An AFDD (arc fault detection device) is a protective device to BS EN 62606 that detects the signature of a series or parallel arc and disconnects the circuit before the arc starts a fire. Regulation 421.1.7 of BS 7671:2018, as amended by Amendment 2 in 2022 and carried into BS 7671:2018+A4:2026, requires AFDDs on single-phase a.c. final circuits supplying socket-outlets up to 32 A in four types of premises: higher risk residential buildings over 18 m, houses in multiple occupation (HMOs), purpose-built student accommodation and care homes. Everywhere else they are recommended. AFDDs are tested with their own test button, not an instrument, and are recorded in the AFDD column of the schedule.
- Required: socket circuits up to 32 A in HMOs, purpose-built student accommodation, care homes and residential buildings over 18 m. Recommended: the same circuits in all other premises.
- The device standard is BS EN 62606. Most UK products are combined AFDD/RCBO units for a single circuit.
- Testing is by the integral test button; there is no MFT arc test. The RCD part of a combined unit is tested normally.
- On an EICR, absence where 421.1.7 now requires them is generally a C3; absence in a normal house is not a coded observation.
- Nuisance tripping on a modern AFDD usually points to a real problem on the circuit.
What is an AFDD and what does it protect against?
An arc fault detection device is an electronic protective device that analyses the high-frequency noise superimposed on the load current, recognises the pattern produced by a sustained electrical arc, and opens the circuit. It fills a gap left by MCBs and RCDs. An MCB trips on overcurrent, an RCD trips on current leaking to earth, but a series arc, at a loose terminal, a damaged conductor or a crushed flex, draws less current than the load itself and leaks nothing to earth. It simply heats the surrounding material until it ignites.
Electrical Safety First has long identified faulty wiring and connections as a leading cause of accidental fires in dwellings, which is why BS 7671 introduced AFDDs as a recommendation in the 18th Edition in 2018 and made them a requirement for specific higher-risk premises in Amendment 2. The glossary entry on AFDDs covers the device itself; this guide is about where the regulation applies and how the devices are dealt with on certificates.
Where does Regulation 421.1.7 require AFDDs?
The regulation applies to single-phase a.c. final circuits supplying socket-outlets with a rated current not exceeding 32 A. It does not apply to lighting, to fixed appliance circuits such as cookers and showers, or to three-phase circuits, though a designer can extend AFDD protection to those if they choose.
| Premises | Status under 421.1.7 | Notes |
|---|---|---|
| Higher risk residential buildings (over 18 m in height) | Required | Applies to socket circuits in individual dwellings and common parts |
| Houses in multiple occupation (HMOs) | Required | HMO as defined in the Housing Act 2004; includes many shared houses and bedsits |
| Purpose-built student accommodation | Required | Cluster flats, studios and common kitchens |
| Care homes | Required | Residential and nursing care premises |
| All other premises | Recommended | Includes single-family houses, flats below 18 m, offices, shops and workshops |
Since Amendment 2 the regulation simply recommends AFDDs for all other premises; the 2018 edition's list of examples (sleeping accommodation, fire risk from stored materials, combustible construction such as timber frame, fire-propagating structures, irreplaceable goods) remains a sensible designer's cue rather than a requirement, and a decision against AFDDs in a timber-frame house is worth recording.
The requirement applies to new installations and to new circuits, additions and alterations designed to the current edition. It is not retrospective: an existing HMO wired to the 17th Edition does not become non-compliant, which is the basis for the EICR coding below. HMO landlords already have duties under the EICR landlord regulations, and licensing officers increasingly ask about AFDDs, so the required-versus-recommended distinction matters.
What does BS EN 62606 cover?
BS EN 62606 is the product standard for AFDDs. It sets the arc signatures the device must detect, the times within which it must disconnect for given arc currents, and the conditions under which it must not trip, which is the part that limits nuisance operation. The standard requires an integral test facility, which is why every AFDD has a test button, and it covers devices for use up to 63 A on single-phase supplies.
In the UK almost all AFDDs sold are combined units providing arc detection, residual current and overcurrent protection in one module; a few manufacturers offer a stand-alone AFDD that sits upstream of a conventional MCB. Consumer units with the non-combustible enclosure required by Regulation 421.1.201 accept them, though AFDD/RCBO modules are often wider than a standard RCBO and the board may need more ways than a like-for-like count suggests.
How are AFDDs tested and recorded?
There is no instrument test for arc detection. A multifunction tester cannot safely generate a real arc, and BS EN 62606 does not define a field test current. Verification of the AFDD function during initial verification and periodic inspection is therefore by the manufacturer's test button, which injects a simulated arc signature into the detection circuit and should trip the device. Press it, confirm the device trips, reset, and record the result.
Where the AFDD is combined with an RCBO, the residual current part is tested exactly as any other RCD: half IΔn no trip, IΔn within 300 ms, five times IΔn within 40 ms for a 30 mA device providing additional protection. The RCD testing guide covers the method. Test times are recorded in the RCD columns of the schedule of test results as normal.
On the schedule of circuit details, the current BS 7671 model forms in Appendix 6 include an AFDD column alongside the overcurrent device and RCD columns: enter the standard (BS EN 62606) and rating against the circuit. On the schedule of test results the functional test result is a tick in the AFDD test button column. Where a board mixes AFDD/RCBOs on socket circuits and plain RCBOs elsewhere, the column entries make that clear to the next inspector.
How is the absence of an AFDD coded on an EICR?
This is the question electricians ask most, and the honest answer is that the codes depend on the premises and that GN3 and Electrical Safety First Best Practice Guide 4 leave room for judgement.
| Situation | Usual approach | Reasoning |
|---|---|---|
| Ordinary house or flat, no AFDDs | No code; optional note in the observations that AFDDs are recommended by 421.1.7 | AFDDs are only recommended; the installation is not deficient |
| HMO, student accommodation, care home or high-rise, installation predates A2 (2022), no AFDDs | C3 | Not compliant with the current edition for these premises, but not dangerous by itself; improvement recommended |
| Same premises, new circuits or a new consumer unit installed after A2 without AFDDs where required | C3, with a note that the recent work should have included them; consider whether the EIC for that work was correct | Non-compliance with the edition in force at the time of the work |
| AFDD fitted but fails its test button | C2 | A protective device that has failed is potentially dangerous; replace |
| AFDD fitted on a circuit but bypassed or with its output linked out | C2 | Deliberate defeat of a protective device |
The reasoning follows the general rule for EICR codes: non-compliance with a requirement introduced after the installation was built is not a C2 unless it creates danger. A missing AFDD removes a layer of fire protection a new installation would have; it does not create an immediate hazard. C3 fits the premises where the regulation now applies, and no code, or an informative note, fits where it does not. Do not leave an HMO landlord with an Unsatisfactory report on missing AFDDs alone; do tell them what the current standard requires so they can plan the upgrade with the next consumer unit change.
What causes AFDD nuisance tripping?
Arcs occur in normal operation: motor brushes arc continuously, a plug withdrawn under load arcs briefly, and some dimmers and switched-mode power supplies produce high-frequency noise. BS EN 62606 requires the device to tolerate these, and current products do, but three situations are worth knowing.
- Genuine faults that look like nuisance. A loose neutral in a socket, a chafed flex behind a washing machine or a scorched terminal in a fused spur produce exactly the signature the device is looking for. Inspect the terminations before condemning the device.
- Long circuits with a lot of connected electronics. Summed high-frequency noise can approach the detection threshold; splitting the circuit usually resolves it.
- Early or non-compliant devices. Imported devices without genuine BS EN 62606 certification may trip on ordinary loads. Check the marking.
Explain to the client that an AFDD tripping is information, as an RCD tripping is, and record any investigation in the report.
What should the designer consider on new work?
For a new consumer unit in an HMO, student block, care home or high-rise, the socket circuits up to 32 A must have AFDD protection and the EIC should show it. Elsewhere the decision is the designer's, and the certificate documents it. Count board ways for double-width modules; remember an AFDD does nothing for an arc upstream of it, so the tails and board terminations still rely on workmanship; and state on the EIC whether AFDDs are provided and, where not, that the client was advised. If the work is notifiable under Part P, the EIC and the notification should agree on the devices installed.
How Certio helps
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