Surge Protection Devices (SPDs): The BS 7671 Default Most Old Boards Don't Have

A neighbour's house gets struck by lightning. You're three doors down. The electrical surge travels along the underground supply cable, enters every property on the street, and arrives at your consumer unit a few microseconds later. Without a surge protection device, that voltage spike pushes through your fixed wiring and destroys whatever's plugged in: the boiler control board, the smart thermostat, the EV charger, the induction hob's power module, the fridge inverter. The repair bill runs into thousands. With a working SPD at the consumer unit, the surge is clamped at the board and shunted to earth before it reaches anything downstream. The device sacrifices itself. You replace a £30 cartridge and carry on.

This is what an SPD does. Since 28 March 2022, BS 7671 Amendment 2 has made it the default for domestic installations. Most homes built before then don't have one. Most homeowners don't know what it is.

What an SPD actually protects against

A surge protection device is a voltage-limiting component that fits inside (or alongside) your consumer unit. When the supply voltage spikes above a safe threshold, the SPD conducts the excess to earth, holding the voltage on your fixed wiring within tolerable limits.

The threats it handles are not what most people imagine. Direct lightning strikes on buildings are rare and need a separate lightning protection system (LPS): conductors, air terminals, earth electrodes, the lot. An SPD doesn't replace an LPS and never has. What it protects against is mundane and common:

• Indirect lightning surges. A strike to a tree, a substation, an overhead line, or another building several hundred metres away induces a transient on the local grid. That transient travels.
• Switching transients. When the distribution network reconfigures itself, when a large industrial load switches off, when a transformer trips, the grid experiences brief over-voltages. These are the most frequent cause of low-level surge damage.
• Internal switching. Heat pumps starting up, EV chargers cutting in, induction motors stopping. The kit you've added to your own house generates transients on its own circuits.

A single severe event can take out multiple appliances simultaneously. Industry estimates put surge-related damage at £2,000 – £4,000 per incident across affected appliances. An SPD costs £25 – £55 as a component or under under £100 added to a new consumer unit specification.

The BS 7671 Amendment 2 change

Before 2022, the regulations required a risk assessment for SPDs. Single dwellings were specifically exempt unless someone made a positive case for protection. Almost no one did. SPDs were effectively never fitted in domestic installations.

Amendment 2 (effective 28 March 2022) revised BS 7671 Regulation 443.4.1 and removed the single-dwelling exemption. The new position: SPD protection is required by default for all premises types, including domestic. An installer can omit it only if a documented risk assessment justifies the omission, and any such omission must be recorded in writing on the installation certificate. Most safety services in modern homes (mains-wired smoke alarms, fire detection, intruder alarms) trigger a separate mandatory requirement under Reg 443.4 regardless.

In practice, the loophole has narrowed to nothing. Modern homes contain EV chargers, heat pumps, smart heating controls, networked smoke alarms, solar PV inverters, battery storage. Each of these tips the cost-benefit calculation toward "fit it." Most installers now fit SPDs as standard rather than document refusals.

The regulation applies in England, Wales, Scotland, and Northern Ireland identically. BS 7671 is adopted by all UK jurisdictions. There is no regional variation here.

Type 1, Type 2, Type 3: which one for your home

The classification system describes where in the installation the SPD sits and what magnitude of surge it handles. The naming is unhelpful for homeowners and conflated by online sources. Here's what each one actually is.

For most UK homes, the answer is a Type 2 at the consumer unit. That's it. The regulatory text in BS 7671 Reg 534.4.1.3 is specific: Type 1 is mandatory only when the building has an LPS, or when overhead supply feeds a building that has an LPS. An overhead service entry by itself does not require Type 1.

Rural properties on overhead supply, hilltop locations, and farmsteads with TT earthing systems are the cases where installers commonly fit a combined Type 1+2 module instead. Combined units are double-width on the DIN rail and roughly three to five times the cost of a Type 2. Many "Type 1" products marketed for domestic use are in fact combined Type 1+2+3 devices, which keeps the conversation simpler.

Type 3 plug-in surge strips are not a substitute for a board-level SPD. They protect only what's plugged into them, against residual transients that the upstream Type 2 hasn't fully clamped. Use them in addition for a home server, hi-fi, or home cinema. Don't use them instead.

TT earthing: the rural complication

Most UK homes are on TN-C-S (PME) earthing, supplied by an underground cable. The Distribution Network Operator provides the earth via the supply neutral. The standard "CT1" SPD configuration handles this fine.

Properties on TT earthing (the supply has no DNO-provided earth, and the home relies on its own earth electrode in the ground) are different. The standard CT1 wiring of an SPD will allow leakage current to ground through the device under normal conditions, which trips the upstream RCD as a nuisance fault. The fix is the CT2 configuration: a different internal topology that places the SPD components downstream of the RCD rather than parallel to it. Most domestic SPD modules from FuseBox, Hager, and Wylex are available in or convertible to CT2 wiring.

If your supply enters via overhead cables and you have an earth electrode (a copper rod driven into the ground), you're probably on TT. Confirm with your electrician before they specify the SPD. Get the CT2 wrong and you'll be chasing nuisance trips for the rest of the installation's life.

How the SPD actually fits in the consumer unit

A Type 2 SPD is a single or double DIN module that clips onto the rail next to the main switch, occupying one or two of the slots otherwise used by RCBOs. It connects to:

• Live (brown): via a short tail to a 63A Type B MCB, which provides upstream overcurrent protection. The MCB sacrifices itself if the SPD short-circuits at end of life. Some single-module designs have internal thermal disconnection and don't need the upstream MCB, but most installers fit one anyway.
• Neutral (blue): to the neutral busbar.
• Earth (green and yellow): via a 6mm² conductor direct to the main earth terminal (MET), as short as physically possible.

Cable lengths matter more than they do anywhere else in the consumer unit. The combined live-plus-earth conductor path from the SPD's terminals should ideally be under 0.5m and must not exceed 1m. Every additional centimetre adds inductance to the suppression path, which raises the let-through voltage during a surge. A device rated to clamp at 1.5kV with short tails will let through 2-3kV with sloppy 1.5m loops. The protection works on physics, not goodwill.

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The status indicator: a homeowner-actionable check

This is the practical detail no competitor explains and the one thing you should know how to do yourself.

Every modular SPD has a status indicator window on its front face. On most current designs (Hager, Wylex, FuseBox, BG, MK), the window shows green when the device is operational and switches to red when the internal Metal Oxide Varistor (MOV) has degraded past its safe threshold or has been depleted absorbing a surge. The change is permanent. Once the window is red, the device is no longer providing protection.

What you do:

1. Look at the window when you next open the consumer unit cover (or every six to twelve months). It should be green. After any thunderstorm, look again.
2. If it's red, the SPD has failed protective. The downstream installation is no longer protected against surges, but it remains safe to use. There's no immediate danger; you simply have no surge protection until the cartridge is replaced.
3. Note the model number printed on the device. Then call a registered electrician. The job is short: isolate the consumer unit, swap the cartridge (most modern SPDs are modular and don't require replacing the whole device), recertify with a Minor Works Certificate.

Replacement cartridges run £25 to £55, brand-specific. An MK Sentry cartridge is around £24.44 at Screwfix; a Wylex cartridge runs around £52.20 at TLC Direct. Labour for a competent electrician on a swap call-out is typically £80 – £150 depending on location.

The cartridge gets replaced after any surge event that triggers it, or every 10 years as a precaution if the indicator hasn't changed. Most homeowners never look. Make a habit of checking after thunderstorms.

Whole-house SPD vs plug-in surge strips

These are not the same thing and they don't replace each other.

A whole-house Type 2 at the consumer unit clamps voltage across the entire fixed wiring system. It protects every circuit, every socket, every hard-wired appliance: the boiler controls, the smart heating system, the EV charger, the cooker, the lighting circuits, the smoke alarms, the freezer that's ten metres from the board.

A plug-in surge strip (Type 3) is a small additional protection layer at the device end. It deals with residual transients that the Type 2 didn't fully clamp, particularly for equipment more than 10 metres from the consumer unit on a long cable run. It can also protect against transients generated locally on that final circuit.

The two work together. The Type 2 does 90% of the job. A Type 3 strip behind a sensitive piece of kit (server, NAS, expensive AV equipment, home cinema receiver) covers the last 10%. A Type 3 alone is not regulatory compliance and not adequate protection for the rest of the house.

Cheap "surge protected" extension leads sold at supermarkets are often just Type 3 with low energy ratings. They protect the four sockets on the strip and nothing else.

Cost: what you actually pay

The decision tree comes down to whether you're specifying a new consumer unit or retrofitting an existing one.

For a new consumer unit (extension, rewire, board upgrade): specify a board with the SPD already integrated at factory. The cost difference is small and the wiring is done correctly out of the box. A BG Fortress 8-way 16-module board with integrated Type 2 SPD and dual RCD is around £80.99 at Screwfix. A 12-way version is £98.99. MK Sentry equivalents run £89.99 for 8-way and £121.49 for 12-way. Across all major brands the range for a domestic-scale board with integrated SPD is £65 to £205 board-only at trade prices.

The £20 – £45 figure in our graph is the additional component cost the manufacturer charges for an SPD-included board over an otherwise identical SPD-free version. On a £100 board, you're paying about a third more for the SPD. Worth every penny.

For a retrofit into an existing compliant board: a standalone Type 2 SPD module costs £25 to £55 as a part. Add the upstream 63A Type B MCB and 6mm² earth tail and you're at parts cost £35 – £75. With a registered electrician's labour, Part P notification, and Minor Works Certificate, the all-in installed cost is £150 to £300.

For a retrofit where the existing board has no spare DIN slot: an enclosed standalone SPD unit fits alongside the consumer unit and connects via a short tail. The Lewden CGSRG1VCU is £81 at TLC Direct. The Wylex WYREC2SPD with built-in isolator is £155.34. Installed, expect £200 – £350.

For overhead-supply rural properties needing combined Type 1+2: the FuseBox combined kit is around £142.20 as parts. Installed, £250 – £450.

For a full consumer unit replacement (older plastic board, no SPD, no per-circuit RCD protection): the right answer is often to replace the whole board rather than retrofit. £380 – £520 covers a dual-RCD configuration including SPD. £520 – £720 covers an all-RCBO board. Both include parts, labour, testing, certification.

The most economical position by far is to specify a board with integrated SPD when you're already changing it. Adding an SPD as a separate later job costs three to five times the marginal component cost.

Part P: this is notifiable work

Any work that adds, alters, or replaces a circuit at the consumer unit, or modifies the consumer unit itself, is notifiable under Part P of the Building Regulations in England and Wales (and equivalent regulations in Scotland and Northern Ireland). Adding an SPD to your existing consumer unit is alteration work at the board. It's notifiable.

In practice, this means one of two routes:

1. Use a Part P registered electrician (NICEIC, NAPIT, ELECSA, or a similar competent person scheme). They self-certify the work and notify Building Control on your behalf. You receive an Electrical Installation Certificate or Minor Works Certificate plus a copy of the Building Regulations Compliance Certificate from the scheme provider, typically within 30 days. This is the standard route.

2. Notify Building Control yourself before the work and have them inspect. This requires you (or whoever does the work) to provide design, install, and test certification, and pay Building Control's fee. The route exists but is impractical for a homeowner-led job.

You cannot legally do this work yourself unless you're competent and notify it correctly. The penalty for unnotified electrical work is unenforceable in most cases but creates real problems at house sale conveyancing: solicitors ask for certification, missing certificates lead to retrospective inspections, indemnity insurance, or remedial work demands.

For a new extension, the SPD goes in as part of first fix electrics and is captured in the broader Part P certification for the project. There's no separate notification.

Common mistakes

Assuming an SPD is lightning protection. It's not. An SPD handles indirect transients arriving on the supply cable. A direct strike to your roof needs a Lightning Protection System (BS EN 62305): air terminals, down conductors, earth electrodes. The two are complementary, not interchangeable. Most UK homes don't need an LPS. All of them now need an SPD.

Buying a plug-in surge strip and thinking the house is protected. A plug-in strip protects only what's plugged into it, against residual transients close to the equipment. It does not protect the boiler, the cooker, the EV charger, the central heating controls, or anything else hard-wired. The whole-house SPD at the consumer unit is what protects the fixed installation.

Not specifying SPD when buying a new consumer unit. Boards without integrated SPD are still sold. If you tell a wholesaler "12-way dual RCD board" without specifying SPD, you might get one without. Ask for "with integrated Type 2 SPD" explicitly. The cost difference is small. Adding it later costs many times more.

Ignoring the indicator window. The window is the only feedback the device provides. Red means no protection. Plenty of homeowners have an SPD that has tripped during a surge years ago and remains red, providing zero protection until someone notices. Look at it occasionally.

Long earth tails. A Type 2 SPD wired with a one-metre loop of green-yellow earth cable is roughly half as effective as one wired with a 200mm direct run. Cable physics, not aesthetics. Insist on short tails or accept that your protection is degraded.

Running cables behind the SPD module. The DIN slot adjacent to the main switch is the right place for the SPD. Some installers tuck the SPD into a corner slot to "save space," running long busbar tails to reach it. The performance penalty is substantial. The SPD goes next to the main switch.

Treating the EICR C3 as an emergency. A C3 code for missing SPD is advisory. The installation is Satisfactory. You have time to plan a proper consumer unit upgrade rather than a hasty retrofit. Some electricians present the C3 with more urgency than the code warrants because it's a billable opportunity. Don't be rushed.

Mixing manufacturer SPDs and consumer units without checking compatibility. Reg 536.4.203 requires confirmation that protective devices from different manufacturers are mutually compatible. If you're retrofitting a Hager SPD into a Wylex board, the electrician needs to verify suitability. The pragmatic answer is to use the same brand of SPD as the board manufacturer.

Where you'll need this

• First fix electrics - the SPD goes into the consumer unit during first fix, alongside the RCBOs and main switch
• Electrical layout planning - the board specification including SPD type and module count is decided before ordering

These decisions sit on any extension, rewire, or board-change project. The simple rule: if the board is being touched, an SPD goes in. New build, new extension, garage conversion, loft conversion, full rewire, or simple consumer unit upgrade in an unchanged house. The regulatory position is the same in every case, and the cost-benefit is overwhelming when the work is being done anyway.