What is the difference between an AFDD and an RCBO?

An RCBO protects against earth leakage, overload and short circuit. An AFDD detects the high-frequency signature of an electrical arc, which none of those functions can see, and disconnects before the arc starts a fire. Most AFDDs on the UK market are combined devices that include RCBO functions, so one unit gives all four protections for a circuit.

How do you test an AFDD?

AFDDs to BS EN 62606 have an integral test button that simulates an arc and confirms the electronics and trip mechanism work. There is no field instrument test equivalent to an RCD trip-time test. The schedule of test results has a column to record that the AFDD test button operated correctly, and the RCD element is tested normally.

Do AFDDs cause nuisance tripping?

Early devices could trip on brushed motors, such as vacuum cleaners and drills, whose commutators produce arcing. Devices to BS EN 62606 are designed to distinguish that from a fault arc, and nuisance tripping is now uncommon. Persistent tripping usually indicates a genuine loose connection or damaged cable and should be investigated, not dismissed.

Should I fit AFDDs on a consumer unit change in a normal house?

BS 7671 recommends it but does not require it outside the premises listed in Regulation 421.1.7. Offer them, explain the fire-protection benefit and the cost, and record the client's decision. On an EICR, the absence of AFDDs in an ordinary dwelling is not a defect and should not be coded.

What is an AFDD? Arc fault detection devices explained

Q: Are AFDDs mandatory in the UK?

Since BS 7671 Amendment 2 in 2022, Regulation 421.1.7 requires AFDDs on single-phase AC final circuits supplying socket-outlets up to 32 A in higher-risk residential buildings, houses in multiple occupation, purpose-built student accommodation and care homes. Everywhere else they are recommended, not required. Amendment 4 in 2026 keeps that position.

An AFDD (arc fault detection device) is a protective device to BS EN 62606 that monitors a circuit for the high-frequency current signature of an electrical arc, caused by a loose terminal, a damaged cable or a failing appliance flex, and disconnects the supply before the arc can start a fire. BS 7671 Regulation 421.1.7, introduced by Amendment 2 in 2022, requires AFDDs on socket-outlet circuits up to 32 A in higher-risk residential buildings, houses in multiple occupation, purpose-built student accommodation and care homes, and recommends them elsewhere.

Key takeaways

An AFDD detects arcing faults that MCBs and RCDs cannot see.
Regulation 421.1.7 makes them mandatory in four categories of residential premises; recommended in all others.
Most UK AFDDs are combined with an RCBO, so one device per circuit covers arc, earth fault, overload and short circuit.
They are tested with the integral test button; there is no instrument trip test.
Amendment 4 (published 15 April 2026) does not change the AFDD requirement.

What is an AFDD in practice?

A series arc occurs when a conductor is broken or a connection is loose and current jumps the gap; a parallel arc occurs between conductors through damaged insulation. A series arc draws less current than the normal load, so an MCB never sees an overload, and because nothing leaks to earth an RCD never trips either. The arc can ignite surrounding material long before any conventional device operates.

The AFDD contains a microprocessor that samples the current waveform and looks for the characteristic pattern of an arc, distinct from the noise of a drill motor or a dimmer, and trips when it recognises one. In the UK most are combined AFDD/RCBO units that fit a standard consumer unit.

When are AFDDs required?

Premises	Circuits	Requirement under Reg 421.1.7
Higher-risk residential buildings (over 18 m)	Single-phase socket-outlet circuits up to 32 A	Mandatory
Houses in multiple occupation (HMOs)	Single-phase socket-outlet circuits up to 32 A	Mandatory
Purpose-built student accommodation	Single-phase socket-outlet circuits up to 32 A	Mandatory
Care homes	Single-phase socket-outlet circuits up to 32 A	Mandatory
All other premises	Any final circuit	Recommended

The requirement applies to new work in those premises from 28 September 2022, when Amendment 2 took effect. It is not retrospective: an existing HMO without AFDDs is not in breach, and on an EICR their absence is at most a C3 in the premises listed and not an observation elsewhere. Amendment 4, published 15 April 2026, leaves Regulation 421.1.7 in place; its headline changes concern battery storage, ICT earthing, Power over Ethernet and medical locations.

How are AFDDs tested and recorded?

AFDDs are verified with the integral test button, which injects a simulated arc signal and confirms the electronics and trip mechanism operate. No field instrument generates a real arc, so the schedule of test results has a single column recording that the AFDD test button operated.

Where the device is a combined AFDD/RCBO, the RCD element is tested as any other RCD: no trip at ½ × IΔn, trip within 300 ms at 1 × IΔn and within 40 ms at 5 × IΔn for a 30 mA device, and Zs is compared to the maximum for the overcurrent element. A combined device therefore has entries in the RCD columns and the AFDD column.

On installation, AFDDs need a neutral connection and more space than an MCB, each protects one final circuit from its origin, and the manufacturer's limits on circuit length and load type apply.

Common mistakes with AFDDs

Coding their absence on an EICR in an ordinary house. They are recommended, not required, so there is nothing to code.
Fitting them only on some socket circuits in an HMO consumer unit change; Regulation 421.1.7 covers all socket-outlet circuits up to 32 A.
Confusing the AFDD test button with the RCD test button on a combined device, and recording only one.
Assuming the device protects against shock. Only the RCD element of a combined device does.
Treating repeated tripping as a faulty device. It usually means a real loose connection on the circuit.

Related terms

How Certio helps

Certio records this on the BS 7671 model forms without the typing: photograph the board to draft the circuit schedule, point the camera at a Megger MFT-X1 to capture readings, or dictate them. Starter is free for 7 days.

Written by the Certio team. Last checked 14 September 2026.