What Is an AFDD? Arc Fault Detection Devices Explained for 2026

An AFDD (arc fault detection device) is a protective device to BS EN 62606 that watches the current waveform on a final circuit for the signature of a dangerous arc, from a loose terminal, a crushed cable or a frayed flex, and disconnects the circuit before the arc can set fire to anything around it. BS 7671 Regulation 421.1.7, brought in by Amendment 2 in 2022, makes AFDDs mandatory on single-phase socket-outlet circuits up to 32 A in high rise residential buildings, HMOs, purpose-built student accommodation and care homes, and recommends them everywhere else. The product standard requires the device to trip within 1 s on a 2.5 A arc and within 0.12 s on a 32 A arc.

How does an AFDD actually detect an arc?

A series arc happens when a conductor is broken or a termination is loose and current has to jump the gap: a cracked ring final conductor behind a socket, a screw that was never tightened, a flex that has been trapped under a chair leg for five years. Because the arc is in series with the load, the current can never exceed what the load draws. A 2 kW heater on a 32 A circuit arcing at a loose terminal pulls under 9 A, so the MCB sees nothing wrong, and because the current goes out on L and comes back on N, the RCD sees nothing either. The arc meanwhile runs at a few thousand degrees, and that is how a fire starts behind a skirting board.

A parallel arc is between L and N, or L and earth, through damaged insulation. Those usually have enough current to operate the overcurrent device eventually, but the BEAMA guide makes the point that a high-impedance arc through carbonised insulation can sit well below an MCB's magnetic trip while still being hot enough to ignite dust or timber.

The AFDD has a microprocessor sampling the current waveform many thousands of times a second. It is looking for the ragged, high-frequency pattern of a sustained arc, with its randomly varying half-cycles and the characteristic "shoulder" where the current fails to restrike at each zero crossing, and it is trained to ignore the clean, short arcs of a brushed drill motor, a switched-mode power supply, a dimmer or a fluorescent fitting. BS EN 62606 Table 1 sets the maximum break times: 1 s at a 2.5 A test arc, 0.5 s at 5 A, 0.25 s at 10 A, 0.15 s at 16 A and 0.12 s at 32 A and 63 A. The 2.5 A floor is deliberate. BEAMA explains it comes from an energy threshold of about 100 J at a 40 V arc voltage, which is the point at which arc energy becomes a realistic ignition source. A series arc below 2.5 A does not have to trip the device.

The product standard also includes a battery of unwanted-tripping tests with motors, SMPS loads, dimmers and lamps, which is why the "AFDDs nuisance trip on everything" reputation from the early American AFCI days does not carry over cleanly. Where a UK AFDD keeps tripping, the EngX threads on the subject nearly always end with a real fault: a fridge compressor with a failing winding, a microwave with a damaged lead, a backbox with a loose line conductor nobody could see.

Where does BS 7671 require AFDDs?

Regulation 421.1.7 in BS 7671:2018+A4:2026 reads, in summary, that arc fault detection devices conforming to BS EN 62606 shall be provided for single-phase a.c. final circuits supplying socket-outlets with a rated current not exceeding 32 A in the four premises below, and that for all other premises their use is recommended. The source for the categories is the regulation itself; the definitions in the right-hand column are what the legislation and the IET notes point to.

Three things trip people up on the table. First, the regulation names socket-outlet circuits, not sockets. A 6 A lighting circuit with one socket for a TV booster is still a lighting circuit and is outside the mandatory list, as several EngX contributors have pointed out, even though the fixed-wiring risk it carries is the same. Second, the rule is single-phase only: BEAMA's fact sheet notes that three-phase AFDDs exist in BS EN 62606, but 532.6 does not require them. Third, 32 A is a ceiling on the circuit's rated current, so a 20 A radial and a 32 A ring are both caught; a 40 A or 45 A socket circuit feeding a hob plate is not, because it is not a socket-outlet circuit in the first place.

Amendment 4, published 15 April 2026 and sole current edition once A2 and A3 were withdrawn on 15 October 2026, made two AFDD edits. It reworded 421.1.7(a) from "higher risk" to "high rise residential buildings", which the EDA's April 2026 summary confirms does not change where the devices are needed, and it added Regulation 722.421.1.7.201, which says AFDDs are not required for circuits supplying EV charging equipment to the BS EN 61851 series with compliant socket-outlets or vehicle connectors. That closes the argument about whether a 32 A commando socket on an EV circuit in a block of flats needed an AFDD. Our EV charger certificate guide covers the rest of Section 722.

Regulation 532.6 then carries the selection and erection detail: the device shall comply with BS EN 62606 and be placed at the origin of the circuit to be protected, with a note excepting busbar trunking to BS EN 61439-6 and powertrack to BS EN 61534. Each AFDD protects one final circuit. You cannot protect a whole board with one upstream AFDD the way you can with a 100 mA time-delayed RCD.

Is the requirement retrospective, and what about additions and CU changes?

BS 7671 is not retrospective, so an HMO wired in 2015 without AFDDs is not in breach and its landlord is not obliged to retrofit them. The regulation bites on work you design and certify after September 2022. Three cases come up every week:

A new socket circuit in one of the four premises types needs an AFDD, full stop, and it goes on the EIC. An extension to an existing socket circuit is murkier. Regulation 132.16 requires that an addition or alteration does not impair the safety of the existing installation and that the existing installation is adequate for it, which is about earthing, bonding and the rating of existing equipment rather than about retrofitting new protective measures. The IET's position on EngX has been that the new cable and new sockets you add are new work, and a strictly conservative reading says the circuit they extend should now be AFDD protected. In practice most contractors offer the AFDD/RCBO swap as a priced option on the Minor Works and record the decision in the comments on existing installation box. Our Minor Works guide covers what that certificate can and cannot carry.

A consumer unit change is the clear one. You are designing the protective devices for every circuit from its new origin, so in an HMO, PBSA, care home or high rise flat every single-phase socket circuit up to 32 A comes off the new board on an AFDD. That means a bigger enclosure (combined devices are one or two modules, but you still lose the economy of a shared RCD), a neutral to every device, and a price perhaps a few hundred pounds above a competitor's quote that ignores 421.1.7. Put the regulation number on the quote so the customer can see why. The consumer unit replacement guide has the rest of the checklist, and the EICR for HMOs guide covers the inspection side.

How do you test and record an AFDD?

There is no instrument test for the arc detection function. Neither BS EN 62606 nor BS 7671 requires one, no field instrument generates a controlled arc, and BEAMA's testing fact sheet is explicit on the point. What you do is press the test button, watch the device trip, and reset it. The standard lets manufacturers meet the self-check requirement two ways: a manual button, which the recipient should press every six months, or an automatic self-test that runs at switch-on and at least daily and only trips the device if it finds its own electronics faulty. Hager's single-module devices, for example, self-test every hour and show a failed self-test on an LED. Where a device has both, follow the manufacturer's instructions for the button.

On a combined AFDD/RCBO the residual element is tested like any RCD: no trip at ½ IΔn, trip within 300 ms at IΔn for a general 30 mA device (130 to 500 ms for an S-type), and the result goes in the RCD disconnection time column. The 5 × IΔn test has not been a BS 7671 requirement since A2:2022, and the A4 model forms have one RCD time column, so do not go hunting for a 40 ms figure. The RCD testing guide goes through 643.8 in detail.

The IET's A4:2026 model EICR, dated 15 April 2026, records all this in three places. Inspection item 4.23 is "Provision of arc fault detection device(s) (AFDD) where required" and cites 421.1.7, 532.6 and 651.2(e). Column 29 of the schedule of test results is the RCD test button and column 30 is the AFDD manual test button, with a footnote reminding you that not all AFDDs have one. The RCD columns (13 to 16 in circuit details, 28 and 29 in test results) carry the residual element. So a combined device fills the RCD columns, column 30 and item 4.23. The schedule of test results guide walks every column.

Insulation resistance is the other trap. An AFDD has electronics between L and N and, in combined units, a residual-current sensing circuit as well. BEAMA recommends disconnecting AFDDs before the 500 V d.c. test. Regulation 643.3.2 gives the alternative: where equipment likely to influence or be damaged by the test is connected, test at 250 V d.c. and still require 1 MΩ. Either way, check the manufacturer's data sheet first, because some makers void the warranty on devices tested at 500 V. The insulation resistance guide covers the 643.3 sequence.

What EICR code is a missing or faulty AFDD?

Electrical Safety First's Best Practice Guide 4 (Issue 7.4, August 2026) gives C3, improvement recommended, for the absence of AFDD protection where Regulation 421.1.7 requires it. The logic is the same as for a missing SPD under 443.4.1: the installation is not dangerous, and C2 means potentially dangerous, so a C3 is the right ceiling. That C3 only exists in the four building types. In a standard house or a single-let flat the regulation only recommends AFDDs, a recommendation not followed is not a non-compliance, and nothing goes on the report. You can mention them in the recommendations box; you should not give them a code.

A faulty AFDD is different. If the test button does not trip the device, that is a protective device failing to operate and it is coded on its merits, typically C2 where it also carries the RCD element for a socket circuit (loss of additional protection) and never left blank. Repeated tripping with no fault found is not an observation on the device until you have exhausted the circuit: the EngX "Tackling AFDD tripping" thread is a good worked example of an electrician swapping a fridge-freezer between two AFDD circuits until it declared itself. Our observation codes guide and the writing observations guide have the wording.

What does an AFDD cost and what are the options?

Prices checked on 25 October 2026. The ex-VAT figures are from Sparks Direct; the electrical4less listing did not state whether VAT was included, so treat those as indicative.

Trade commentary puts the premium over a plain RCBO at roughly £40 to £90 a circuit, so a six-socket-circuit HMO board carries a few hundred pounds of extra material before you touch the enclosure size. Nearly every AFDD sold in the UK is a combined AFDD/RCBO with a 30 mA Type A residual element; stand-alone AFDDs that pair with a separate MCB exist in BS EN 62606 but are rare on UK shelves. Several makers now do single-module versions, which is what lets an all-AFDD board fit a standard domestic enclosure. BEAMA's answer on cost is that RCBOs followed the same curve, expensive at launch and commodity-priced within a decade.

Why bother outside the mandatory list?

The BEAMA guide quotes over 17,000 electrical fires a year in UK homes, with 12 per cent of 2017/18 domestic fires attributed to the electrical distribution itself (wiring, cables, plugs) and almost 23 per cent to faulty appliances and leads. Those are precisely the two fault populations an AFDD addresses and an MCB plus RCD does not. The honest counter-argument, raised regularly on EngX, is that the UK has not yet published fire statistics that isolate the arcs an AFDD would have caught, so the benefit is inferred from the physics and from the product standard tests rather than measured in the field. Both points belong in the conversation with a customer who is choosing between a £400 board and a £800 one.

Where an AFDD earns its keep without any regulation telling you so: timber-framed and thatched properties, flats with a long run of 1970s PVC socket circuits behind plasterboard, any home with a sleeping occupant who could not self-evacuate, and ring finals you have found with a high r1 or an open leg once already. Offer it, write the decision on the certificate, and move on.

How Certio helps

Certio produces the EICR, EIC, Minor Works certificate and PAT records on the BS 7671 model forms and turns them into branded PDFs. Photograph the consumer unit and the circuit schedule drafts itself; point the phone camera at a Megger MFT-X1 and the reading goes straight into the schedule of test results, or dictate it while your hands stay on the probes. Suggested observation wording with the regulation reference is there for you to check and confirm. The judgement stays yours; the typing goes. Starter is £4.99 a month, charged when you sign up.

Written by the Certio team. Last checked 25 October 2026.