Surge Protection Devices (SPDs): What Electricians Need to Know Under BS 7671 (2026)

What an SPD Is and What It Protects Against

A Surge Protection Device is a component fitted into an installation to limit transient overvoltages — short, very high voltage spikes lasting microseconds — and divert the resulting surge current safely to earth. It is not a replacement for an RCD or an MCB. Those protect against shock and overcurrent. An SPD protects equipment.

Transients come from two main sources. The first is lightning — a direct strike is rare, but a strike to overhead lines or nearby ground can couple thousands of volts onto the supply. The second, and far more common in everyday installations, is switching events: large inductive loads (motors, pumps, lifts), the grid switching capacitor banks, and faults clearing elsewhere on the network all inject spikes onto the cable. A modern home is full of sensitive electronics — heat pump and EV charger control boards, solar inverters, smart heating, computers, TVs, fridges with PCBs — and a transient that an old resistive load would have shrugged off can quietly degrade or destroy these. SPDs clamp those spikes before they reach the equipment.

The BS 7671 Position: Regulation 443

The requirement for surge protection sits in Section 443 of BS 7671 (18th Edition, Amendment 2). In plain terms, Regulation 443 requires you to determine whether protection against transient overvoltages is needed — and the default route to that decision is a risk assessment.

Historically the regulation listed consequences (loss of human life, public services, commercial activity, large numbers of co-located people) where SPDs were effectively mandatory, and a risk-assessment calculation for everything else. The practical shift that matters for the working spark is this: under the current edition, for the great majority of installations the assessment now points toward fitting an SPD. Rather than running a full calculation on every job, the simpler reading is that you should provide surge protection unless a specific exception applies or the customer makes an informed decision to decline it.

That exception is the de minimis case: where the installation only serves equipment that is neither sensitive to transients nor expensive to replace, and the consequences of a failed bit of kit are trivial, an SPD can reasonably be omitted. A detached single-socket outbuilding feeding a basic light and a strimmer charger is a candidate. A modern home full of electronics, a heat pump, solar PV or an EV charger is not — there, the answer is almost always "fit one".

The key behavioural change: don't silently leave an SPD out to shave the quote. Either fit it, or have the documented conversation and record that the customer declined. The decision has to be a deliberate, recorded one — not an omission.

SPD Types and Where They Go

SPDs are classified by Type, and the Type tells you both the kind of surge it's built to handle and where in the installation it belongs. Getting this right is the part most commonly fudged on site.

Type 1

Type 1 SPDs are designed to handle the partial lightning current from a direct strike. They are fitted at the origin of the installation and are required where the building has a structural Lightning Protection System (LPS) or is fed by an overhead line in an exposed location. Type 1 devices are tested with a 10/350 µs impulse current waveform, which represents a direct lightning event. For most domestic consumer unit changes you will not be fitting a Type 1 — they belong to buildings with a designed LPS, typically commercial, industrial or tall structures.

Type 2

Type 2 is the workhorse and the one you'll fit on the vast majority of jobs. It is installed at the consumer unit or main distribution board and protects against the residual energy that gets past upstream protection and against transients generated within the supply network — i.e. switching surges, the everyday threat. Type 2 devices are tested with an 8/20 µs impulse waveform. When someone says "the board needs an SPD", in a normal house they mean a Type 2.

Type 3

Type 3 SPDs provide fine, local protection and are fitted close to particularly sensitive or valuable equipment — within a few metres of the load. They handle the lower residual let-through voltage that a Type 2 at the board can't fully clamp by the time the spike has travelled down the final circuit. Think a Type 3 in a socket or a fused spur next to a server, a high-end AV rack or sensitive medical/lab equipment. Type 3 is never fitted on its own — it must always sit downstream of a Type 2.

Quick Reference: SPD Types

Coordination of Types

Where more than one Type is installed, they must be coordinated so that they share the surge energy correctly and the downstream device is never overwhelmed before the upstream one operates. The general principle is a cascade: Type 1 at the origin, Type 2 at the board, Type 3 at the load, with each device handling progressively lower residual energy.

Coordination is not automatic. Use devices from the same manufacturer's coordinated range, or follow the manufacturer's coordination tables for minimum cable lengths between devices. If a Type 2 and a Type 3 sit too close together with no decoupling, they can fight each other and the protection performance suffers. Combined Type 1+2 devices simplify the origin where an LPS is present, but you still need to coordinate with any downstream Type 3.

Installation Considerations

The 0.5 metre rule — connecting conductor length

This is the single most important practical detail and the one most often got wrong. An SPD only works if its let-through voltage at the protected equipment is low — and the inductance of the connecting conductors adds voltage during the surge. BS 7671 advises that the total length of the connecting conductors (the line/neutral conductors to the SPD plus the protective conductor from the SPD to the main earthing terminal) should be kept as short as possible, and ideally not exceed 0.5 metres total.

Every extra centimetre of conductor adds inductive voltage drop on top of the SPD's clamping voltage — roughly 1 kV per metre during a fast transient. Long, looped or carefully "tidied" tails wrapped around the enclosure can add hundreds of volts to the protected voltage and partially defeat the device. Keep the runs direct and short, use adequately sized conductors (typically 6 mm² minimum, or as the manufacturer specifies), and if 0.5 m genuinely can't be achieved, use the manufacturer's recommended connection method such as a V-wiring or Kelvin connection arrangement to minimise the effect.

Upstream overcurrent protection / backup fuse

An SPD needs an overcurrent protective device (OCPD) upstream of it — either an integral one, a dedicated backup MCB/fuse, or reliance on the main switch/incomer where the device rating permits. At end of life, or under a fault, the SPD can fail short; the backup OCPD disconnects it so it doesn't become a fire risk or take out the whole installation. Always follow the manufacturer's stated maximum backup fuse rating — fit a device rated above that and the SPD's short-circuit protection is no longer guaranteed.

Consumer unit space and status indication

SPDs take up module width in the board — typically one to four modules depending on the device and whether a separate backup MCB is needed. On a tight replacement board this matters: plan the way count before you order. Most Type 2 SPDs have a status window (green/red flag) showing whether the device is still functional; mount it where it's visible, and tell the customer that a red indicator means the SPD has done its job protecting them once too often and needs replacing — the rest of the installation is fine, but the surge protection is spent.

Recording the Decision on the Certificate

This is the bit that protects you. Whether or not you fit an SPD, BS 7671 requires the decision to be documented. On an Electrical Installation Certificate (EIC) or Minor Works, the design section should record that the requirement for surge protection has been considered, and the outcome.

• If you fit one, note the Type and that protection against transient overvoltage has been provided in accordance with Section 443.
• If the customer declines, record that surge protection was assessed, recommended and declined by the client in writing. Get that decline acknowledged — an email or a signed note on the quote is enough, but it must exist.
• If a de minimis exception applies, note the basis for omission (equipment not sensitive, low consequence) so the reasoning is traceable.

An undocumented omission is the worst outcome. If a transient later wipes out a heat pump and the certificate is silent on surge protection, you're defending a decision you never recorded. A one-line note turns it into a deliberate, defensible call.

Cost Implications for the Customer

Set expectations early so the SPD isn't a surprise line on the invoice. A Type 2 SPD module itself is typically £40–£90 trade, plus a backup MCB if required, plus a little labour and a module or two of board space. On a consumer unit change, adding the SPD usually lands around £80–£150 on the customer's bill — a small fraction of what they'd pay to replace a damaged heat pump controller, inverter or AV system.

Frame it as insurance for their electronics, not as an upsell. When you compare £100-ish against the cost of a fried EV charger or solar inverter, most customers take it without hesitation. The ones who decline have made an informed choice — which is exactly what the regulation wants, provided you record it.

How SPDs Complement RCDs and AFDDs

It's worth being clear with customers that these devices do different jobs and aren't interchangeable:

• RCDs / RCBOs protect people from electric shock and reduce fire risk by detecting earth leakage. They do nothing for transient overvoltages.
• AFDDs (Arc Fault Detection Devices) detect dangerous series and parallel arcing faults in wiring, reducing fire risk from damaged or loose conductors.
• SPDs protect equipment from transient overvoltage spikes. They don't detect leakage or arcing.

A modern, well-specified board increasingly carries all three: RCBOs on the ways for shock and discrimination, AFDDs where required by the risk profile, and a Type 2 SPD at the board for surge protection. They layer up rather than overlap — each covers a hazard the others don't.

FAQ

Is an SPD mandatory on every consumer unit change?

Not literally mandatory in every case, but the practical answer is close to it. Regulation 443 requires the decision to be made by risk assessment, and for typical modern installations that assessment points to fitting one. Omit it only where a genuine de minimis exception applies or the customer makes an informed, recorded decision to decline.

Which Type do I fit on a normal house?

A Type 2 at the consumer unit covers the common domestic case. You only need a Type 1 if the building has a Lightning Protection System or an exposed overhead supply, and a Type 3 only where specific sensitive equipment needs extra local protection downstream of the Type 2.

Why does the 0.5 metre rule matter so much?

Conductor inductance adds voltage during a fast surge — roughly 1 kV per metre. Long connecting tails raise the voltage that reaches the protected equipment and can partially undo the SPD. Keep total connecting conductor length to 0.5 m or under, or use the manufacturer's recommended short-connection method.

What do I write on the certificate if the customer says no?

Record that surge protection was assessed and recommended in accordance with Section 443 and was declined by the client in writing. Keep the email or signed note. Never leave the certificate silent on the matter.

Does the SPD need its own fuse?

It needs an upstream overcurrent protective device — integral, a dedicated backup MCB/fuse, or the incomer where ratings allow — sized at or below the manufacturer's stated maximum backup rating, so the SPD disconnects safely at end of life.