AFDDs & Arc Fault Detection: What Electricians Need to Know (2026)

What Is an AFDD and What Does It Protect Against?

An AFDD is an electronic protective device that detects the electrical "signature" of a dangerous arcing fault and disconnects the circuit before it can start a fire. It sits in the consumer unit, usually combined with an MCB or RCBO so a single module gives you overload, short-circuit, earth-fault and arc-fault protection in one width.

The critical point — and the one most customers don't grasp — is that AFDDs protect against faults that conventional protective devices simply cannot see. There are two types of arcing fault:

• Series arcing: a break in a single conductor — a loose terminal, a nicked cable under a carpet, a cracked flex on an appliance. The current flowing is normal or below the rating of the circuit, so an MCB never trips, but heat builds at the arc and can ignite surrounding material.
• Parallel arcing: an arc between live and neutral, or live and earth, where insulation has degraded or been damaged. This can draw current — but often not enough, fast enough, to trip an MCB on its instantaneous element.

A standard MCB protects against overload and short-circuit. An RCD (or RCBO) protects against earth leakage to protect people from shock. Neither is designed to detect a low-energy arc that produces heat without tripping the protection. Arcing faults are a recognised major cause of electrical fires in dwellings, which is exactly why the regulations moved to address them.

What BS 7671 Actually Says

The relevant regulation is 421.1.7 in BS 7671 (the IET Wiring Regulations, 18th Edition). Before Amendment 2 it read as a relatively soft recommendation — AFDDs were noted as one means of providing additional protection against fire caused by arc faults, and their use was left largely to the designer.

Amendment 2 (published 2022) strengthened this considerably. Regulation 421.1.7 now recommends that AFDDs are provided for final circuits supplying socket-outlets rated up to 32A in certain higher-risk premises. The categories specifically called out include:

• Higher-risk residential buildings (broadly, taller residential blocks)
• Houses in multiple occupation (HMOs)
• Purpose-built student accommodation
• Care homes

This is the part that trips people up: even after Amendment 2, the regulation uses the word "recommended" rather than "shall be provided." In BS 7671 language those two phrases are worlds apart. "Shall" is a hard requirement; "recommended" means the designer must consider it and make a documented decision. So AFDDs are not blanket-mandatory in standard domestic work. In premises outside the listed higher-risk categories, the decision rests on a risk assessment and the designer's judgement.

For the working electrician this matters because you are frequently the designer on smaller jobs — a consumer unit change, a new circuit, a rewire. Whoever holds the designer role has to consider arc-fault protection and be able to justify the decision either way.

Where Are AFDDs Recommended vs a Risk-Assessment Decision?

Always work from the current published edition of BS 7671 and the relevant guidance — this table is a quick orientation, not a substitute for the regulation text.

How AFDDs Work

An AFDD continuously monitors the waveform of the circuit it protects. A healthy load has a recognisable current and voltage pattern; an arcing fault introduces a distinctive high-frequency "noise" — irregular bursts and broadband interference characteristic of an electrical arc. The device uses on-board electronics and signal processing to distinguish a genuine dangerous arc from the harmless arcing that happens normally every day, such as a switch operating, a brushed motor running, or a vacuum cleaner starting.

When the device recognises the signature of a hazardous series or parallel arc, it disconnects the circuit. Because the detection is electronic rather than purely thermal or magnetic, an AFDD can react to a fault that produces heat without ever producing the level of current that an MCB needs to operate.

Installation Considerations on the Tools

Combined AFDD/RCBO Devices and Board Space

Most manufacturers now sell AFDDs as a single module that combines arc-fault detection with RCBO functionality (overload, short-circuit and 30mA RCD protection). That keeps the device to a single or, in some ranges, a wider module footprint, but board space is a real planning factor. On a tight consumer unit change you may find that fitting AFDDs to several socket circuits pushes you to a larger enclosure than the customer expected. Check the way count before you quote.

Cost Per Device

This is the headline objection. A combined AFDD/RCBO module costs substantially more than a standard MCB or even a standard RCBO — typically in the region of £40–£90+ per device at trade depending on brand and range, against a few pounds for an MCB. Across a board with several socket circuits that is a meaningful line on the quote, and it is one you need to explain to the customer rather than bury.

Nuisance Tripping and Good Installation Practice

Early AFDD designs developed a reputation for nuisance tripping, where the device disconnected on harmless arcing from certain appliances. Modern devices are considerably better at discriminating, but installation quality still matters. Good practice that reduces unwanted tripping includes:

• Following the manufacturer's instructions precisely — wiring, torque settings and load notes are device-specific.
• Ensuring terminations across the whole circuit are clean and tight, since loose terminals are exactly the series-arc condition the device is built to catch.
• Being aware of older or noisy appliances that may need investigation if a device trips repeatedly.
• Functional testing using the device's test button and recording the result.

AFDD vs MCB vs RCD vs RCBO at a Glance

Recording the Decision on the Certificate

Because Regulation 421.1.7 leaves arc-fault protection as a recommendation requiring judgement outside the listed higher-risk premises, the designer's decision must be visible. Where AFDDs are not fitted, the sensible approach is to note on the Electrical Installation Certificate (or Minor Works / EIC as applicable) that arc-fault protection was considered and the basis on which it was or was not provided. If you, as the designer, departed from a recommendation, that is the kind of decision that belongs in the documentation rather than in your head.

This protects you. If there is ever a question after a fire or an insurance claim, a clear record that you assessed arc-fault protection and made a reasoned decision is far stronger than silence. Treat it the same way you treat any other design choice you have to stand behind.

Cost Implications for the Customer

The hard part of AFDDs is rarely the wiring — it's the conversation. A customer comparing two consumer unit quotes will see one that's a few hundred pounds higher and want to know why. Your job is to explain that AFDDs add genuine fire protection that nothing else in the board provides, and that the extra cost reflects the device price, not padding.

Where AFDDs are a recommendation rather than a requirement, it is good practice to give the customer the choice: present a board with and without arc-fault protection, explain the difference in plain terms, and let them make an informed decision that you then record. For higher-risk premises in the listed categories, the recommendation should be your default specification.

How AFDDs Fit the Wider Consumer Unit and EICR Landscape

AFDDs sit alongside the broader move to safer consumer units — metal enclosures, RCD or RCBO protection on circuits, and surge protection devices (SPDs) where required. On an EICR you would not normally code an installation purely for the absence of AFDDs, because they are a recommendation rather than a requirement for most premises; the absence of arc-fault protection is not in itself a defect on a standard dwelling. But on higher-risk premises, or where a designer specified them, their condition and presence form part of the overall picture of an installation that meets current best practice.

For the customer, the practical takeaway is that a modern board increasingly bundles several layers of protection. AFDDs are the layer aimed specifically at fires from arcing faults that the other devices were never built to catch.

FAQ

Are AFDDs a legal requirement in the UK?

Not as a blanket rule. Under BS 7671 Amendment 2, Regulation 421.1.7 recommends AFDDs for socket circuits up to 32A in specific higher-risk premises (higher-risk residential buildings, HMOs, purpose-built student accommodation and care homes). Elsewhere it is a designer's judgement based on a risk assessment. "Recommended" is not the same as "shall be provided."

Do I need to fit AFDDs on a standard domestic consumer unit change?

Not automatically. For a normal single-occupancy dwelling it is a recommendation to consider, not a requirement. Assess the installation, discuss it with the customer, and record the decision on the certificate.

Why won't an MCB or RCD catch an arc fault?

An MCB responds to overload and short-circuit current; an RCD responds to earth leakage. A series arc — like a loose terminal — can run at normal current while generating dangerous heat, so neither device sees a reason to trip. The AFDD detects the arc's electrical signature instead of relying on current level.

How much does an AFDD add to a job?

Combined AFDD/RCBO modules typically cost around £40–£90+ each at trade, far more than an MCB. Across several socket circuits that adds up, so quote it as a clear, separate line and explain the protection it buys.

Will AFDDs cause nuisance tripping?

Modern devices discriminate well between harmless everyday arcing and genuine faults. Most unwanted trips trace back to a real wiring issue or a faulty appliance — which is the point. Follow the manufacturer's instructions, keep terminations tight, and test on commissioning.