Understanding arc fault detection devices (AFDDs)

Helpful video reference. We use John Ward's video "The £1300 AFDD consumer unit" as the video reference here. John Ward (jwflame) is a qualified electrician based in Dorset who has been making UK electrical education videos for over a decade. This one shows a real consumer unit fitted with AFDDs on every circuit and puts the cost in plain terms — worth watching before you get quotes for a board change.

1. What is an arc fault?

Most electrical faults are obvious to the protection in your consumer unit: a short circuit draws thousands of amps and trips the MCB, or a fault to earth draws enough current to operate the RCD. Arc faults are different.

An arc fault is an unintended electrical discharge jumping across a gap, typically inside cable insulation that has been pinched, nicked or aged until the insulation has partially broken down. The current involved can be surprisingly modest — well below the MCB trip threshold — but the arc itself reaches temperatures of thousands of degrees. That is more than enough to ignite insulation, timber joists or carpet.

Series arc faults (within the conductor path) are the most dangerous. They occur when a conductor is broken or poorly connected: inside a screw terminal that has worked loose, at a damaged flex, or in an older cable where the insulation has become brittle. The arc keeps the circuit alive — lights still work, appliances still run — whilst silently generating heat.

2. How an AFDD works

An AFDD sits in the consumer unit in place of a plain MCB or RCBO. It contains electronics that monitor the current waveform on the circuit in real time. Normal loads (motors, dimmers, switch-mode power supplies) each produce a characteristic waveform that the AFDD learns to recognise as benign.

An arc fault produces a high-frequency burst superimposed on the normal waveform — a signature that ordinary protective devices cannot see. When the AFDD detects that signature, it disconnects the circuit within milliseconds.

Modern AFDDs also incorporate RCD protection (30mA) and overcurrent protection, so the single device gives arc, shock and overload protection in one unit. That is why they are bigger and heavier than a plain MCB, and why they cost considerably more.

3. How AFDDs differ from RCDs

It is worth being clear about what each device does, because they are often discussed together:

• RCD (Residual Current Device): detects current imbalance between the live and neutral conductors — the signature of current leaking to earth (usually through a person or a fault). Trips at 30mA for personal protection. Does not detect series arc faults, which produce almost no earth leakage.
• AFDD (Arc Fault Detection Device): detects the high-frequency waveform signature of arc faults. Protects against fire, not electric shock. Does not replace the RCD — the two work together.

A modern AFDD-plus-RCD combination device gives you both protections in a single unit. A plain AFDD without RCD protection would still leave you unprotected from shock.

4. What BS 7671 Amendment 2 says

Amendment 2 to BS 7671:2018 came into force on 28 September 2022. It introduced Regulation 421.1.7, which states that AFDDs should be installed to protect socket outlet circuits in:

• Houses in multiple occupation (HMOs)
• Student accommodation
• Care homes and other buildings with sleeping accommodation open to the public
• Houses converted into flats

For a single-family dwelling, the regulation uses "should" rather than "shall", which in BS 7671 language means it is a strong recommendation rather than a hard requirement. In practice, many electricians working on new consumer unit installations in family homes now include AFDDs as standard — or at minimum offer them as a clearly costed option — because Amendment 2 made them the expected baseline for new work.

If you own or rent out a property that falls into the HMO or converted-flat category, AFDDs on socket circuits are no longer optional for new or replacement work.

5. What an AFDD consumer unit looks like and costs

An AFDD-equipped consumer unit looks broadly the same as a standard board from the outside. The individual devices in the slots are larger than plain MCBs, which sometimes means fewer circuits fit in a standard enclosure — a wider or twin-gang board may be needed.

At trade prices in 2024 to 2025, a combined AFDD/RCBO device for a single circuit typically costs £40 to £80 per slot. A family home with ten socket circuits might spend £400 to £800 on the devices alone before labour, board, trunking and certification are added. This is why John Ward's video is titled "The £1300 AFDD consumer unit" — the final installed price can be significantly higher than a standard RCD or RCBO board.

There is nothing wrong with the price: it reflects real components doing a real job. But it is worth understanding what you are paying for, and worth getting a couple of quotes from registered electricians so you can compare what protection each one includes.

6. What to do if your AFDD trips and will not reset

An AFDD that trips is telling you it detected something. Do not simply keep resetting it. Unlike a plain MCB that trips on overload (switch off some appliances and reset), an AFDD trip may indicate a genuine arc fault: a loose connection inside a socket, damaged cable under flooring, or a failing appliance flex.

Switch off and unplug everything on the circuit, then try resetting. If it holds, plug in appliances one at a time to find the faulty one. If it continues to trip with nothing connected, or if it trips immediately, call an electrician. The alternative to taking that seriously is ignoring a device that is trying to prevent a house fire.

When to call us

If you are getting quotes for a consumer unit replacement in Sandwich or east Kent, Richard can talk you through what protection level makes sense for your property — whether that is a standard RCBO board, AFDDs on socket circuits, or a full AFDD fit-out for an HMO. No pressure, no upsell. Just a clear explanation of what each option does and what it costs.