AFDD: When Arc Fault Detection Becomes Mandatory in London (2026)

An Arc Fault Detection Device watches the waveform on a circuit for the distinctive high-frequency signature of an arcing fault — typically a series arc inside a loose terminal, a stapled-through cable, or a damaged flex behind furniture. Conventional MCBs and RCDs cannot see series arcs because the current stays within normal load range while the arc burns at 1,000°C+ in a sealed wall void.

AFDDs sit in the consumer unit looking like a wider RCBO. They use embedded signal processing to distinguish a genuine arc from the normal switching noise of vacuum cleaners, drills and dimmers. When they detect a sustained arc signature, they trip in under 1 second.

The fire data is what made this a regulatory issue. Series arc faults are the suspected ignition source in roughly 25% of UK domestic electrical fires — a category previously labelled "cause undetermined" because the evidence burns away.

BS 7671:2018 Amendment 4 (effective for new work from October 2026) requires AFDD protection on all final circuits in: HMOs and houses of multiple occupation regardless of size, residential care homes and supported living accommodation, schools and nurseries, and buildings of "higher fire risk construction" — which in practice means timber-frame, SIPs panels, and CLT structures.

For all other premises AFDDs remain "recommended" — meaning C3 (improvement recommended) on an EICR. The wording is taken from BS 7671 Regulation 421.1.7.

Inner London boroughs with high HMO density — Newham, Tower Hamlets, Hackney, Brent — are already seeing licence renewal officers ask whether AFDDs have been fitted at the most recent board change. They cannot demand retrofit, but they can refuse to accept a recently-installed non-compliant board.

A 2024 LFB investigation in a Stratford HMO traced a fatal fire to a loose backstabbed connection inside a socket behind a bedhead — a textbook series arc. The MCB never tripped because the current draw stayed under 13A. An AFDD would have caught the signature within seconds.

A care home fire in Croydon the same year started at a damaged extension lead under a resident chair. Again, current was nominal. The arc carbonised the insulation and ignited the upholstery before staff noticed.

These are the cases A4:2026 is responding to. They are not edge cases.

A combined AFDD+RCBO from Hager, Wylex or MK costs the trade about £45–£55 per device versus £12–£18 for a standard Type A RCBO. On a 10-way HMO board that adds roughly £350 in materials.

Total installed delta for a compliant A4:2026 HMO board upgrade with AFDDs is around £950–£1,100, versus £750 for a standard Type A RCBO board. We quote a fixed £999 for an HMO-compliant 6–12 way AFDD board including SPD and certification.

For new builds the price gap is closing fast. Three of the major manufacturers now ship dual AFDD+RCBO modules as a single device, which has cut wiring time and brought the per-circuit premium down from £50 (2022) to about £35 (2026).

A4:2026 allows the designer to omit AFDD where a documented risk assessment shows the circuit cannot present a meaningful arc-fault hazard — for example, dedicated final circuits feeding fixed equipment with no flexible cabling. In practice this exemption is narrow and rarely worth invoking.

Existing installations are not required to retrofit. The standard applies to new work — fuse board upgrades, rewires, and circuit additions. An EICR on a pre-A4 installation will code an AFDD-absent socket circuit C3 in an HMO, not C2.

Where an installation undergoes "major alteration" the new circuits must comply but existing untouched circuits can remain. The interpretation of "major alteration" is going to drive disputes in 2026–2027.

AFDDs are not yet mandatory in owner-occupier homes, but three scenarios justify retrofitting at the next board change. First, timber-frame extensions or loft conversions where a series arc in a wall void could spread undetected. Second, properties with elderly or vulnerable occupants who would struggle to evacuate quickly. Third, homes with significant flexible cable loads — home offices with multiple extension leads, workshops, hobby rooms.

For everyone else, AFDD on the kitchen and bedroom sockets is a reasonable middle path: £70 added to a board upgrade, protection on the two highest-risk circuits.