Arc Fault Detection Devices (AFDDs) — 18th Edition Amendment 2, Wiring, and Selection for UK Electricians

Arc fault detection devices are one of the most significant additions to UK electrical installation practice in recent years. Introduced into BS 7671 via Amendment 2 (2022) to the 18th Edition Wiring Regulations, AFDDs are now required in specific high-risk premises and are strongly recommended across a wider range of installations. This guide explains how AFDDs work, where they are mandatory, how to wire them, and how to select the correct device for a given circuit.

What Is an Arc Fault?

An arc fault is an unintentional electrical discharge between two conductors or between a conductor and earth. Unlike a short circuit — which creates a large fault current that trips an MCB or blows a fuse almost instantly — arc faults can be characterised by relatively low fault currents that nonetheless generate intense localised heat, far above what would melt the surrounding insulation.

Arc faults are typically caused by:

• Series arc faults — a break or high-resistance connection in a conductor (damaged cable, loose terminal, corroded connection) where arcing occurs across the gap in series with the load
• Parallel arc faults — conductor insulation damaged by mechanical impact, pest damage, nail penetration, or over-bending, creating a fault between line and neutral or line and earth
• Ground arc faults — arcing between a live conductor and an earthed surface, often in cavities where the heat cannot easily be detected

The temperature of an electrical arc can exceed 6,000 °C. A series arc fault in a void, cavity wall, or under a floorboard can ignite surrounding material long before the fault current reaches the MCB trip threshold. The Health and Safety Laboratory and Fire Statistics reports consistently cite electrical faults in fixed wiring as a leading cause of accidental house fires.

How an AFDD Works

An AFDD continuously monitors the waveform of current on the circuit it protects. Normal load current — even from motors, dimmers, and switched-mode power supplies — has a characteristic waveform that AFDD processors learn to distinguish from the high-frequency noise signature of an arc.

The detection algorithm analyses:

• High-frequency current components (typically in the 10 kHz – 1 MHz range) superimposed on the 50 Hz power waveform
• Random, repetitive burst patterns consistent with intermittent arcing
• Half-cycle asymmetry and other waveform anomalies characteristic of series or parallel arcs

When the algorithm identifies a sustained arc fault signature, the AFDD disconnects the circuit within milliseconds — fast enough to prevent ignition in most scenarios. Modern AFDDs are designed to respond to arc fault conditions within 2.5 ms after the detection threshold is crossed.

Crucially, AFDDs are not a substitute for RCDs or MCBs. They protect against a specific type of fault that conventional overcurrent and residual current devices cannot reliably detect. The three devices serve complementary purposes:

• MCB — short circuit and overload protection
• RCD/RCBO — shock protection via earth fault current detection
• AFDD — fire protection via arc fault detection

Amendment 2 Requirements — Where Are AFDDs Mandatory?

Amendment 2 to BS 7671:2018, published in 2022 and taking effect for new installations from 28 September 2022, introduced Regulation 421.1.7. This regulation states that AFDDs shall be provided for final circuits supplying socket outlets up to 32 A in:

• Houses in multiple occupation (HMOs)
• Purpose-built student accommodation
• Care homes
• Houses converted to flats
• Any premises where sleeping accommodation is provided (hotels, hostels, boarding houses)

For all other domestic premises, Amendment 2 includes a strong recommendation that AFDDs should be provided for socket outlet circuits. Where a client declines AFDD protection in a domestic property, the electrician should document the discussion and any signed declination. The Approved Document P and associated guidance notes make clear that risk communication is the installer's professional responsibility.

For commercial and industrial premises, risk assessment drives the decision. Premises with significant quantities of flammable materials, historic buildings with concealed wiring, or locations where a fire would have disproportionate consequences are candidates for AFDD protection on relevant circuits.

AFDD Types and Form Factors

AFDDs are available in several configurations to suit different installation types:

Type A — Detects AC Series and Parallel Arc Faults

Responds to both series arcs (high-resistance in-line faults) and parallel arcs (line-to-neutral or line-to-earth). Suitable for most residential and light commercial circuits. This is the most commonly installed type for socket outlet and ring final circuit protection.

Type A+

Enhanced detection including pulsating DC components — required for circuits feeding equipment with rectifiers or power electronics where pulsating DC arc signatures may occur. Increasingly relevant given the proliferation of EV chargers and solar inverters at lower circuit levels.

Plug-In AFDD

Designed to mount directly in a consumer unit DIN rail alongside existing MCBs. One of the most common retrofit approaches — the AFDD sits in a standard module width position, connects between the busbar and the downstream MCB, and provides arc fault protection for the whole circuit. These are available from manufacturers including Hager (AFD516), Eaton (AFDD-63), ABB, and Schneider Electric.

AFDD+RCBO Combined Modules

A combined AFDD and RCBO in a single DIN-rail module. Offers overcurrent, earth fault, and arc fault protection in one device. Simplifies consumer unit layouts in new installations and is particularly practical for new build or full rewire projects. The combined format occupies 2–3 module spaces depending on the manufacturer.

AFDD+MCB Combined Modules

Combined AFDD and MCB without RCD functionality — intended for use downstream of a whole-board RCD (split-load boards) or where a separate RCD is installed. Less common in practice given the industry move toward individual RCBO protection.

Circuit Applications and Ratings

AFDDs are rated by their nominal current and intended circuit type. Common ratings include:

• 10 A AFDD — lighting circuits, spur circuits
• 16 A AFDD — socket outlet circuits (ring final circuits are typically protected at 32 A, but spurs may use 16 A)
• 20 A AFDD — dedicated circuits for washing machines, dishwashers, and similar fixed appliances
• 32 A AFDD — ring final circuits, the primary circuit type in UK domestic installations

The AFDD must be rated to match the MCB or RCBO with which it is used. Mismatched ratings can result in the MCB tripping before the AFDD engages detection, or in nuisance tripping from the AFDD on circuits with normal inrush characteristics.

Installation and Wiring

Wiring an AFDD follows the standard approach for DIN-rail protection devices, with the AFDD inserting in series between the supply (busbar) and the load (circuit cable).

New Consumer Unit Installation

When specifying a new consumer unit for an HMO or a property where the client accepts AFDD protection, select a board with combined AFDD+RCBO modules for each socket outlet circuit. The resulting board is straightforward to install and meets both Regulation 421.1.7 and the Chapter 41 shock protection requirements simultaneously.

Retrofit into Existing Consumer Unit

Where an AFDD is being added to an existing board (e.g., rewiring a room in an HMO, or upgrading for a client):

1. Isolate the supply to the consumer unit fully — both the isolator and the main switch.
2. Identify the circuit(s) to receive AFDD protection — typically all socket outlet final circuits up to 32 A.
3. Check available module space. A plug-in AFDD occupies at least one additional module width. Combined AFDD+RCBO may require replacement of the existing MCB/RCBO.
4. Remove the existing MCB and replace with AFDD+RCBO, or install a plug-in AFDD upstream of the existing MCB if space permits.
5. Where space is insufficient, the consumer unit may need to be replaced with a larger enclosure — this is common in older properties with compact boards.
6. Reconnect the circuit conductors to the output of the AFDD (or AFDD+RCBO), ensuring correct polarity and secure terminations.
7. Test as described below before restoring supply.

Terminals and Polarity

AFDDs monitor the live (line) conductor and, in most designs, also sense the neutral. Both line and neutral must pass through the AFDD — this is not optional. Where a plug-in AFDD connects via a busbar clip for the live feed, ensure the neutral is also routed through the device as per the manufacturer's wiring diagram. Failure to route both conductors through the AFDD will prevent correct operation and may introduce detection errors.

Testing and Commissioning

All AFDDs must be tested after installation to confirm correct operation. Testing is also required at subsequent periodic inspections.

Functional Test

All AFDDs include a test button that simulates an arc fault condition internally. Pressing the test button should cause the device to trip and disconnect the circuit. This test should be performed:

• After initial installation, before handover
• At each periodic inspection (EICR)
• Where nuisance tripping has occurred, to confirm the device responds correctly and is not faulty

EICR Inspection Codes

From the date Amendment 2 came into effect, inspectors should note the absence of AFDDs on circuits where they are mandated (HMOs, care homes, etc.) as a Code C2 (potentially dangerous) observation. In premises not covered by the mandatory requirement, the absence of AFDDs should be noted as a recommendation or Code C3 (improvement recommended), depending on the risk profile of the installation and the nature of the wiring.

The IET Guidance Note 3 (Inspection and Testing) provides further detail on AFDD codes and documentation. See also the guidance on EICR and landlord electrical safety requirements for context on how AFDDs are treated in periodic reporting.

Nuisance Tripping — Causes and Solutions

One of the practical challenges with AFDD installation is nuisance tripping — the device disconnecting the circuit in response to high-frequency noise that resembles an arc fault signature, even though no actual fault is present. Sources of legitimate high-frequency noise on circuits include:

• Variable-speed motor drives (power tools, washing machine motors, vacuum cleaners)
• Switched-mode power supplies in laptops, phone chargers, and LED drivers
• Dimmers (leading-edge and trailing-edge phase-cut dimmers can generate significant switching transients)
• EV chargers with high-frequency DC ripple
• Some fluorescent lamp ballasts and neon transformers

Modern AFDDs use adaptive algorithms designed to distinguish load-generated noise from genuine arc signatures, and nuisance trip rates have improved significantly in the latest generation of devices. However, if nuisance tripping occurs:

1. Identify the appliance or load that correlates with the trip event — switch loads off one at a time and monitor.
2. Check firmware version — some manufacturers issue updates that refine the detection algorithm for specific load types.
3. Consider circuit segregation — where a high-noise load (e.g., a workshop with power tools) is on a dedicated circuit, an AFDD may be omitted on that circuit if it is not a socket outlet final circuit subject to the mandatory requirement, and documented in the circuit schedule.
4. Check for loose connections throughout the circuit — a genuine arc fault may be causing the trips.

Coordination with Other Protective Devices

AFDDs must be coordinated with the upstream and downstream protection:

• Upstream MCB/fuse — the AFDD must operate before the upstream device on arc fault detection. The rated operating current of the AFDD must not exceed that of the upstream protective device.
• Downstream RCD — where a whole-board RCD is used (split-load board), the AFDD must not interfere with RCD operation on genuine earth faults. Combined AFDD+RCBO avoids this coordination issue entirely by providing both functions in series.
• Downstream SPD — if a Type 2 SPD is installed at the board, the AFDD and SPD coordinate normally. See the article on surge protection devices for SPD selection guidance.

For consumer unit design on new builds and full rewires, the preferred configuration is an AFDD+RCBO combined module per circuit on all socket outlet final circuits, a Type 2 SPD at the board, and a main switch isolator. This provides overcurrent, earth fault, arc fault, and surge protection from a single compact installation.

Labelling and Documentation

Circuits protected by AFDDs must be clearly labelled in the consumer unit schedule. The Electrical Installation Certificate (EIC) or Minor Works Certificate should record:

• Presence of AFDD on each protected circuit
• Device type (AFDD, combined AFDD+RCBO, etc.)
• Manufacturer and model reference
• Test button tested and operated correctly — confirmed on handover

Where AFDDs are installed in response to an EICR recommendation or as part of remedial work in an HMO, a new Minor Electrical Installation Works Certificate or, for more extensive work, a partial EIC should be issued.

Cost and Client Communication

AFDDs add cost to a consumer unit installation. Combined AFDD+RCBO modules typically cost £25 each at trade price, compared with £8 for a standard RCBO. For a consumer unit with eight socket outlet circuits, this represents an additional £140 in device costs, plus any additional labour or enclosure costs.

When presenting this to clients, the key messages are:

• For HMOs and qualifying premises, AFDD protection is now a legal requirement for socket circuits — non-compliance is a Building Regulations offence and creates significant liability.
• For domestic clients, the additional cost is modest relative to the risk mitigation — a domestic fire can cause tens of thousands of pounds of damage and potentially endanger lives.
• Insurance implications — some insurers are beginning to ask about AFDD protection in new policies. Providing a completed EIC demonstrating AFDD installation may benefit the client at renewal.

For the related protection devices covered in this guide, see the articles on RCD, RCCB, and RCBO protection and consumer units, MCBs, and RCBOs. For surge protection coordination, see surge protection devices for UK electricians.

Products for This Guide

No combined AFDD+RCBO modules are currently stocked in the APM catalogue — these are typically special order from Hager, Eaton, or ABB. The following RCBO and consumer unit products are suitable for use in AFDD-ready installations: