Amendment 4 redrafted Regulation 421.1.7 so that AFDDs (Arc Fault Detection Devices) conforming to BS EN 62606 are now mandatory — not just recommended — in four premises types, and added a dedicated AFDD line to the redrafted Condition Report inspection schedule (item 4.23). This guide maps every A4 AFDD touch-point: where required, where recommended, where prohibited, and how to record it.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
A4:2026 redrafted Regulation 421.1.7 so AFDDs (to BS EN 62606) are now mandatory on single-phase AC final circuits supplying socket-outlets up to 32 A in high-rise residential buildings, HMOs, purpose-built student accommodation and care homes. For all other premises they are recommended. A4 also added EICR inspection item 4.23 to confirm AFDD operation.
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An Arc Fault Detection Device (AFDD) is a protective device that detects the characteristic high-frequency current signatures of arcing faults — both series arcs (loose terminals, broken conductors) and parallel arcs (cable damage, line-to-line or line-to-neutral arcing). Conventional MCBs trip on overload or short-circuit; RCDs trip on residual current. Neither reliably detects the low-current, intermittent arcs that cause most electrical fires.
BS 7671 has recognised AFDD as a protective device for some years, but A4:2026 went further: it redrafted Regulation 421.1.7 to make AFDDs a requirement (not just a recommendation) in named premises, and it added a dedicated AFDD line to the redrafted Condition Report inspection schedule — item 4.23, "Confirmation of indication that AFDD(s) are operational". So every A4-compliant EICR now explicitly records whether AFDD operation has been confirmed, and flags where AFDDs should have been fitted but were not.
A series arc forms in line with the load — a loose terminal, a broken strand, a damaged flex — and the current stays at or below the load current, so overcurrent protection never operates. A parallel arc forms between conductors (line-to-line or line-to-neutral), often from crushed or nailed cable. An AFDD is designed to recognise both signatures.
Every key change in the 2026 amendment on one page. AFDDs, TN-C-S protection, new schedule columns, model forms. Pinned on your van dash.
Regulation 421.1.7 of BS 7671:2018+A4:2026 is the controlling rule. The redrafted wording reads that AFDDs conforming to BS EN 62606 "shall be provided" for single-phase AC final circuits supplying socket-outlets with a rated current not exceeding 32 A in four specific premises types. "Shall" makes this a requirement, not a recommendation, in those premises.
The requirement is scope-limited: it applies to single-phase AC final circuits supplying socket-outlets rated up to 32 A. Where AFDDs are used, Regulation 421.1.7 requires them to be placed at the origin of the circuit being protected (for busbar systems to BS EN 61439-6 and powertrack to BS EN 61534, the AFDD may sit elsewhere). Fitting an AFDD does not remove the need for the other protective measures in BS 7671.
For all premises other than the four named above, Regulation 421.1.7 recommends AFDDs for single-phase AC final circuits supplying socket-outlets up to 32 A — it does not mandate them. On an EICR, a missing AFDD where 421.1.7 requires one (e.g. an HMO socket circuit) is typically a C2; where AFDDs are only recommended, their absence is usually an improvement recommendation (C3). See EICR Code C2 — Potentially Dangerous for how these are coded.
Outside the standard installations covered by 421.1.7, specific Part 7 locations have their own AFDD rules — for example, Regulation 710.421.1.201 sets installation requirements for AFDDs within medical locations (Section 710), and the EV charging rules in Section 722 (below) provide a conditional exemption.
A4:2026 strengthened the medical-location prohibitions. AFDDs are explicitly prohibited in:
When an AFDD is discovered on a medical IT system circuit during EICR, the responsible person or installer shall remove or disable the AFDD to comply with Regulation 710.421.1.7.101. The change is recorded on the circuit documentation and the client informed.
Regulation 722.421.1.7.201 states that AFDDs are not required for circuits supplying EV charging equipment conforming to the BS EN 61851 series that incorporate socket-outlets or vehicle connectors conforming to BS EN IEC 62196-2. The exemption reflects the fault-detection capability built into compliant charge-point electronics.
For the wider EV charging picture — RCD types, PEN-fault protection and the rest of Section 722 — see the BS 7671 Amendment 4 (2026) overview.
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On the A4:2026 Condition Report inspection schedule, item 4.23 reads "Confirmation of indication that AFDD(s) are operational" and cross-references Regulations 421.1.7, 532.6 and 651.2(e). The inspector confirms, across the inspected installation, that fitted AFDDs are operational — and identifies where AFDDs are required but absent. Item 4.23 is typically completed as one of:
Item 4.23 ties three regulations together: 421.1.7 (the requirement to provide AFDDs), 532.6 (the selection and erection rules for AFDDs — installed at the origin of the final circuit, in AC single-phase circuits not exceeding 230 V, conforming to BS EN 62606) and 651.2 (periodic inspection). The inspection item is the point where all three are checked on a Condition Report.
A4:2026 redrafted the model forms so that the single-page generic schedule of test results was split into a separate schedule of circuit details and a separate schedule of test results. AFDD information is captured per circuit alongside the rest of the protective-device data. Practically, that means recording, circuit by circuit:
A4 standardised the data to be captured, but the exact box layout and any internal numbering vary between certificate templates and software. The key is that the schedule of test results carries the per-circuit AFDD status and that it agrees with item 4.23 on the inspection schedule.
Functional testing of AFDDs falls under the functional-testing requirements of Part 6 (Regulation 643.10): where an AFDD is installed, the effectiveness of any manually operated test facility shall be verified in accordance with the manufacturer's recommendations. This functional check does not replace the type-test carried out to the device's product standard. The On-Site Guide also advises that an AFDD with a manual test facility should be operated six-monthly by the user.
Where an installation includes an AFDD with a manual test facility, the user instructions should tell the occupier to test it six-monthly by pressing the test button — and to seek expert advice if the device does not disconnect. It is good practice to leave a clear AFDD test notice with the documentation at handover.
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Elec-Mate's digital EICR ships the A4:2026 model form: item 4.23 AFDD inspection, the redrafted schedule of test results, and per-circuit AFDD status.
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