What Is an AFDD and Does My Consumer Unit Need One?

The 18th edition of the IET Wiring Regulations (BS 7671) introduced a requirement for Arc Fault Detection Devices (AFDDs) on certain circuits in certain property types. AFDDs are a relatively new addition to the protective device family, sitting alongside MCBs, RCDs, and RCBOs in modern consumer units. Most homeowners and even some electricians are still unsure exactly what they do and when they are needed. This guide explains what an AFDD is, what fault it protects against, when the regulations require one, what it costs to fit, and whether your existing consumer unit needs upgrading.

What is an AFDD?

An Arc Fault Detection Device is a protective device fitted in the consumer unit. It looks similar to an RCBO (a combined MCB and RCD in one unit) but contains additional electronic detection circuitry that monitors the waveform of the current flowing through it. When the device detects the characteristic signature of an electrical arc, it disconnects the circuit before the arc can develop into a fire.

An AFDD provides three layers of protection in a single device:

• Overcurrent protection (the same function as an MCB): disconnects the circuit if the current exceeds the rated value.
• Earth fault protection (the same function as an RCD): disconnects the circuit if a current imbalance indicates an earth fault.
• Arc fault detection (unique to AFDDs): disconnects the circuit if the current waveform indicates an electrical arc.

The third function is what makes the AFDD different. Standard MCBs and RCDs do not detect arcing; they only detect overcurrent or earth faults. An arcing fault that does not produce sufficient current to trip an MCB and does not cause an earth fault can still ignite materials around the cable, which is the leading cause of electrical fires in homes.

What is an electrical arc and why does it matter?

An electrical arc occurs when current jumps across a small gap in a conductor: a loose connection at a socket terminal, a damaged section of cable insulation, a broken strand in a flex, or a partially burnt connection. The arc generates intense localised heat (often above 5,000 degrees Celsius) that can ignite cable insulation, plaster, dust, or any material in immediate contact with the arc.

Arcing faults are the leading identified cause of electrical fires in UK homes. They are particularly difficult to detect with conventional protective devices because:

• The arc current may be relatively small (a few amps), well below the trip threshold of an MCB.
• The arc may be a series fault with no path to earth, so RCDs do not detect it.
• The arc may be intermittent, occurring only briefly before resolving and recurring later.

An AFDD specifically watches for the high frequency current signatures that distinguish arcing from normal load behaviour. The device disconnects the circuit before the arc can sustain long enough to cause a fire.

When does the 18th edition require AFDDs?

The 18th edition first introduced AFDD requirements as a recommendation. Subsequent amendments tightened the position. Under the current Wiring Regulations as of 2026, AFDDs are required on the following circuits in the following property types:

Mandatory AFDD installations

AFDDs are required on socket outlet circuits in:

• Higher Risk Buildings (typically buildings 18 metres or 7 storeys and above).
• Houses in Multiple Occupation (HMOs).
• Purpose built student accommodation.
• Care homes and similar residential institutions.

For these property types, AFDDs are not optional. The Wiring Regulations require them on relevant circuits during any new installation or major rewire.

Recommended AFDD installations

AFDDs are recommended on socket outlet circuits in:

• Other residential properties (single occupation homes, family rentals).
• Educational premises.
• Places of public assembly.

For these property types, AFDDs are not strictly mandatory but are increasingly fitted as a safety upgrade. Insurance companies and lender solicitors are increasingly favourable to properties with AFDD protection.

Do I need to retrofit AFDDs to my existing consumer unit?

Generally, no. The Wiring Regulations apply to new installations and major alterations rather than retroactively to existing installations. An existing consumer unit installed before AFDDs were required can continue in service, and the EICR will not flag the absence of AFDDs as a C2 issue on a standard residential property.

You should consider AFDD retrofit in three situations:

1. You are replacing the consumer unit anyway. If the unit is being replaced (because of an EICR finding, an extension, an EV charger installation), specifying AFDDs on socket circuits adds modest cost and provides significant additional safety.
2. The property is converted to HMO or student accommodation. The Wiring Regulations require AFDDs on the relevant circuits when the property changes use to one of the categories where AFDDs are mandatory.
3. You are doing a major rewire. A major alteration triggers the current Wiring Regulations on the affected circuits.

For a Scottish landlord property that is not HMO licensed, the absence of AFDDs is not in itself a Repairing Standard issue. Adding AFDDs is a positive upgrade rather than a compliance requirement.

AFDD cost in Aberdeenshire

AFDD pricing in 2026:

• Single AFDD device: £80 to £150 fitted, depending on the consumer unit brand and the device specification.
• Adding AFDDs to a new consumer unit installation (typical 4 socket circuits): £320 to £600 above the standard consumer unit price.
• Full AFDD board (every circuit on AFDD): £600 to £1,000 above the standard consumer unit price.
• Retrofitting AFDDs into an existing consumer unit: only viable if the existing unit can accommodate them. Otherwise the unit needs to be replaced, in which case the figures above apply.

For a typical Aberdeenshire 3 bedroom home installing AFDDs on the four socket outlet circuits at the time of consumer unit replacement, the total cost is approximately £350 to £550 above the base consumer unit price.

Are AFDDs worth fitting voluntarily?

For most Scottish residential homeowners, the answer is increasingly yes. The cost is modest in the context of a consumer unit upgrade, and the safety benefit is genuine. The case for AFDDs is strongest where:

• The property has older wiring that may have deteriorated insulation in places.
• The property has many sockets in heavily used locations (kitchens, home offices, family rooms).
• The household includes children or vulnerable occupants for whom the additional safety margin matters.
• The property is a long term investment where the value of the upgrade compounds over years.
• The property is in a remote location where any fire response would be slower than in urban areas.

Faithful Spark advises on AFDD specification at the survey stage based on your property’s specific characteristics and your priorities.

AFDDs in HMO properties: a specific note

For HMO landlords in Aberdeen and Aberdeenshire, AFDDs on socket circuits are now mandatory under the Wiring Regulations for any consumer unit replacement or major alteration. HMO licence applications increasingly include reference to electrical safety standards which the AFDD requirement supports.

If your HMO consumer unit was installed before the AFDD requirement and has not been altered since, the existing unit can remain in service for the time being. However, when the unit is next replaced (which is likely to be at the next major EICR remedial work or at the next significant alteration), AFDDs will be required as part of that replacement.

Frequently asked questions

Will an AFDD trip when I plug in a vacuum cleaner or hairdryer?

No, in normal operation. Modern AFDDs are designed to distinguish between the harmless current waveforms of motor appliances (which produce some harmonics on starting) and the characteristic signature of a sustained electrical arc. False trips do happen occasionally with very old or faulty appliances, but they are uncommon in normal use of modern equipment.

Can I add AFDDs to my old consumer unit?

It depends on the unit. Some older consumer units can accept AFDDs as direct replacements for existing MCBs or RCBOs of the same brand. Most older units cannot. In practice, retrofitting AFDDs to a unit that does not natively support them usually means replacing the unit altogether, in which case the cost of a new unit with AFDDs from the start is similar.

Is the EICR going to flag the absence of AFDDs?

For standard residential properties, no. The EICR does not flag the absence of AFDDs as a C2 finding on a standard owner occupied home or single household rental. For HMO licensed properties or other categories where AFDDs are mandatory, the EICR may flag the absence as a C2 if the unit was replaced after the AFDD requirement came into force.

Are AFDDs different from RCBOs?

Yes. An RCBO combines overcurrent protection and earth fault protection. An AFDD adds arc fault detection on top of those two functions. AFDDs are technically a superset of RCBOs in protection terms, with the additional arc detection element. They are also significantly more expensive per device than basic RCBOs.

Do AFDDs need testing as part of an EICR?

Yes. Where AFDDs are fitted, the EICR includes a functional test to confirm the device responds correctly. The test is built into modern testing instruments and adds no significant additional time to the inspection.

Book a consumer unit survey including AFDD options

Faithful Spark provides NICEIC certified consumer unit installations across Aberdeen and Aberdeenshire including AFDD options on relevant circuits. We advise on whether AFDDs are required by the Wiring Regulations for your property and whether voluntary AFDD fitting makes sense for your specific situation. See our pillar guide on consumer unit upgrades in Aberdeen.

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Faithful Spark Electricians. NICEIC approved. Local Aberdeen team. Consumer unit upgrades, AFDD installations, and Electrical Installation Certificates for Aberdeen, Peterhead, Ellon, Fraserburgh and across Aberdeenshire.