj asks his insurer, then decides to protect the socket circuits. The IET says that where the risk comes from the structure, as in a timber-framed building, the whole installation should be protected, so she discusses the other circuits with him too.^{[1]IET, Arc Fault Detection Devices (AFDD)}

What people get wrong

People often think an AFDD is the same as an RCD. It is not. An RCD detects current leaking to earth, which protects people from shock. An AFDD detects arcing, which is a fire risk, and it works in a different way.

A second mistake is to think AFDDs are now compulsory in every home. They are not. NICEIC puts the requirement on four building types, and the IET describes a recommendation for the rest.^{[1]IET, Arc Fault Detection Devices (AFDD)[2]NICEIC, Understanding BS 7671:2018 + Amendment 4:2026}

The bottom line

An AFDD detects the arcing that precedes many electrical fires and cuts the circuit. It is a recommendation for most houses and a requirement on socket circuits in four building types. It usually means a new consumer unit, not an add-on. Whether to fit one is a risk decision to make with a registered electrician.