What is an SPD (surge protection device) and do I need one? (UK guide)

The short answer

An SPD (surge protection device) is a component wired into a consumer unit that protects the circuits and connected equipment from transient overvoltages — brief, high-energy voltage spikes on the supply network caused by events such as lightning strikes, utility switching operations, or large inductive loads switching off nearby. The SPD clamps the spike by diverting its energy to earth before it can travel down the circuit cables to your equipment. Under BS 7671 18th Edition (Amendment 2, 2022), an SPD is required on most new or replacement domestic consumer units unless a risk assessment specifically concludes it is not needed — which means in practice that virtually all new and replacement boards in the UK now include one. Adding an SPD to an existing board typically costs in the region of £80–£150 including fitting, depending on the type and the electrician's rates.

SPDs were once considered optional extras or commercial requirements. Since Amendment 2 to BS 7671 18th Edition they are effectively standard for new and replacement domestic consumer units. This page explains what they do, how they are classified, and how to assess whether you should add one to your existing installation.

SPD key facts

Full nameSurge protection device
Protects againstTransient overvoltages (voltage spikes)
How it worksClamps spike, diverts energy to earth
Required byBS 7671 18th Edition Amd 2 for most new boards
Typical add-on cost~£80–£150 including fitting (indicative)
ClassificationType 1, Type 2, Type 3 (by installation position)

What causes transient overvoltages and why they matter

The UK electricity supply is nominally 230 V AC, but the voltage on the network is not perfectly smooth. Brief spikes — lasting microseconds to milliseconds but potentially reaching hundreds or thousands of volts — can be induced on the cables by:

Lightning: a strike on or near the overhead network can couple a voltage spike into the supply cable. A direct strike to a building's lightning protection system can also inject energy into the earthing system, which can propagate through the installation.
Utility switching: operations on the distribution network — capacitor banks switching, fault clearing — generate transients that propagate through the supply.
Internal switching: large inductive loads such as motors and transformers being switched off generate back-EMF spikes; these are usually lower in amplitude but more frequent.

Modern electronics — computers, televisions, routers, EV charger control boards, smart-home devices — are increasingly sensitive to these spikes. The semiconductor components inside them can be damaged or destroyed by voltages well below those that would trip a circuit breaker or blow a fuse, because the damage mechanism is different: the spike destroys insulation or junction material in microseconds before any overcurrent device can respond.

How an SPD works

Most consumer unit SPDs contain a component called a metal oxide varistor (MOV) — a semiconductor device whose resistance drops dramatically when the voltage across it exceeds a threshold. In normal operation, the MOV presents a very high resistance and carries negligible current. When a spike arrives, the MOV's resistance falls, it conducts the spike current to earth, and the voltage at the consumer unit terminals is clamped to a safer level.

SPDs are classified by where in the system they are installed:

Type 1: installed at the service entrance, between the supply and the consumer unit, typically where there is a lightning protection system. Handles the highest energy levels.
Type 2: the most common domestic type, installed inside or adjacent to the consumer unit. Handles the energy remaining after a Type 1, or stands alone where there is no lightning protection.
Type 3: point-of-use devices (in socket strips, for example), providing supplementary protection close to sensitive equipment.

For a typical domestic installation without a lightning protection system, a Type 2 SPD at the consumer unit is what BS 7671 now requires.

A note on SPD lifespan: an SPD absorbs energy when it operates. After a significant surge, or after many smaller surges, the MOV material degrades and the device offers less protection. Most consumer unit SPDs include an indicator window (green = healthy, red = end of life) or a remote signalling terminal. If your SPD indicator shows red, the device needs replacing — it will no longer clamp future spikes.

When BS 7671 requires an SPD — and when it does not

Amendment 2 to BS 7671 18th Edition (published 2022) introduced Regulation 443.4, which requires the installation of protection against transient overvoltages where the consequences of overvoltage could result in serious injury to, or loss of, human life, or could result in significant financial or data loss. For most domestic installations the amendment requires either an SPD or a documented risk assessment concluding that one is not needed.

In practice, the risk assessment threshold is rarely met for domestic properties — the factors that would justify omitting an SPD (very short cable runs entirely underground, remote location with no overhead supply, very low consequence of equipment loss) do not apply to most UK homes. As a result, registered electricians installing or replacing a consumer unit will almost always include an SPD as standard.

If you have an older board that predates Amendment 2, there is no retrospective requirement to retrofit an SPD — but it is worth considering if you have sensitive or expensive electronics in your home.

Frequently asked questions

Is an SPD the same as a surge protector plug strip?

Not quite. A consumer unit SPD (Type 2) is a permanent device wired into the board that protects all circuits in the installation. A plug-in surge protector strip is a Type 3 device providing supplementary point-of-use protection for the equipment plugged into it. They work on the same MOV principle but are installed at different points in the system and handle different energy levels. For comprehensive protection, a board-level Type 2 SPD is the primary measure.

How long does an SPD last?

An SPD does not have a fixed service life like a battery. How long it lasts depends on how many transients it has absorbed and their magnitude. The indicator window on the device shows its status; once it shows the end-of-life signal the device should be replaced. A board-level SPD that has never seen a significant surge may last many years; one that has absorbed several large spikes may reach end of life sooner.

Does adding an SPD protect my appliances against all power problems?

No. An SPD protects against brief transient overvoltages — spikes. It does not protect against sustained overvoltage (which would be a DNO supply fault), undervoltage, or power cuts. For sustained overvoltage protection, a different type of device (voltage monitoring relay) would be needed. Most appliances tolerate normal supply variation without damage; transient spikes are the specific risk an SPD addresses.

Sources & further reading

Figures on this page are typical UK ranges drawn from published sources and depend on your specific property and installation. They are guidance, not a quotation.