AFDDs Explained: What Amendment 4 Means for Your Consumer Unit

If your consumer unit was installed before 2020, AFDDs were probably not in the conversation. The device was not mandatory under earlier editions of BS 7671 and uptake in the UK domestic market was slow. Amendment 4 (in force from 15 April 2026) has made AFDDs mandatory for specific higher-risk property types, and a firmer recommendation for the rest. An AFDD watches the waveform on a circuit for the electrical signature of a developing arc, and disconnects before the local heat at the fault can become a fire.

What an arc fault is, and why other devices miss it

The everyday protective devices in a UK consumer unit cover three types of fault:

• The MCB (Miniature Circuit Breaker) trips on overload or short circuit, high-current events where something has drawn far more than the circuit is rated for.
• The RCD (Residual Current Device) trips on earth leakage, where current is escaping the intended live-neutral loop, usually to earth or through a person.
• The RCBO (Residual Current Breaker with Overcurrent) combines both of the above in a single device on a single circuit.

What none of those devices see is an arc fault. An arc fault is an electrical discharge across a small gap, typically at a loose terminal, a pinched or damaged cable, a worn socket contact, or a failing appliance flex. The current involved is often well below the MCB rating, sometimes a few hundred milliamps, and the earth leakage is often below the RCD trip threshold. But the local heat at the fault point can reach several hundred degrees, quite enough to ignite cable insulation, plasterboard dust, or combustible materials near the fault.

Fire investigators have traced a significant proportion of domestic electrical fires to arc faults that ran at currents below the threshold of any protective device in the consumer unit. The AFDD was developed to close that gap. It monitors the waveform of the current in real time, looking for the distinctive high-frequency signature of arcing, and disconnects the circuit when it sees it.

What changed in A4:2026

The requirement has tightened over three amendments. A2:2022 introduced AFDDs as a recommendation. A3:2024 extended the recommendation. A4:2026 is the first to make them a mandatory requirement, and only for specific property types:

• Houses in Multiple Occupation (HMOs)
• Purpose-built student accommodation
• Care homes and similar healthcare settings
• High-rise residential buildings above a specified height threshold

The requirement covers final circuits supplying socket outlets rated up to 32 A. For these property types, a new consumer unit installed from 15 April 2026 must have AFDD protection on every socket circuit.

For a standard single-household dwelling, AFDDs remain strongly recommended but not mandatory. You do not have to retrofit AFDDs to an existing installation. A new consumer unit upgrade on a family home can be specified with or without AFDDs, and your electrician should be able to explain the cost-benefit for your specific property.

Where an AFDD sits in the board

An AFDD is a DIN-rail device roughly the same size as an RCBO and takes one slot in the consumer unit per protected circuit. In practice you will see it installed in one of a few forms. The original spec was a dedicated AFDD upstream of an MCB, taking two DIN slots per circuit. A combined AFDD plus RCBO in a single slot, handling arc-fault, earth-leakage and overcurrent protection together, is the more common modern choice. Where the main board cannot accept a compatible AFDD at all, the workable option is a small sub-consumer-unit alongside it, housing the AFDDs and feeding a subset of circuits.

Which of those applies to your property comes down to the existing board's manufacturer and model. Hager, Schneider, MK and Wylex all produce AFDD devices for their current ranges. An older Wylex or MK board from the mid-2000s will not accept a current AFDD, so the practical answer there is a full consumer unit upgrade rather than a piecemeal retrofit.

Cost and trade-offs

AFDDs are more expensive than RCBOs. Trade cost at time of writing is around £70 to £120 per device depending on brand and rating, with combined AFDD-plus-RCBO devices costing more again. Labour is 30 to 45 minutes per device plus full testing, so retrofitting every socket circuit on a 10-circuit domestic board meaningfully adds to the cost of a consumer unit upgrade.

Whether it's worth it depends on the property. On older housing stock where a rewire isn't practical and the fixed wiring is suspect, I lean into AFDDs. They catch the specific failure mode older cable tends to die of. The same thinking applies to properties at the edge of HMO rules or with a student-let history: even where A4:2026 doesn't classify them as mandatory, the liability picture after a fire is sharper than it used to be. Installations with a lot of electric heat (showers, immersion elements, heat pumps) run warmer terminals, which raises arc-fault risk.

By default I wouldn't specify AFDDs on a post-1990s owner-occupier board with no risk factors, on a landlord EICR remedial where the scope already covers what the regulations require, or on a property already scheduled for a full rewire inside the next 3 to 5 years.

Retrofit or replace?

It depends on the board you have. A modern Hager or Schneider installed 2020 onwards with spare ways is straightforward: AFDDs go in circuit by circuit, typically alongside other planned work, by swapping the existing RCBO for a combined AFDD+RCBO in the matching range. Pre-2020 metal consumer units that are full or near-full are a compatibility question. Some older Hager and MK boards accept current-generation AFDDs, others do not, and a photograph against the manufacturer's current catalogue usually gives a definitive answer within 10 minutes.

A plastic consumer unit of any age, or a pre-2008 metal board, is the wrong starting point for a retrofit at all. The board itself is near end of life, the cost of retrofitting AFDDs to an obsolete range is high, and a full consumer unit upgrade to a modern metal enclosure with RCBO per circuit, Type 2 SPD, and AFDDs on the socket circuits where appropriate is almost always the better spend.

What to ask your electrician

If you are commissioning a new installation or a consumer unit upgrade:

• Does A4:2026 classify this property as one where AFDDs are mandatory?
• If not mandatory, are you recommending AFDDs on the socket circuits, and why or why not?
• Which specific AFDD device are you proposing, and does it match the consumer unit range?
• What is the cost differential versus a spec without AFDDs?
• Will the final EIC and the installation test results demonstrate the AFDDs have been commissioned and tested?

A competent electrician answers each of those questions specifically. A shrug or a vague reassurance is the wrong answer on any of them, and worth pressing on before you sign anything.