What is an
AFDD Board?
A homeowner guide to arc fault detection in modern UK consumer units. What an AFDD actually detects, why MCBs plus RCDs miss it, when BS 7671 requires it plus what an AFDD-protected board costs to install.
An AFDD board is a UK consumer unit fitted with one or more AFDDs (Arc Fault Detection Devices). An AFDD detects the unique high-frequency electrical signature of an arc fault inside the wiring (a loose terminal, a damaged cable or a failing connection) plus disconnects the circuit before that arc starts a fire. BS 7671:2018+A2:2022 requires AFDDs on circuits feeding sleeping accommodation in higher-risk premises such as HMOs, care homes plus high-rise residential blocks. AFDDs are recommended on most other domestic installs.
Four figures
worth knowing
These figures are baked into the design of every AFDD device specified to BS EN 62606 plus installed under BS 7671 in the UK.
Faults Detected
Series arc faults, parallel arc faults plus ground arc faults. Three failure modes that an MCB or RCD will not catch on their own.
BS 7671 Section
Section 421.1.7 sets out where AFDDs are required plus where they are recommended on UK domestic plus commercial installs.
Typical Width
An AFDD or AFDD+RCBO combined device takes 2 module widths on the busbar. Allow extra board space when planning the install.
BS EN Standard
BS EN 62606 is the device standard every AFDD sold for UK use must meet. Always check the device label carries this number.
How an AFDD board
is laid out
An AFDD board has the same backbone as any modern consumer unit. The difference sits at the protective device layer where some or all circuits are protected by AFDDs instead of plain RCBOs.
Main Switch
A double-pole 100A isolator at one end of the board. Standard on any UK consumer unit. AFDD provision does not change the incoming arrangement.
AFDDs (or AFDD+RCBOs)
2-module devices clipped to the busbar covering the circuits that BS 7671 requires AFDD protection on. Modern devices combine AFDD plus RCBO.
RCBOs Per Other Circuit
Circuits that do not require AFDD protection (cooker, shower, EV charger) sit on standard 1-module RCBOs alongside the AFDD-protected ones.
Earthing Terminal
The main earthing terminal where the supply earth lands plus where main protective bonding to gas plus water originates. Every CPC traces back here.
AFDDs catch the kind of fault that starts most electrical fires
To understand why AFDDs matter, you have to understand what an MCB plus an RCD actually detect. An MCB trips on overload (too much current) plus on short circuit (a sudden direct line-to-neutral fault). An RCD trips on earth fault (current escaping to earth via a person or a damaged conductor). Between them they cover most fault types that present an immediate shock or thermal risk.
What they do not catch reliably is the slow, low-current arc fault. An arc fault happens when current jumps a small gap in a damaged or loose connection rather than flowing cleanly through a conductor. The classic causes are a loose terminal screw at a socket back-box, a nail or screw partially driven through a buried cable, a rodent-chewed flex behind a built-in oven or a degraded extension lead. The current draw stays low. The MCB sees nothing. The RCD sees nothing. The arc keeps generating heat. Insulation chars. A fire eventually starts.
How an AFDD detects what others miss
An AFDD has electronic detection circuitry that monitors the high-frequency signature of the current flowing through the protected circuit. Arc faults produce a very specific noise pattern across the current waveform that does not appear under normal load. The AFDD watches for that pattern. When it sees a sustained arc signature it disconnects the circuit, normally within a fraction of a second. The fire never starts.
A modern AFDD device is electronic plus complex compared to a mechanical MCB or magnetic RCD. That complexity is why the device standard BS EN 62606 exists. It defines the test signals an AFDD must respond to, the immunity it must show to false trips plus the long-term reliability it must demonstrate. Every AFDD legally sold in the UK must carry that standard number on the label.
Where BS 7671 requires AFDDs
BS 7671:2018+A2:2022 Section 421.1.7 requires AFDD protection on final circuits feeding socket outlets in the following premises:
- Higher-risk residential buildings (HRRBs) as defined in the Building Safety Act 2022. This typically means residential blocks at or above 18 metres or 7 storeys.
- Houses in Multiple Occupation (HMOs) licensed under the Housing Act 2004.
- Purpose-built student accommodation.
- Care homes plus similar premises with sleeping accommodation for occupants who may not be able to evacuate easily.
Outside these premises AFDDs are recommended rather than mandated on most other domestic installs. The recommendation language in 421.1.7 is strong though plus most modern boards being fitted on full rewires now include AFDD protection on at least the bedroom circuits even where the premises do not strictly require it.
AFDD plus RCBO combined
The neatest devices on the market combine AFDD plus RCBO functions in one 2-module body. That single device gives you arc fault detection plus earth fault detection plus overload plus short circuit protection on a single circuit. On a full RCBO board this is the cleanest way to add AFDD protection to specific circuits without having to redesign the rest of the install.
Cost-wise, an AFDD or AFDD+RCBO sits at roughly 3 to 5 times the price of a plain RCBO. That is the main reason AFDD protection on every circuit is rare in domestic installs even where regulations encourage it. The practical compromise most electricians offer is AFDD on circuits feeding sleeping accommodation plus plain RCBO on the rest of the board.
AFDD board install
cost ranges
AFDD devices add roughly £70 to £120 per protected circuit on top of the equivalent RCBO. The cost varies sharply with how many circuits are AFDD-protected.
AFDD Board Cost Bands
Prices include parts, labour, EIC certification plus Building Control notification. Remedial work uncovered during testing is quoted separately.
Four moments that brought
AFDDs into BS 7671
AFDDs have been used in North America since the early 2000s. Their UK adoption arrived in four broad steps.
US Adoption
NEC adopts AFCI requirements on US bedroom circuits. Decade of real-world data builds the case for European adoption.
18th Edition Published
BS 7671:2018 introduces AFDDs as a recommendation in UK domestic installs. Section 421.1.7 lays the groundwork.
Amendment 2 Strengthens
Amendment 2 to BS 7671:2018 makes AFDDs mandatory on socket circuits in HMOs, care homes plus higher-risk residential buildings.
AFDD+RCBO Integration
Combined AFDD+RCBO devices in 2-module bodies become widely available. Specifying AFDD on key circuits becomes routine on rewires.
Four facts about
AFDD boards
Mandatory in HMOs plus HRRBs
If you let an HMO, run a care home or own a flat in a high-rise residential block, AFDDs on socket circuits are required not optional.
Recommended on bedrooms
Even on standard houses, BS 7671 strongly recommends AFDD protection on circuits feeding sleeping accommodation. Most rewires now include them.
Test the trip button monthly
AFDDs are electronic. Press the test button monthly to confirm correct operation. The label on every device tells you the test interval.
Plan extra board space
AFDD or AFDD+RCBO devices are 2 modules wide. A 16 way enclosure with 4 AFDDs plus 8 RCBOs uses 16 module slots, the same as 16 standard RCBOs.
Consumer Unit Upgrades in Milton Keynes
C-Lec Electrical fits AFDD plus RCBO consumer units to BS 7671 across Milton Keynes, Bedford plus the surrounding Bedfordshire area. We cover landlord HMO compliance, single dwelling rewires plus targeted AFDD upgrades on existing boards.
AFDD-protected board vs
RCBO-only board
Both are compliant in their right context. AFDD is mandatory on certain premises plus recommended on others. RCBO-only is fully compliant on most standard domestic installs.
Strongest fire protection
- ●Detects series, parallel plus ground arc faults that MCB plus RCD cannot catch.
- ●Required on socket circuits in HMOs, HRRBs, care homes plus student accommodation.
- ●Higher parts cost. AFDD or AFDD+RCBO sits at 3 to 5 times the price of a plain RCBO.
- ●Targets the most common cause of UK domestic electrical fires (loose plus damaged connections).
- ●Best fitted on bedroom plus living circuits even on standard homes if budget allows.
Standard 2026 baseline
- ●Each circuit gets dedicated RCD plus MCB protection but no arc fault detection.
- ●Fully BS 7671 compliant on most standard owner-occupied domestic installs.
- ●Lower parts cost. Standard RCBO is roughly 1/4 to 1/3 the cost of an AFDD device.
- ●Catches earth faults plus overloads but not the low-current arcs that start most fires.
- ●Not compliant on premises where Section 421.1.7 mandates AFDDs.
For the wider context on consumer unit types, RCBOs, RCDs plus when an upgrade is genuinely needed, head back to our full guide to consumer units where every common homeowner question is answered in one place.
Back to the Consumer Units Guide
This article sits inside our complete Consumer Units knowledge base. The hub covers everything from board types plus RCBOs through to landlord requirements plus BS 7671 wiring regulations.
More on consumer
unit protection
If you have not seen a full RCBO board broken down before, the explainer on what is an RCBO board sets out how every circuit gets dedicated RCD plus MCB protection. To understand exactly how AFDDs differ from RCDs in the field, our deep dive on AFDD vs RCD covers what each device detects plus where they overlap. For the formal wiring rules behind all of this, the dedicated consumer unit wiring regulations guide breaks down the BS 7671 sections that apply. If you need a board fitted in Milton Keynes or Bedford, our consumer unit upgrades service page is the fastest route to a quote.