What Is an AFDD, and Do You Need Arc Fault Protection?

Arcing at a damaged cable or loose connection can start a fire without ever tripping a fuse, breaker or RCD, and that is the gap an AFDD fills. An arc fault detection device is a protective device fitted in the consumer unit that analyses the current on a circuit for the electrical signature of arcing, and switches the circuit off before the arc can start a fire. Under the current Wiring Regulations, AFDDs are required on socket-outlet circuits of up to 32 A in four types of higher-risk building, and recommended for the same circuits everywhere else, which includes an ordinary house.

What is an arc fault?

An arc is electricity jumping a gap through the air, the same thing you see as a spark, but sustained. The IET describes arcs as visible plasma discharges caused by current passing through a normally non-conductive medium such as air, and they run very hot. Inside a wall or behind a socket, next to timber, dust or insulation, that heat is enough to start a fire.

Arcs usually come from damage or poor connections. The IET lists the most common causes as worn contacts in electrical equipment, damaged insulation, a break in a cable and loose connections. A cable crushed behind a kitchen unit, a screw put through a buried cable when hanging a shelf, or a terminal that has worked loose over years can all arc without anyone noticing until something smells hot.

What an AFDD does that an RCBO cannot

Circuit breakers and RCDs are good at the faults they were designed for, and blind to this one.

• A circuit breaker (MCB) trips when too much current flows. An arc at a loose terminal on a circuit powering a kettle only carries the kettle's normal current, so the breaker sees nothing wrong.

• An RCD or RCBO trips when current leaks to earth. An arc between two points on the same conductor, or between live and neutral, leaks nothing to earth, so the RCD sees nothing either.

An AFDD uses microprocessor technology to analyse the waveform on the circuit and pick out the unusual signature an arc produces. The IET says AFDDs are far more sensitive to arcs than conventional protective devices.

Because they rarely fit older boards, AFDDs are usually added as part of a consumer unit replacement rather than on their own.

AFDD regulations: what BS 7671 requires

The current edition is BS 7671:2018+A4:2026, the IET Wiring Regulations, issued on 15 April 2026. Amendment 3 remains valid until 15 October 2026, after which it is withdrawn. The AFDD rule sits in regulation 421.1.7, which Amendment 2 turned into a requirement for certain buildings in 2022. The IET's published list of the main changes in Amendment 4 does not include AFDDs, and Electrical Safety First's coding guide written for Amendment 4 still names the same four types of premises.

Regulation 421.1.7 requires AFDDs on final circuits supplying socket outlets rated up to 32 A in:

• higher-risk residential buildings (HRRBs), such as high-rise blocks of flats

• houses in multiple occupation (HMOs)

• purpose-built student accommodation

• care homes

For the same circuits in all other premises, including a typical family home, AFDDs are recommended, not required. The IET's consumer guidance puts it plainly: where AFDDs are a recommendation, the customer can choose to leave them out.

Two points follow from that. First, the rules are not retrospective. Electrical Safety First's guidance is that there is no legal requirement to upgrade an existing installation to current standards. Second, on an inspection the absence of AFDDs is only coded in those four types of premises. Electrical Safety First's EICR coding guide lists a missing AFDD there as C3 (improvement recommended). In any other home it is at most an observation, with no code.

If you let an HMO, the rule matters when a board is replaced or socket circuits are added, and it is one reason we suggest landlords plan a board change rather than leave it until something fails. Electrical Safety First's guidance on replacing consumer units lists fitting AFDDs on socket circuits in these higher-risk premises among the things a replacement board should include.

Is an AFDD worth fitting in an ordinary home?

It depends on the building and who is in it. The IET's consumer guidance suggests AFDDs deserve serious consideration where there is:

• sleeping accommodation

• a risk of fire from processed or stored materials

• combustible construction materials

• a structure that could help fire spread, such as a thatched or timber-framed building

• irreplaceable items that a fire would destroy

That list covers more homes than people expect. A timber-framed house, a loft conversion with bedrooms, an older property with a lot of alterations to its wiring, or a home where an elderly relative sleeps downstairs are all cases where we would talk it through. A modern flat with short, recent socket circuits is a weaker case.

Can you retrofit AFDDs to an existing consumer unit?

Usually not. The IET's guidance says AFDDs are typically about twice the size of a standard circuit breaker, so they are unlikely to fit an existing consumer unit for lack of space. In practice, AFDDs go in when a board is replaced, which is also when the cost is lowest because the work is being done anyway.

That does not always mean new wiring. A board change onto sound, tested circuits is covered in can you replace a fuse box without rewiring. On cost, the IET's consumer guidance puts a single AFDD at around £100, so fitting them on every socket circuit in a typical house adds a noticeable amount to the parts bill, and far less than the house they protect.

AFDDs are often discussed alongside surge protection because both are consumer unit add-ons with a fire and damage angle, but they deal with different problems. We cover the other one in our guide to a surge protection device in a consumer unit.

Whether you need an electrician in Telford, Shifnal or Wolverhampton, we fit AFDDs as part of board changes across the West Midlands. If you are not sure whether your home or rental would benefit, SM Electrical can look at the board and the building and give you a straight recommendation.

AFDD questions, answered

Do AFDDs cause nuisance tripping?

An AFDD is looking for the signature of an arc, so a trip deserves attention rather than a quick reset. If an AFDD trips repeatedly on one circuit, treat it as a warning that there may be a damaged cable or loose connection on that circuit, and have it investigated rather than reset it again and again.

Do AFDDs need testing?

The IET's guidance says AFDDs usually have a test button you can press to prove the mechanism works, much like an RCD. Your electrician will also test the installation when the board is fitted and at each inspection.

Will my EICR fail if I do not have AFDDs?

In a normal owner-occupied or single-family rented house, no. The absence of AFDDs is only coded, as C3, in the higher-risk premises listed above, and a C3 is a recommendation rather than a fail. An EICR will still pick up the loose connections and damaged cables that cause arcing in the first place.