Protection · 21 July 2026 · For contractors and project engineers
A surge protective device (SPD) exists to catch the brief, high-voltage spikes — from lightning or from switching on the network — that can destroy electronics in a fraction of a second. Amendment 2 to the 18th Edition changed when you have to fit one, and the change was significant: the old risk-assessment escape route is gone, and far more installations now need protection.
Under normal conditions an SPD does nothing. When a transient overvoltage appears, it conducts briefly to earth, clamping the voltage to a safe let-through level and diverting the surge energy away from the equipment downstream. It protects the insulation and the electronics; it is not a substitute for the fuse or RCD, which handle power-frequency faults.
Regulation 443 was rewritten. The calculated risk-level method — the arithmetic that let you decide an SPD wasn't needed — was removed. Now protection is required where the consequence of a transient overvoltage could be:
And for all other cases, an SPD "shall be fitted" unless the owner of a single dwelling declines it. In practice that means most new work — including homes, which lost their previous exemption — now includes surge protection.
| Type | Protects against | Where |
|---|---|---|
| Type 1 | Partial direct-strike lightning current | Origin, where an external lightning protection system exists |
| Type 2 | Switching and indirect lightning surges | At/near the origin — the common device |
| Type 3 | Residual surges near equipment | Close to sensitive equipment, as a final stage |
Combined devices (for example Type 1+2) are common. On a building with a lightning protection system you'll typically need Type 1 at the origin; most ordinary installations use a Type 2 at the main board, sometimes with Type 3 units local to critical equipment.
SPD provision is part of the protection and coordination design for a board, alongside RCD selection, discrimination and the cable checks. Deciding where SPDs are required across a distribution system, and coordinating the Types, is something we fold into a full design package.
Under BS 7671:2018+A2:2022, in most cases yes. Protection against transient overvoltage is required where the consequences could be serious — injury or loss of life, failure of a safety service, or significant commercial, financial or data loss. For all other situations an SPD must be fitted unless the owner of a single dwelling specifically declines it.
Yes. The earlier calculated risk-level (CRL) assessment route in Regulation 443 was removed in Amendment 2. You now decide on the basis of consequences, not a numerical risk calculation, which in practice means SPDs are fitted far more often than before.
No longer. Amendment 2 removed the special dispensation for single dwellings, so they are treated like every other installation. An SPD is fitted unless the homeowner declines the protection, and that decision should be recorded.
Type 1 handles the partial lightning current from a direct strike and is used where there is an external lightning protection system. Type 2 is the workhorse fitted at or near the origin of the installation to protect against switching and indirect lightning surges. Type 3 is fitted close to sensitive equipment as a final stage. They are often combined (e.g. a Type 1+2 device).
With very short connecting conductors — the total lead length should ideally be no more than 0.5 m, because every extra centimetre adds volts to the let-through during a surge. It needs backup overcurrent protection (unless within the manufacturer's declared rating), and the installation must carry a notice indicating that SPDs are fitted.
For the exact wording, see BS 7671 Regulations 443 and 534 and the IET's Wiring Matters guidance on surge protection.