BS 7671 Guide · Section 4

Fire, arc fault and surge protection

An installation can be shock-safe and still start a fire, trap people on a staircase or cook every appliance in a thunderstorm. This section covers the parts of BS 7671:2018+A4:2026 that deal with heat, arcing, fire spread and transient overvoltage.

Chapter 42: the starting point

Chapter 42 is about electrical equipment as a source of heat. Fixed equipment must not ignite anything nearby, either in normal use or under a foreseeable fault. That means respecting the manufacturer’s clearances, keeping hot surfaces away from combustibles and choosing enclosures that will contain an internal fault.

For domestic premises, consumer units and similar switchgear assemblies must have an enclosure made of non-combustible material, or sit inside a cabinet that is. In practice that means a metal consumer unit. The reason is simple: plenty of house fires start at the board.

The chapter also covers locations with a raised fire risk. These include places where stored materials could catch fire, buildings made of combustible materials and structures that help a fire spread. In those locations, expect tighter rules on enclosure ratings, where cables run and RCD protection. Check Section 422 for the location in front of you rather than relying on memory.

Wherever a cable passes through a fire-separating wall or floor, seal the opening back to that element’s fire resistance (Section 527).

Regulation references: Chapter 42, 421.1, 421.1.201, Section 422, Section 527

Arc fault detection devices

Where AFDDs are required

Regulation 421.1.7 requires an AFDD to BS EN 62606 on single-phase AC final circuits supplying socket-outlets rated up to 32 A in four types of premises:

• High-rise residential buildings (HRRBs): A4 now pins this to a domestic building with at least one storey more than 18 m above ground level.
• Houses in multiple occupation (HMOs).
• Purpose-built student accommodation.
• Care homes.

The last three are unchanged from A2. The change is the HRRB definition. A2 used the term “higher risk residential building”, which people read in different ways. A4 replaces it with a single height test in line with how building regulations describe high-rise domestic buildings. If a building is close to the limit, confirm it with the designer or building control.

Outside those premises, AFDDs are recommended for the same kind of circuit. A recommendation is not a requirement. Even so, it is worth offering them where people sleep, in timber-framed or thatched buildings, and where a fire would destroy something that can’t be replaced.

The EV charging exemption

Regulation 722.421.1.7.201 says AFDDs are not needed on circuits feeding EV charging equipment to the BS EN 61851 series that use socket-outlets or vehicle connectors to BS EN IEC 62196-2. That covers the standard Type 2 charge point. A domestic 3-pin socket used for occasional charging is not covered by the exemption. If it is in an HMO or HRRB, treat it like any other socket-outlet circuit.

What an AFDD does and doesn’t see

An AFDD watches the current waveform for the high-frequency, irregular signature of an arc. It is built to ignore the normal, regular arcing of switches, motor brushes, dimmers and switch-mode supplies.

So an AFDD is an extra layer, not a substitute for good terminations. Torque every screw to the manufacturer’s figure. The AFDD covers faults you can’t see, not the ones you could have avoided.

Regulation references: 421.1.7, 722.421.1.7.201

Supporting cables so they don’t collapse in a fire

Regulation 521.10.202 requires every wiring system to be supported so that it won’t drop prematurely in a fire. This applies to the whole installation, not just escape routes. Cables hanging from melted plastic clips or trunking have trapped firefighters before.

A4 adds a note to make the purpose clear. The rule is there to keep exits and access routes clear during evacuation, rescue and firefighting. It does not require circuits to keep working in a fire. Fire alarm, emergency lighting and other life-safety circuits that must survive a fire are dealt with separately, under Chapter 56 and the relevant British Standards.

• Use metal clips, metal cable ties, steel fixings or supports that are fire-resistant in some other way.
• Plastic trunking or conduit can still be used, but it must not be the only thing holding the cables up. Add fire-resistant fixings inside or alongside it.
• On cable tray and basket, use metal ties at sensible intervals, not only nylon ones.
• Above suspended ceilings, fix cables to the structure. Don’t lay them on the ceiling tiles.

Regulation references: 521.10.202, Chapter 56

Escape routes and fire-fighting shafts

A4 adds a new UK regulation, 422.2.201. It keeps fire-fighting shafts and the protected stairways of protected escape routes free of cables and other electrical equipment, apart from three permitted uses:

1. Essential fire safety systems and their associated systems.
2. General lighting for the area.
3. Socket-outlets provided for cleaning or maintenance.

Anything else, such as risers to flats, landlord supplies passing through, or data and door-entry runs that don’t serve the stair, needs another route.

Regulation 422.2.202 gives a way out for unavoidable runs. Fire-rated cables inside a suitably fire-rated enclosure within a protected escape route can be treated as outside the route. Get the enclosure’s fire rating agreed with the fire engineer or building control before you install.

Appendix 13 has been rewritten and retitled to cover escape routes and fire protection. It is informative guidance that supports Chapter 42. Read it before you design anything in a block of flats or any building with a protected stair.

Regulation references: 422.2.201, 422.2.202, Appendix 13

Firefighter’s switches

Regulation 537.4.2 has been redrafted. A firefighter’s switch is now required wherever the fire engineer specifies one as part of the building’s overall fire strategy. Examples given are outdoor lighting operating at high voltage and indoor discharge lighting. Check 537.4.2 for the voltage thresholds that bring an installation into scope.

The redraft puts the decision with whoever owns the fire strategy. Ask for it at design stage. If the fire strategy calls for a switch, follow 537.4.2 and any requirements from the local fire and rescue service on position, colour and marking.

Regulation references: 537.4.2, Table 537.4

Surge protection devices

When you need them

Section 443 sets out when you need protection against transient overvoltages. A4 keeps the approach introduced by A2. SPDs are required where an overvoltage could:

• cause serious injury or loss of life, or
• cause significant financial or data loss.

A2 as first printed also listed failure of a safety service. A corrigendum issued in May 2023 took that item out, so older training notes that hang the decision on mains smoke alarms are out of date.

In every other case, SPDs are the default. They can be left out only if the installation owner declares that any loss would be tolerable and accepts the risk. The old calculated-risk method was removed by A2, and there is no blanket exemption for dwellings. In practice, most homes end up with SPDs, because few owners want to sign away the risk.

Types and where they go

Combined devices (Type 1+2, or 1+2+3) are common and save space. Match the SPD to the earthing system. A device designed for TN-C-S may not suit TT. Never use an AC SPD on PV DC strings, or a DC SPD on the AC side.

Connecting leads

Surge current is fast, so lead inductance matters more than resistance. Every extra length of lead adds voltage on top of the SPD’s protection level. Section 534 asks you to keep the total connecting lead length (live conductor to SPD, plus SPD to earth) to about 0.5 m where you can, and never more than 1 m. Run the leads together and keep them straight.

Section 534 also sets minimum conductor sizes. For example, a Type 2 SPD’s lead to earth needs at least 6 mm² copper, and a Type 1 needs at least 16 mm². Follow the manufacturer’s datasheet where it asks for more. Fit the overcurrent device the SPD manufacturer specifies. That way a failed SPD is disconnected without taking out the whole supply.

Regulation references: Section 443, 443.4, Section 534

For shock protection and RCD selection, see Protection against electric shock. For the A4 changes as a whole, see What changed in Amendment 4. For SPD and AFDD rules on charge points and PV, see EV, solar, batteries and heat pumps.