Electrical

SPDs: Surge Protection Devices

A surge protection device diverts transient overvoltages — from lightning or switching — safely to earth before they reach your equipment, and BS 7671 Regulation 443 sets out when the risk means you have to fit one.

Updated 20 Aug 20268 min read

Transient overvoltages last microseconds but reach thousands of volts, and they arrive from lightning strikes on or near the supply network and from switching events on the grid. They damage the increasingly sensitive electronics in modern buildings — controls, IT, appliances — and Surge Protection Devices are the means of clamping them before they do harm.

This guide explains what an SPD does, the difference between Type 1, Type 2 and Type 3 devices and where each belongs, and the Regulation 443 risk assessment that determines whether protection is required for a given installation.

Key takeaways

An SPD limits transient overvoltages by diverting the surge current to earth, protecting downstream equipment.
Type 1 handles direct lightning currents at the origin (buildings with a lightning protection system); Type 2 handles switching and indirect surges at the distribution board; Type 3 protects sensitive equipment locally.
Types are often combined — a Type 1+2 device at the origin, Type 3 near delicate loads.
Regulation 443 decides when an SPD is required, based on the consequences of an overvoltage and, in some cases, a risk calculation.
Where required, the SPD needs correct connection and its own overcurrent protection per the manufacturer and BS 7671.

What a surge is and what an SPD does

A transient overvoltage — a surge — is a very short spike, microseconds long, that can reach many times the normal supply voltage. The two main sources are lightning (a direct strike or, far more often, a strike near the overhead network coupling a surge onto the line) and switching transients from the grid and large loads switching on and off. Either way, the spike travels down the supply into the installation.

An SPD sits between the live conductors and earth and stays effectively invisible at normal voltage. When a transient arrives, the SPD's components conduct almost instantly, diverting the surge current to earth and clamping the voltage to a level the downstream equipment can survive. Once the transient passes, the SPD returns to its high-impedance state. It's a diverter, not a filter — it shunts the energy away rather than blocking it.

Types 1, 2 and 3

The three types handle progressively smaller surges closer to the equipment. Type 1 SPDs are for the origin of installations where a direct lightning strike is a risk — principally buildings with a lightning protection system (LPS) — and are built to handle the partial lightning current itself. Type 2 SPDs are fitted at the main or sub-distribution board and handle switching transients and the residual surges that get past a Type 1 or arrive without a direct strike; they're the workhorse for most installations. Type 3 SPDs are local, installed close to sensitive equipment, and deal with the lower-energy transients that remain — used in addition to a Type 2, never on their own.

In practice the types are coordinated and often combined. A common arrangement is a Type 1+2 combined device at the origin of a building with an LPS, then Type 3 units at sensitive equipment. A typical installation without an LPS uses a Type 2 at the consumer unit, with Type 3 added where particularly delicate electronics warrant it. Coordination matters: the upstream device takes the big hit, the downstream device cleans up what's left.

Type by position

Type 1 at the origin against direct lightning (LPS buildings); Type 2 at the board for the everyday surges; Type 3 local to delicate kit. Type 3 always works alongside a Type 2, not instead of it.

Regulation 443: when an SPD is required

BS 7671 Regulation 443 governs whether protection against transient overvoltages is needed. The decision turns on the consequences of an overvoltage. Protection is required where the consequence could affect human life (for example safety services, or medical care in a building), public or commercial services, or a large number of co-located individuals — and for other cases the decision is made on a risk basis.

For those other cases, Regulation 443 provides a risk assessment: a calculated risk level (based on the length of the supply line and regional lightning activity) is compared against a tolerable threshold, and if the calculated risk exceeds it, SPD protection is required. There's also the practical route of deciding to provide protection regardless where the cost of the SPD is low relative to the value and importance of the equipment it protects — often the sensible call in domestic work.

Consequence first, then risk

If an overvoltage could affect life, key services or lots of people, an SPD is required outright. Otherwise Reg 443 gives you a risk calculation — or you can simply fit one because it's cheap insurance.

Installing an SPD correctly

An SPD only works if it's connected and protected properly. It needs its own overcurrent protection unless the upstream device already provides it within the manufacturer's stated rating, and it must be connected according to the earthing system (the connection configuration differs between TN and TT arrangements).

Connection lead length is critical and easily overlooked: the total length of the connecting conductors should be kept as short as possible — commonly aiming for well under half a metre — because the inductance of long leads adds voltage during the surge and undermines the protection. Follow the manufacturer's wiring configuration and lead-length guidance; an SPD wired with long, looping tails protects far less than the same device installed tightly.

SPDs alongside other protection

An SPD protects against overvoltage — it does nothing for overcurrent, earth faults or arcing, so it complements the MCBs, RCDs and any AFDDs rather than overlapping them. In a modern consumer unit it typically appears as a dedicated SPD module at the origin, with its own connection to the busbar and earth.

The trend is towards fitting SPDs as standard, especially given the value of connected electronics in a typical building and the relatively low cost of a Type 2 device. Where Regulation 443 requires protection it's mandatory; where it doesn't, it's frequently still worth recommending, framed to the client as inexpensive protection for expensive and safety-relevant equipment.

Frequently asked questions

What does a surge protection device do?

It diverts transient overvoltages — short, high-voltage spikes from lightning or grid switching — safely to earth, clamping the voltage to a level downstream equipment can survive. It's a diverter that conducts only during the surge, then returns to being effectively invisible.

What's the difference between Type 1, 2 and 3 SPDs?

Type 1 handles direct lightning current at the origin of buildings with a lightning protection system. Type 2 handles switching and indirect surges at the distribution board — the workhorse for most installs. Type 3 is a local device for sensitive equipment, always used alongside a Type 2.

When does BS 7671 require an SPD?

Regulation 443 requires protection where an overvoltage could affect human life, key public or commercial services, or many co-located people. For other cases it provides a risk assessment based on supply line length and lightning activity, or you can fit one anyway where it's cheap relative to the equipment protected.

Why does SPD connection lead length matter?

Long connecting leads add inductance, which raises the voltage the equipment sees during a surge and undermines the protection. The connecting conductors should be kept as short as possible — commonly under half a metre total — following the manufacturer's guidance.

From guidance to action

Electrical Installation CertificateRecord SPD provision and its details on a new installation certificate.