AFDD (Arc Fault Detection Device): How It Works and Where It Is Required
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An arc fault detection device (AFDD) is a protective device that watches the current waveform of a final circuit and disconnects it when it recognises the signature of an electric arc. It exists for one job that fuses, circuit-breakers and RCDs cannot do: catching a loose or damaged connection that is arcing away inside a wall, a socket-outlet or a flex long before it has drawn enough current to trip anything else.
The product standard is IEC 62606 (BS EN 62606 in the UK). In North America the comparable device is the AFCI, built to UL 1699. The two do the same thing in principle, but they grew up under different wiring rules, and the differences matter when you read a datasheet or a regulation.
The fault an AFDD is built to find
Picture a 13 A plug-in heater on a 32 A ring final circuit. One terminal screw in a socket-outlet behind the sofa was never fully tightened. Every time the heater cycles, the contact surface heats, oxidises and gets a little worse. Eventually the gap starts to arc. The current through that arc is still the heater current, around 9 A, because the arc sits in series with the load.
Now look at what each protective device sees. The circuit-breaker sees 9 A on a 32 A circuit and has no reason to act. The RCD sees the same current going out on the line and back on the neutral, so there is no residual current to detect. The arc meanwhile runs at several thousand degrees and is a few centimetres from timber, plasterboard paper and PVC. That is a series arc, and it is the main reason AFDDs exist.
A parallel arc between line and neutral, from crushed or nail-damaged cable for example, is different. The current can be high, but it flows in short bursts as the arc strikes and extinguishes, and the RMS value may stay below the magnetic trip level of the breaker. A parallel arc from line to earth is usually caught by an RCD once the leakage exceeds 30 mA, which is why the AFDD is an addition to the RCD and never a replacement for it.
How the detection works
An arc does not draw a clean sine wave. Each half-cycle the arc has to re-strike once the voltage rises far enough, so the current shows flat steps around the zero crossings, and the arc itself generates broadband high-frequency noise. The AFDD samples the current, looks at that high-frequency content and at the shape and duration of the irregularities, and compares the pattern with what normal loads produce.
The hard part of the design is the second half of that sentence. Brushed motors in drills and vacuum cleaners, phase-cut dimmers and switch-mode power supplies all distort the current in their own ways. A good AFDD has to tell those apart from a real arc, which is why the detection is done in firmware and why the devices differ between manufacturers far more than circuit-breakers do.
IEC 62606 sets the maximum break times for a series arc at different load currents:
| Arc current | Maximum break time |
|---|---|
| 2.5 A | 1 s |
| 5 A | 0.5 s |
| 10 A | 0.25 s |
| 16 A | 0.15 s |
| 32 A | 0.12 s |
| 63 A | 0.12 s |
These times are short compared with how long it takes an arcing connection to ignite its surroundings, which is the whole point. The device also runs a self-test at intervals and has a test button that checks the tripping mechanism.
Where BS 7671 requires them
Regulation 421.1.7 of BS 7671:2018+A2:2022 requires AFDDs on single-phase AC final circuits supplying socket-outlets rated up to 32 A in four types of premises:
- higher-risk residential buildings;
- houses in multiple occupation;
- purpose-built student accommodation;
- care homes.
For every other installation the same regulation recommends AFDDs on those circuits. Many designers now fit them where people sleep in timber-framed buildings, where wiring is buried in combustible construction, or where the building would be hard to escape from, even when the premises fall outside the four mandatory categories. If you are working to a later amendment of BS 7671, check that the scope of 421.1.7 has not changed before relying on this list.
The AFDD has to sit at the origin of the circuit it protects, which in practice means the distribution board. It cannot see an arc on the supply side of itself, so a loose connection on the busbar or the main switch is outside its reach.
Choosing and fitting one
Most AFDDs sold in Europe are combined units: an AFDD module joined to an RCBO, or a single 2-module device with arc detection, overcurrent protection and 30 mA residual current protection in one body. A combined unit is usually the sensible choice in a domestic board, where space is tight and every socket circuit needs RCD protection anyway.
A few points to check before ordering:
- Width. Combined AFDD/RCBO units are typically 2 modules wide. On a board populated with single-module RCBOs you may not have the space, so count the ways before quoting.
- Neutral connection. The AFDD needs a neutral for its electronics and for detection. Make sure the neutral of the protected circuit goes through the device and not to the neutral bar directly.
- Breaking capacity and curve. The overcurrent part is an ordinary MCB function with a B or C curve and a rated short-circuit capacity, usually 6 kA or 10 kA. Check it against the prospective fault current at the board.
- Busbar compatibility. Some combined units fit only the manufacturer’s own board and busbar. Mixing brands in a consumer unit is a common reason for a failed inspection.
Testing and the insulation resistance trap
An AFDD contains electronics that are connected between line and neutral. If you run a 500 V insulation resistance test between live conductors with the device in circuit, you can damage it, and you will certainly get a meaningless reading. The usual approach is to switch the AFDD off, which isolates the outgoing circuit, and test the circuit from the load side, or to follow the manufacturer’s instructions exactly where they say otherwise.
Functional testing is simple: press the test button and confirm the device trips. Record that on the schedule of test results. There is no field instrument that generates a calibrated test arc, so there is no equivalent of the RCD trip-time test, and you rely on the device’s self-test and the test button.
When an AFDD keeps tripping
The first assumption should be that it has found something. Look at the indicator; most units show whether the last trip was caused by an arc, overcurrent, residual current or an overvoltage. If it was an arc trip:
- Unplug everything on the circuit and reset. If it trips again with no load, the arc is in the fixed wiring or accessories.
- Check every termination you can reach: socket-outlet terminals, spur units, junction boxes and the device’s own terminals in the board. A loose terminal that has been arcing usually shows discolouration or a melted backbox.
- If it holds with no load, plug items back in one at a time. A damaged flex or a worn plug is a frequent culprit, and occasionally an old appliance with a badly sparking motor.
Shared or borrowed neutrals between circuits upset AFDDs for the same reason they upset RCDs. If a combined unit trips on residual current as well, check that the neutral of the circuit really belongs to that circuit.
AFDD and AFCI compared
| AFDD (IEC 62606) | AFCI (UL 1699) | |
|---|---|---|
| Typical market | Europe, UK, IEC countries | USA, Canada |
| Supply | 230 V single-phase final circuits | 120 V branch circuits, 15 A and 20 A |
| Where required | BS 7671 421.1.7 for the premises listed above; recommended elsewhere | NEC 210.12 for most habitable rooms of dwelling units |
| Usual form | Combined with RCBO or MCB | Combination AFCI breaker, often dual-function with GFCI |
Questions we get asked
Does an AFDD replace an RCD?
No. The AFDD looks for arcs. Protection against electric shock still comes from the RCD and from automatic disconnection by the overcurrent device. A combined unit gives you all three in one device, which is why it can look as if the AFDD is doing the RCD’s job.
Can an AFDD be added to an existing consumer unit?
Usually, if the manufacturer makes an AFDD for that range of board and there is space. Where there is no compatible device, a separate enclosure for the protected circuits is a common answer.
Is an AFDD needed on a lighting circuit?
BS 7671 makes them mandatory only on socket-outlet circuits in the listed premises. Nothing stops you from fitting them on lighting or fixed-equipment circuits, and on long runs of cable buried in timber that can be a reasonable decision.
Related reading: RCBO guide, RCD types, how to read circuit-breaker markings.
References
- IEC 62606:2013+A1:2017, General requirements for arc fault detection devices.
- BS 7671:2018+A2:2022, Regulation 421.1.7.
- UL 1699, Arc-Fault Circuit-Interrupters; NFPA 70 (NEC), section 210.12.