Surge Protection Devices and Arc Fault Detection Devices: What UK Electricians Need to Know Under BS 7671:2022

Surge Protection Devices and Arc Fault Detection Devices: What UK Electricians Need to Know Under BS 7671:2022

Amendment 2 to the 18th Edition Wiring Regulations (BS 7671:2022) introduced two devices that are now firmly part of the modern domestic installation: surge protection devices (SPDs) and arc fault detection devices (AFDDs). Both have been around for years, but the updated regs have moved them from optional extras to expected — and in some cases mandatory — installations.

If you're replacing a consumer unit, carrying out a full rewire, or advising a client on circuit protection, you need to understand what these devices do, when they're required, and what you're actually fitting. This guide covers the practical trade picture.

Surge Protection Devices (SPDs)

What an SPD actually does

A surge protection device clamps transient overvoltages before they can damage connected equipment. Overvoltages — also called transients or spikes — occur when there's a sudden change in the electrical supply: a nearby lightning strike, the switching of a large motor, or supply switching by the DNO. These events produce a brief burst of voltage that can be many times higher than the nominal 230 V supply, lasting microseconds but enough to destroy sensitive electronics, smart controls, or LED drivers.

An SPD works by diverting the surge energy to earth before it reaches the protected circuit. The key component inside most Type 2 SPDs is a metal oxide varistor (MOV), which has a very high resistance under normal conditions but becomes conductive almost instantly when voltage exceeds the clamping threshold.

Types 1, 2, and 3 — and where each goes

SPDs are categorised by where they sit in the installation and what they protect against:

• Type 1 SPD — fitted at the origin of the installation, at or ahead of the main distribution board. Required where the installation has lightning protection or where overhead supply lines are present. Capable of handling partial lightning current (direct strikes to the supply). Tested to impulse current (I_imp) waveform.
• Type 2 SPD — the most common domestic and light commercial type. Fitted at the consumer unit (or sub-distribution board). Protects against induced surges from lightning and from switching transients. Tested to 8/20 µs discharge current (I_max / I_n).
• Type 3 SPD — supplementary protection fitted close to sensitive equipment: at socket outlets, within appliance connection units, or inside equipment itself. Tested to combination waves. Must always be used alongside a Type 2 upstream.

For most domestic consumer unit replacements, a Type 2 SPD is what you'll be fitting. Type 1+2 combined devices are also available for installations where the incoming supply is overhead.

When are SPDs required under BS 7671:2022?

Regulation 443.4 sets out the requirement. The key principle is that an SPD should be installed unless a risk assessment demonstrates that the consequences of overvoltage are tolerable. In practice, this means:

• Where failure of equipment due to overvoltage could result in danger to persons (medical equipment, fire detection systems, emergency lighting)
• Where equipment failure would cause significant financial loss or data loss
• Where the installation is in a location with increased lightning risk or overhead supply

The 18th Edition Amendment 2 made clear that the default should be to fit an SPD in domestic premises, with omission requiring a documented risk assessment. Many certification bodies and building control authorities now expect to see either an SPD fitted or a written justification for its omission in an EICR or installation certificate.

In practice: if you're replacing a consumer unit in a domestic property, fit an SPD. The cost is modest, the protection is real, and it covers your liability if a client's smart home equipment is later damaged by a transient.

Fitting a Type 2 SPD at the consumer unit

Most modern consumer units (18th Edition compliant boards from Hager, British General, Schneider, Wylex, MK) have a dedicated SPD busbar or SPD connection point. The SPD connects between line and neutral (for SP+N types) or between all three phases and neutral (TP+N for three-phase boards), with a separate earth conductor.

Key installation points:

• The SPD should be connected as close as possible to the incomer — ideally within 0.5 m of the main switch
• The earth conductor from the SPD to the earth bar should be kept as short as possible; long earth leads add inductance that reduces clamping effectiveness
• Most SP+N Type 2 SPDs consume 1–2 module spaces in the consumer unit
• SPDs have a thermal disconnector or visual status indicator — always check the indicator window after installation to confirm the unit is active
• SPDs are sacrificial: after a major surge event, the MOV may be degraded. The status indicator will turn red (or the window will change colour) to signal the device needs replacing

SPD products and pricing

The current market for Type 2 single-phase SPDs (SP+N or DP) is competitive. Typical price range for trade:

• MK Sentry 1P+N Type 2 40kA: around £22–27 (Screwfix)
• British General Fortress Type 2 kit: around £25 (Screwfix)
• Wylex DP Type 2 40kA: around £46 (Screwfix)
• Crabtree Starbreaker plug-in Type 2: around £92 (Screwfix) — premium plug-in format for Crabtree boards

For a domestic consumer unit replacement, a basic SP+N Type 2 device in the £22–30 range does the job and is easily justified to the client on both regulatory grounds and equipment protection.

Arc Fault Detection Devices (AFDDs)

What serial arcing is — and why standard RCBOs miss it

An arc fault occurs when current jumps across a gap in a conductor — through damaged insulation, a loose connection, or a pinched cable. Serial arcing (along the conductor, rather than between line and earth) is particularly dangerous because it can sustain enough energy to ignite surrounding materials while drawing current well below the trip threshold of an MCB or RCBO.

A 32 A MCB will trip under a sustained 32 A overload or a fault current many times that value. But a serial arc in a circuit with a few hundred watts of connected load may only draw 5–10 A — enough to carbonise cable insulation and start a fire over time, but nowhere near enough to trip the overcurrent device. Standard RCDs and RCBOs won't detect it either, because there's no significant earth leakage current — the arcing is happening within the live conductors.

An AFDD monitors the current waveform continuously and uses signal processing to detect the characteristic electrical signature of arc discharge: irregular high-frequency current components superimposed on the 50 Hz sine wave. When it identifies arcing, it trips the circuit.

When are AFDDs required?

Regulation 421.1.7 (BS 7671:2022) states that AFDDs shall be provided for:

• Final circuits supplying socket outlets in high-risk locations: premises with sleeping accommodation (HMOs, care homes, student accommodation, hotels, hostels)
• Final circuits supplying socket outlets in locations with increased fire risk where an arc fault could not be readily detected

The regulation specifically targets premises where people may not be awake to notice a smouldering cable before it escalates. For standard owner-occupied domestic premises, AFDDs are recommended but not yet mandatory — though the direction of travel in the regs is clearly towards wider adoption.

Where AFDDs are required, they must protect socket outlet circuits. They are not required for (and in some cases are not suitable for) all final circuits — motor loads, variable speed drives, and some discharge lighting can generate waveform characteristics that cause nuisance tripping.

AFDD formats — standalone vs combined AFDD+RCBO

AFDDs are available in two main formats:

• Standalone AFDD — fits in the consumer unit alongside an existing RCBO or MCB+RCD combination. Less common now as the combined format has become dominant.
• Combined AFDD+RCBO — a single module that provides arc fault detection, residual current protection (30 mA), and overcurrent protection. This is the standard approach for new installations. Fits in a standard consumer unit like an RCBO.

The combined AFDD+RCBO is the practical choice for any consumer unit replacement in premises where AFDDs are required. Fitting a separate AFDD module upstream of each RCBO would consume additional board space and cost more overall.

Key specifications to understand

• Curve — Most AFDDs are B curve (lower magnetic trip threshold), appropriate for resistive and light inductive loads. Check compatibility with the load type before specifying.
• RCD type — Combined AFDD+RCBOs are available in Type A (detects AC and pulsating DC residual current — the standard for most domestic circuits including those supplying EV chargers, solar inverters, and VSD-equipped appliances). Type AC is no longer permitted for new installations under BS 7671:2018.
• Module width — Standard combined AFDD+RCBOs are typically 2 modules wide (18 mm per module = 36 mm total). Check board capacity before specifying a full AFDD+RCBO fit-out.
• Sensitivity — Standard 30 mA for RCD function. The AFDD detection threshold is determined by the device's onboard electronics and is not user-adjustable.

AFDD pricing and product landscape

AFDDs carry a significant cost premium over standard RCBOs. Current market pricing for combined AFDD+RCBO modules:

• Wylex NXS combined AFDD+RCBO (10A or 32A): approximately £100 each (Screwfix)
• Axiom combined AFDD (10A): approximately £56 (Toolstation, on clearance)

A full consumer unit replacement with 8 AFDD+RCBO modules for an HMO could therefore add £400–800 to the materials cost over a standard RCBO board, plus the additional board space required. This should be quoted clearly to the client and justified on the regulatory requirement.

Testing AFDDs

AFDDs have a test button (separate from the RCD test button on combined devices). The AFDD test simulates an arc signature and should trip the device independently of the RCD. Both test functions should be verified:

• Press the RCD test button — trips on residual current
• Press the AFDD test button — trips on arc detection

Some manufacturers require a load to be connected on the circuit for the AFDD test to operate correctly. Check the manufacturer's instructions — an AFDD test that doesn't trip under load may indicate a fault with the device.

On commissioning documentation (EIC), record the AFDD as fitted and note the test result. For EICR purposes, AFDDs in locations where they are required should be recorded as C2 if absent.

SPDs and AFDDs Together: the Modern Compliant Consumer Unit

A consumer unit replacement on a rental property with sleeping accommodation now typically needs:

• A main switch (100A isolator)
• A Type 2 SPD (1–2 modules)
• AFDD+RCBO modules for all socket outlet circuits (Type A, 30mA)
• Standard RCBOs for lighting and fixed appliance circuits where AFDD is not required

This adds both cost and board space to the job versus a pre-18th Edition replacement. Plan board size accordingly — a 20-way or 24-way board is now often necessary where a 12-way or 16-way would previously have sufficed.

Common Questions from Electricians

Do I need an AFDD on a lighting circuit?

Regulation 421.1.7 specifies socket outlet circuits. Lighting circuits are not explicitly required to have AFDD protection under the current regs. However, if a client wants full arc protection or the AHJ (Authority Having Jurisdiction, e.g. building control) asks for it, there is no technical reason not to fit them on lighting circuits — but nuisance tripping from lamp dimmer circuits and certain LED drivers is more likely, so select carefully.

Can I use a Type 3 SPD instead of Type 2?

No. Type 3 SPDs (plug-in surge protectors, socket suppressor strips) are supplementary protection only and must always be used with a Type 2 device upstream. They are not a substitute for a Type 2 at the consumer unit.

How long does an SPD last?

MOV-based SPDs degrade with each surge event they absorb. Under normal conditions with no major transients, a Type 2 SPD can last 10–20 years. After a significant surge, the status indicator changes and the device should be replaced. Include a note on the EIC/minor works certificate that the SPD status indicator should be checked periodically.

What happens if an AFDD nuisance trips?

Nuisance tripping on AFDDs is a real issue, particularly on circuits supplying older dimmers, variable speed drives, or equipment with high inrush current. If a client reports persistent nuisance trips, first confirm there are no genuine arc faults (check connections, cable condition). If the installation is sound, the device may need replacement with a model from a different manufacturer — AFDD sensitivity and filtering algorithms vary between brands. Document any investigation.

Summary

SPDs and AFDDs are now a normal part of electrical installation work in the UK. The regulatory picture is clear: SPDs are expected in all new and replacement domestic installations, with omission requiring documented justification. AFDDs are required on socket outlet circuits in premises with sleeping accommodation and are the direction of travel for wider domestic adoption.

Understanding how these devices work, where they go, and how to test them correctly is essential for anyone carrying out consumer unit replacements or rewires on rental and HMO properties. The premium on materials is real — factor it into your quote, explain it to the client, and record everything on your certification paperwork.

For SPDs, AFDDs, RCBOs, and consumer units, APM Electricals stocks a full range of circuit protection devices from leading brands at trade prices. Browse online or call the team for availability.

Shop Consumer Units at APM Electricals — Trade Counter, Acton W3

APM Electricals stocks a full range of surge protection devices and consumer units for trade professionals. Same-day collection from our Acton trade counter.

• FuseBox SPD In Line T2 Surge Protection Unit 100A
• Axiom Consumer Unit 12 Way 100A

Browse our full Consumer Units range at apmi.uk. Visit us at 24 Western Avenue, Acton, London W3 7TZ or call 020 8702 8080.