AFDDs Explained
Arc fault protection,
to BS 7671 Reg 421.1.7.
AFDDs conforming to BS EN 62606 detect the tiny, sustained sparks that MCBs and RCDs miss — cutting power before an electrical fire can start. Here's exactly what BS 7671:2018+A4:2026 requires, where they're mandatory and where they're recommended.
What arc faults do
The fires you cannot see.
An arc fault is a small, sustained electrical spark caused by damaged cable, a loose terminal, a crushed flex or a faulty appliance. Traditional protective devices — MCBs, RCDs, SPDs — deal with overcurrent, earth leakage and overvoltage, but none of them can identify an arc. AFDDs use microprocessors to analyse the current waveform and recognise the unique signature, duration and irregularity of a dangerous arc, while ignoring the harmless arcs produced by everyday switching, motors and appliances.
- Detects both series and parallel arc faults
- Protects fixed wiring and plugged-in equipment (including extension leads)
- Differentiates real arc faults from normal switching arcs
- Complements RCDs, MCBs/RCBOs and SPDs — does not replace them
Common causes of arc faults
- Frayed or rodent-damaged cable
- Loose screw terminals in sockets, joints or accessories
- Trapped or crushed appliance flexes
- DIY alterations leaving damaged conductors
- Old rubber or cloth cabling breaking down
- Cables outside prescribed zones or with tight bend radii
BS 7671:2018+A4:2026 — Regulation 421.1.7
Where AFDDs are mandatory.
Reg 421.1.7 mandates AFDDs conforming to BS EN 62606 on single-phase AC final circuits supplying socket-outlets with a rating not exceeding 32A, in four specific installation types. For all other premises, AFDDs on the same type of circuit are strongly recommended.
Higher Risk Residential Buildings (HRRB)
Residential buildings over 18m in height or more than six storeys — whichever is reached first (Reg 421.1.7 Note 1).
Houses in Multiple Occupation (HMO)
Shared houses and flats let to multiple unrelated occupants — AFDDs mandatory on new socket-outlet circuits up to 32A.
Purpose-built student accommodation
Halls of residence and similar student housing — mandatory arc-fault protection on socket circuits up to 32A.
Care homes
Residential care premises where evacuation is more complex and occupants may be vulnerable to fire.
Recommended everywhere else
For all other premises — including standard domestic homes — BS 7671 recommends AFDDs for single-phase AC circuits supplying socket-outlets up to 32A. In BS 7671 language, a "recommendation" is one of several options that is particularly suitable — the implication being that you should do it unless there's a good reason not to.
Type of work
When the AFDD requirement kicks in.
In the four listed installation types, any work involving new socket-outlets requires AFDDs at the origin of the circuit.
New builds
All new socket-outlet circuits in listed premises require AFDDs at the origin.
Rewires
Full or partial rewires that introduce new socket-outlet circuits must include AFDD protection.
Additions & alterations
Extending a socket-outlet circuit in a listed premise triggers the requirement.
Consumer unit replacements
A CU change in a listed premise is the ideal (and required) point to add AFDDs.
Maintenance exception
A like-for-like replacement of a damaged socket-outlet for maintenance purposes may be carried out without adding an AFDD. This exception exists so maintenance stays affordable and gets done — but it only covers a straight swap of the accessory with no other alterations to the circuit.
Device options
AFDD, AFDD+RCBO, or combined protection.
Electrical fires can be caused by faults that need more than one protective device. Modern consumer units let you combine functions in a single module — saving space and simplifying the wiring.
Standalone AFDD
A dedicated arc-fault detection module, paired with a separate MCB/RCBO in the consumer unit. Useful when retrofitting AFDDs into an existing compliant board.
AFDD + RCBO combined
One module providing arc-fault, overload/short-circuit and earth-leakage protection. The most common and cost-effective way to meet Reg 421.1.7.
SPD + AFDD + RCBO
Full protection stack: arc faults, earth leakage, overcurrent and surge protection all built into a single compliant consumer-unit layout.
Industry guidance
BS 7671 and BS EN 62606.
AFDD requirements are set out in BS 7671:2018+A4:2026, Chapter 42 and specifically Regulation 421.1.7. The devices themselves must conform to BS EN 62606. Alongside fitting an AFDD, correct cable installation in prescribed zones, proper bend radii, correct terminal tightness and periodic EICR inspection all remain essential to minimising the risk of arc faults in the first place.
Reg 421.1.7 at a glance
BS 7671:2018+A4:2026
- Applies to single-phase AC final circuits supplying socket-outlets up to 32A.
- AFDDs must conform to BS EN 62606.
- Mandatory in HRRBs, HMOs, purpose-built student accommodation and care homes.
- Recommended for all other premises on the same circuit type.
- Must be installed at the origin of the circuit (with a specific busbar/powertrack exception — Note 2).
- Detects both series and parallel arc faults; complements MCBs, RCDs and SPDs.
FAQ
Common AFDD questions.
What is an AFDD?
An AFDD (Arc Fault Detection Device) conforming to BS EN 62606 uses microprocessor technology to analyse the waveform of the current and identify the signature of a dangerous arc fault — the sort of sparking inside walls, sockets or flexes that standard MCBs and RCDs can't see. When an arc fault is detected the device disconnects the circuit before it can ignite surrounding material.
Where does BS 7671 mandate AFDDs?
BS 7671:2018+A4:2026 Regulation 421.1.7 requires AFDDs to BS EN 62606 on single-phase AC final circuits supplying socket-outlets with a rating not exceeding 32A in four installation types: Higher Risk Residential Buildings (HRRB), Houses in Multiple Occupation (HMO), purpose-built student accommodation, and care homes. For all other premises, AFDDs are recommended for the same type of circuit.
What is a Higher Risk Residential Building (HRRB)?
Note 1 of Regulation 421.1.7 defines an HRRB as, in principle, a residential building over 18 metres in height or more than six storeys (whichever is reached first). These buildings are treated as higher-risk because of the complexity of evacuation and the range of occupants who may need to escape.
What type of work triggers the AFDD requirement?
Any work in a listed installation type that involves new socket-outlets — new builds, rewires, additions and alterations to socket-outlet circuits, and consumer unit replacements. A like-for-like replacement of a damaged socket-outlet for maintenance does not, on its own, require the addition of an AFDD.
Can an AFDD replace an RCD or circuit breaker?
No — they protect against different faults and work together. Overcurrent (short-circuit and overload) is handled by MCBs/fuses, earth leakage by RCDs/RCBOs, overvoltage by SPDs, and dangerous electric arcs by AFDDs. Modern combined RCBO+AFDD modules cover all four protection functions in one device.
Do AFDDs work on ring final circuits?
Yes — they protect against parallel arc faults in the fixed wiring and both series and parallel arcs in the equipment plugged into the ring. The one fault an AFDD cannot detect on a ring is a series fault, because current flows around the other leg — but a break in continuity there is unlikely to develop into a dangerous arc.
Where must the AFDD be installed?
Regulation 421.1.7 requires the AFDD to be installed at the origin of the circuit it is protecting — typically in the consumer unit or distribution board. There is a specific exception for busbar systems to BS EN 61439-6 and Powertrack systems to BS EN 61534 (see Note 2 of Reg 421.1.7).
Upgrade your protection
Fuse board upgrade with AFDDs.
Whether you need a full consumer-unit replacement or advice on where Reg 421.1.7 applies to your property, Paul will design a BS 7671-compliant layout and quote it straight.