Summary

Arc Fault Detection Devices represent one of the most significant additions to BS 7671 in recent years. Introduced in the 18th Edition (2018), they target a specific and previously undetectable failure mode: the series arc fault. An arc fault occurs when a damaged or loose wire generates an electrical discharge (arc) that can ignite surrounding materials — the arc may not draw enough current to trip an MCB, and the intermittent nature means an RCD may not trip either. House fires caused by wiring faults kill approximately 50 people per year in the UK, and series arc faults are a contributing factor.

The regulatory status is important to understand clearly. Regulation 421.1.7 uses 'shall be installed' — but only for a specific list of higher-risk premises: Higher Risk Residential Buildings (HRRBs — residential buildings over 18m or six storeys, whichever is met first), Houses in Multiple Occupation (HMOs), purpose-built student accommodation, and care homes. This mandatory scope has not changed in substance across A2:2022, A3:2024, and A4:2026, but it is commonly mis-stated (including in earlier guidance and some trade commentary) as applying more broadly to "sleeping accommodation in new residential buildings" generally — it does not. For an ordinary new-build house or flat outside those four categories, AFDDs on bedroom socket circuits remain 'recommended', not mandatory, though NICEIC/NAPIT and most contractors treat it as best practice regardless. This distinction matters when discussing the requirement — and its cost — with clients and building control.

BS 7671:2018+A4:2026 was published on 15 April 2026 and consolidates all prior amendments (A2:2022, A3:2024); the previous consolidated edition is withdrawn on 15 October 2026. A4:2026 also added a specific exemption for EV charging equipment conforming to BS EN 61851 (Regulation 722.421.1.7.201) and prohibits AFDDs in medical locations of Group 0 and Group 2.

The practicalities of AFDD installation — including load compatibility (see afdd nuisance tripping for fault-finding), position in the consumer unit, and coordination with SPDs — require careful attention. AFDDs are more expensive than standard RCBOs (typically 3–5× the cost) and the nuisance tripping risk means specification and load selection must be done carefully.

Key Facts

Quick Reference Table

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AFDD Type Arc Detection RCD Type Typical Application Notes
Type A Series arcs only RCD Type A Bedroom socket circuits (basic compliance) Minimum per BS 7671
Type F (combined) Series + parallel RCD Type A All domestic socket circuits Preferred; better protection
Type A + SPD Series + surge RCD Type A Combined SPD+AFDD in one module Available from Eaton, Hager
Type F + SPD Series + parallel + surge RCD Type A Full protection single device Premium; most comprehensive

Detailed Guidance

Regulatory Context — What is Actually Required?

Mandatory scope (higher-risk premises only): Regulation 421.1.7 of BS 7671:2018+A4:2026 requires AFDDs conforming to BS EN 62606 for single-phase AC final circuits supplying socket-outlets rated up to 32A specifically in: Higher Risk Residential Buildings (HRRBs — residential buildings over 18m in height or six storeys, whichever is met first), Houses in Multiple Occupation (HMOs), purpose-built student accommodation, and care homes.

This wording — 'shall be installed' — is a mandatory requirement in those four categories and will be treated as such by electrical certification bodies (NICEIC, NAPIT, ELECSA) and by building control officers. An EICR or Electrical Installation Certificate for a qualifying building that omits AFDDs on relevant socket circuits will require a code C2 or C3 deficiency notation. Outside these four categories — including the great majority of ordinary new-build houses and low-rise flats — AFDDs are recommended, not mandated by this regulation, though many contractors fit them as standard practice.

Existing installations: No mandatory requirement for existing installations outside the four categories above. When extending or modifying an existing installation, best practice is to improve to current standards — which NAPIT and NICEIC guidance suggests includes AFDD installation on any new or extended bedroom socket circuit. However, this is guidance, not regulation, except where the building itself falls within the mandatory HRRB/HMO/student accommodation/care home scope.

Sleeping accommodation and scope notes: Where AFDDs are specified (mandatorily or as a recommendation), the relevant circuits are typically bedrooms in dwellings, including children's rooms, guest rooms, and any room designated for sleeping. Living rooms, kitchens, hallways, and bathrooms are not "sleeping accommodation" for this purpose. A4:2026 also added a specific exemption for EV charging equipment conforming to BS EN 61851 (Regulation 722.421.1.7.201) and a prohibition on AFDDs in Group 0 and Group 2 medical locations.

Type A vs Type F — Selection Guidance

Type A (series arc fault detection only): Detects arcing that occurs in a single conductor — for example, a loose connection at a socket terminal where the wire intermittently contacts and disconnects, creating a series arc (current path: L → arc → L continues in circuit). This is the most common type of arc fault in domestic wiring.

Type F (combined — series and parallel): Additionally detects arcing between two conductors (L to N, L to E). Parallel arc faults can occur where two cables run together and both have degraded insulation, or where a cable is pinched between floorboards causing both insulation layers to be damaged. Type F provides more comprehensive protection and is recommended in BS 7671 commentary.

Practical recommendation: Specify Type F (combined) for all new domestic bedroom socket circuit AFDDs. The additional cost over Type A is typically £10–20; the additional protection is significant. For circuits with known dimmer or motor loads, check the specific AFDD brand's compatibility list (see afdd nuisance tripping).

Consumer Unit Installation — Position and Wiring

AFDDs are installed in the consumer unit on the DIN rail, connected in the same position as an MCB or RCBO would be:

DIN rail space:

Neutral routing: Unlike a standard MCB (which connects only to the live busbar), an AFDD requires individual neutral connection. This means the neutral routing must be planned carefully — each AFDD-protected circuit has its own neutral cable running from the circuit wiring back to the AFDD's neutral terminal, then to the neutral bar. This is identical to RCBO wiring.

Busbar compatibility: AFDDs are typically DIN rail-mounted and connect to the consumer unit busbar via standard MCB connection. Verify the AFDD brand is compatible with the consumer unit busbar (Hager AFDDs work best in Hager consumer units; Eaton in Eaton units; MK in MK units). Cross-brand compatibility exists but confirm with manufacturers.

Coordinating AFDDs with SPDs

When both an SPD (see spd installation) and AFDDs are installed in the same consumer unit:

  1. Position the SPD upstream of (before) all AFDDs — the SPD should see the surge before it reaches the AFDD
  2. Check SPD-AFDD compatibility: some SPD brands can interact with AFDD detection algorithms. Consult both SPD and AFDD manufacturer's documentation for known incompatibilities.
  3. Combined SPD+AFDD devices: available from Eaton (AFDD-B-SPD), Hager, and others — these integrate Type 2 SPD and AFDD in a single 3-module device. This is the cleanest solution where both are required.

Testing an AFDD

Test button (arc fault simulation):

RCD element test (integral RCD):

IR test of protected circuit:

Commissioning — What to Document

On a new installation with AFDDs, the Electrical Installation Certificate (EIC) should record:

An NICEIC or NAPIT registered electrician must issue the EIC. Part P notification is required for new consumer unit installation or new final circuit installation.

Frequently Asked Questions

Do I need AFDDs on lighting circuits?

No. Regulation 421.1.7 specifies socket outlet circuits only. Lighting circuits are not included in the scope of the regulation. However, lighting circuits with socket outlets incorporated (e.g., a pendant with an integral socket) should be considered in scope for that element.

My client wants to save money — can I skip AFDDs on bedroom circuits in a new build?

It depends on the building. If it's a Higher Risk Residential Building, HMO, purpose-built student accommodation, or care home, no — Regulation 421.1.7 says 'shall be installed' for those categories, and building control or your NICEIC/NAPIT certification body will flag an omission as a deficiency. For an ordinary new-build house or low-rise flat outside those categories, AFDDs are a recommendation rather than a mandatory requirement, so it is technically possible to omit them — but the practical cost of AFDDs on a 2-bedroom flat (2–4 AFDD circuits) is £200–600 for devices, a small proportion of overall electrical installation cost, and the protection provided is genuine and material. Most contractors fit them as standard regardless of whether the regulation mandates it for that specific building.

Can I install a Type F AFDD on a circuit with an existing dimmer switch?

Possibly — check the AFDD manufacturer's compatibility list first. If the dimmer is a trailing-edge (electronic) type from a compatible brand, it will likely work without nuisance tripping. Leading-edge (TRIAC) dimmers are more problematic. If the client insists on a specific dimmer brand not on the compatibility list, the alternative is to put the dimmer on a dedicated lighting circuit (lighting circuits are not required to have AFDD) and supply the sockets from the AFDD-protected ring.

Regulations & Standards