An AFDD (arc fault detection device) to BS EN 62606 recognises the electrical 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+A4:2026 requires AFDDs on single-phase a.c. final circuits supplying socket-outlets rated up to 32 A in four premises types: high rise residential buildings (over 18 m or more than six storeys), houses in multiple occupation, purpose-built student accommodation and care homes. Everywhere else they are recommended. Amendment 4, published 15 April 2026, kept the scope and reworded the building definition; the previous edition is withdrawn on 15 October 2026. AFDDs are tested with their own test button, not a portable instrument, and recorded in the AFDD column of the schedule.
- Required: socket-outlet final circuits up to 32 A in HMOs, purpose-built student accommodation, care homes and high rise residential buildings. Recommended: the same circuits in every other premises.
- A4:2026 did not change the scope. "Higher risk residential building" became "high rise residential building", defined as over 18 m or more than six storeys, whichever is reached first.
- The device standard is BS EN 62606, which covers devices up to 63 A single-phase. Most UK products combine arc detection with an RCBO in one module.
- There is no instrument test for the arc function; the integral test button is the verification. The RCD part is tested at IΔn as normal.
- On an EICR, no AFDDs where 421.1.7 now requires them is a C3 (Electrical Safety First Best Practice Guide 4); in an ordinary house it is not a coded observation.
- The regulation is not retrospective, but a new socket circuit in one of the four premises types must have one, and the EIC should say so.
- Nuisance tripping on a compliant device usually points to a real fault on the circuit.
What is an AFDD and what gap does it fill?
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 arc, and opens the circuit. It covers a fault that neither an MCB nor an RCD can see. A series arc at a loose terminal, a crushed flex or a nail through a cable draws less current than the load itself and leaks nothing to earth, so both conventional devices stay closed while the arc heats whatever is next to it until it ignites.
| Device | Detects | Blind to |
|---|---|---|
| MCB or fuse | Overload and short-circuit current | Series arcs, earth leakage below the trip current |
| RCD or RCBO | Current leaking to earth (IΔn) | Series arcs, line-to-neutral parallel arcs with no earth path |
| AFDD | High-frequency arc signature, series and parallel | Nothing an MCB or RCD catches that the combined unit doesn't also catch |
Government fire statistics for England consistently put faulty electrics and appliances among the largest causes of accidental dwelling fires, which is why the 18th Edition introduced AFDDs as a recommendation in 2018 and Amendment 2 turned that into a requirement for the higher-risk premises in 2022. The glossary entry at What is an AFDD? Arc fault detection devices explained covers the device itself; this guide is about where the regulation bites 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, showers and immersion heaters, or to three-phase circuits, and a fused spur feeding fixed equipment is not a socket-outlet. A designer can extend AFDD protection to any of those if they choose.
| Premises | Status under 421.1.7 | Notes |
|---|---|---|
| High rise residential buildings | Required | Over 18 m or more than six storeys, whichever comes first; socket circuits in the dwellings and the common parts |
| Houses in multiple occupation (HMOs) | Required | HMO as defined in the Housing Act 2004; includes most shared houses and bedsits |
| Purpose-built student accommodation | Required | Cluster flats, studios and shared kitchens |
| Care homes | Required | Residential and nursing care premises |
| All other premises | Recommended | Single-family houses, flats in lower blocks, offices, shops and workshops |
The 2018 edition listed examples where AFDDs were worth considering, such as sleeping accommodation, premises with a fire risk from stored materials, combustible construction like timber frame, fire-propagating structures and premises holding irreplaceable goods. Since Amendment 2 those examples no longer appear in the regulation, but they remain a sensible designer's cue. A decision against AFDDs in a timber-frame house is worth recording on the certificate.
The requirement applies to new installations and to new circuits 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 carry duties under the Landlord EICR Regulations in England: The Electrician's Guide, and licensing officers increasingly ask about AFDDs, so the required-versus-recommended distinction matters when you write the report. See EICR for HMOs: Rules, Scope and Common Findings in England for the wider HMO picture.
What changed in Amendment 4:2026?
Less than the marketing suggests. NICEIC's summary of A4 confirms that the AFDD requirement was not removed or widened: the term "higher risk residential building" was reworded to "high rise residential building (HRRB)", and the definition is a residential building over 18 m in height or in excess of six storeys, whichever is reached first. That aligns BS 7671 with the Building Safety Act's threshold, and it matters on the ground: a seven-storey block below 18 m is in scope.
| Point | BS 7671:2018 (2018) | A2:2022 and A3:2024 | A4:2026 |
|---|---|---|---|
| AFDDs in the four premises types | Recommended | Required | Required |
| AFDDs everywhere else | Recommended for listed examples | Recommended | Recommended |
| Building term | Not defined | Higher risk residential building | High rise residential building, over 18 m or more than six storeys |
| Circuit scope | Socket circuits | Single-phase a.c. socket-outlet final circuits ≤ 32 A | Unchanged |
| Status | Superseded | Withdrawn 15 October 2026 | Published 15 April 2026; current |
Work designed after 15 October 2026 should be certified against A4. Certificates and reports issued in the transition period should state which edition the installation was assessed against, as the model forms already require.
What does BS EN 62606 cover?
BS EN 62606 is the product standard. It sets the arc signatures the device must detect, the disconnection times for given arc currents, and, just as important, 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 not exceeding 240 V.
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 an AFDD that sits upstream of a conventional MCB. Consumer units with the non-combustible enclosure required by Regulation 421.1.201 accept them, but many AFDD/RCBO modules are wider than a standard RCBO and a board can need more ways than a like-for-like count suggests. Count the modules before you order the enclosure; Consumer Unit Replacement: Which Certificate and What to Check covers the rest of a board change, and RCBO vs RCD Split-Load Boards: Which to Fit and Why explains why an all-RCBO board is the natural home for them.
Check the marking. A compliant device carries BS EN 62606 (or EN 62606 / IEC 62606), the manufacturer, the rated current and, for a combined unit, the RCD type and IΔn. Imported modules with no traceable certification are the source of most complaints about AFDDs tripping on ordinary loads.
How are AFDDs tested and recorded?
There is no instrument test for arc detection. No portable instrument can safely generate a real arc on a live circuit, and BS EN 62606 does not define a field test current. Verification of the arc-detection function at initial verification and on periodic inspection is therefore by the manufacturer's test button, which injects a simulated arc signature into the detection electronics and should trip the device. Press it, confirm the device trips, reset, and record the result. IET Guidance Note 3 treats this as a functional test, the same category as an RCD test button.
| Function of a combined AFDD/RCBO | How it is verified | Where it is recorded |
|---|---|---|
| Arc detection | Integral test button; device must trip | AFDD test button operation column on the schedule of test results |
| Residual current | Test at IΔn with the instrument (Regulation 643.8); trip within 300 ms for a general device; test button also pressed | RCD disconnection time and RCD test button columns |
| Overcurrent | Confirm rating and type suit the circuit and Zs is within the maximum (Regulation 411.4.4) | Overcurrent device columns; Zs column |
Since Amendment 2, BS 7671 requires the RCD test at rated residual current only; the ½IΔn and 5×IΔn tests are no longer required by the standard, although many inspectors still run 5×IΔn on 30 mA devices providing additional protection and it does no harm. The method is in RCD Testing Explained: Types, Test Currents and Trip Times.
On the schedule of circuit details, the current model forms in Appendix 6 include AFDD columns alongside the overcurrent device and RCD columns: enter the standard (BS EN 62606) and the rating against each circuit that has one. Where a board mixes AFDD/RCBOs on socket circuits and plain RCBOs elsewhere, the column entries make that clear to the next inspector. How to Fill In a Schedule of Test Results, Column by Column walks through the whole form column by column.
How is the absence of an AFDD coded on an EICR?
This is the question electricians ask most, and the answer depends on the premises. Electrical Safety First Best Practice Guide 4 (Issue 7) lists the absence of AFDDs where Regulation 421.1.7 now requires them as C3, and that has become the settled position across NICEIC and NAPIT guidance.
| Situation | Usual code | Reasoning |
|---|---|---|
| Ordinary house or flat, no AFDDs | No code; optional note that 421.1.7 recommends them | AFDDs are only recommended; the installation is not deficient |
| HMO, student accommodation, care home or high rise building wired before A2 (2022), no AFDDs | C3 | Not compliant with the current edition for these premises, not dangerous by itself; improvement recommended |
| Same premises, new socket circuits or a new consumer unit installed after A2 without AFDDs | C3, with a note that the recent work should have included them | Non-compliance with the edition in force at the time of the work; the EIC for that work was wrong |
| AFDD fitted but does not trip on its test button | C2 | A protective device that has failed is potentially dangerous; replace it |
| AFDD bypassed, linked out or replaced with a plain MCB on a circuit that required it | C2 | Deliberate loss of a required protective measure |
| AFDD fitted, trips on test, but on the wrong circuit type (for example lighting only) | No code for the device; C3 for the socket circuits it should be on | The socket circuits still lack the required protection |
The reasoning follows the general rule in EICR Observation Codes Explained: C1, C2, C3 and FI: 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. Do not leave an HMO landlord with an Unsatisfactory report on missing AFDDs alone. Do tell them, in the observation wording, what the current standard requires, so the upgrade can be planned with the next board change. How to Write EICR Observations That Stand Up has wording that reads well to a landlord and a licensing officer.
Worked example. A four-bedroom licensed HMO, board changed in 2019 to a dual-RCD split-load unit, no AFDDs. Socket circuits are on 30 mA RCDs, Zs values are fine, no other defects. The report is Satisfactory, with one C3: "No arc fault detection devices provided for socket-outlet final circuits. Regulation 421.1.7 of BS 7671:2018+A4:2026 requires AFDDs for these circuits in a house in multiple occupation. Recommend AFDD protection is provided at the next consumer unit alteration."
Does adding a socket in an HMO need an AFDD?
The IET's own forum has chewed over this one at length, and the practical position is this. A new socket-outlet final circuit up to 32 A in one of the four premises types must have an AFDD, because a new circuit is designed to the current edition and the EIC should show it. Adding a socket to an existing circuit is an alteration: BS 7671 is not retrospective, the regulation applies to circuits rather than to individual accessories, and the addition does not by itself oblige you to fit an AFDD. What it does oblige you to do is confirm the existing circuit is adequate for the addition and tell the client, on the Minor Works certificate and in writing, that the circuit lacks the protection the current edition requires. A fused spur feeding fixed equipment is outside the regulation altogether because it is not a socket-outlet. See Minor Works Certificate: When to Use It (and When Not To) for what belongs on the certificate.
In a care home or student block the duty holder, not the electrician, owns the fire-risk assessment. Putting the AFDD point in writing to them is what protects you.
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.
| Symptom | Likely cause | What to do |
|---|---|---|
| Trips when one appliance runs | Chafed flex, damaged plug, arcing internal connection in the appliance | Inspect and test the appliance; it is the appliance, not the AFDD |
| Trips at random on a long circuit with a lot of electronics | Summed high-frequency noise near the detection threshold | Split the circuit; check every termination on the way round |
| Trips on any load | Loose neutral at a socket or spur, scorched terminal, or an uncertified device | Inspect terminations first; check the BS EN 62606 marking |
| Trips only when the RCD button is pressed | You are testing the RCD function, which is working | Nothing; record the RCD result |
Explain to the client that an AFDD tripping is information, as an RCD tripping is, and record what you found in the report. A device that has caught a scorched socket terminal has done exactly what it was fitted for.
What should the designer consider on new work?
For a new consumer unit in an HMO, student block, care home or high rise building, the socket circuits up to 32 A must have AFDD protection and the EIC should show it against each circuit. 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, main switch and busbar terminations still rely on workmanship and a torque driver. Where you fit a surge protective device under Regulation 443, the AFDDs and the SPD share the same board and the same Type of RCD questions; Surge Protection (SPD) Requirements: Section 443 Explained covers that. 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; see Part P notification: what to notify, how, and by when.
Sources: BS 7671:2018+A4:2026 (Regulations 421.1.7, 421.1.201, 443, 643.8, Appendix 6); BS EN 62606; IET Guidance Note 3 (Inspection and Testing); NICEIC's Amendment 4 summary; Electrical Safety First Best Practice Guide 4, Issue 7; GOV.UK fire statistics for England.
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