What is a surge protection device (SPD)?

Glossary · Consumer units and protection

How a surge protection device works

A surge protection device protects the electrical installation, meaning the consumer unit, wiring and accessories, from transient overvoltages. It also protects sensitive equipment connected to the installation, such as computers, televisions and washing machines.^{[1]IET, Surge Protective Devices (SPD)} The IET says the effect of a surge can be instant failure, or damage that only shows over a longer period.^{[1]IET, Surge Protective Devices (SPD)} You can read how this looks at the socket on the power surge page.

A transient overvoltage is a short surge of electricity. The IET says natural ones come from indirect lightning strikes, for example a strike on an overhead power or telephone line nearby. Man-made ones come from the switching of motors and transformers and some types of lighting. It adds that electric vehicle charging, heat pumps and speed-controlled washing machines have made transients more likely in homes.^{[1]IET, Surge Protective Devices (SPD)}

Types of SPD

The IET describes three types.^{[1]IET, Surge Protective Devices (SPD)}

Combined Type 1 and 2 devices are available and are usually installed in consumer units.^{[1]IET, Surge Protective Devices (SPD)} The IET also notes that SPDs for other incoming services, such as telephone lines and cable TV, exist, because protecting the electrical installation alone can leave another route for surges to get in.^{[1]IET, Surge Protective Devices (SPD)}

Do you need one?

The IET says BS 7671:2018 states that, unless a risk assessment is carried out, protection against transient overvoltage shall be provided where the consequence could, for example, be serious injury or loss of life. The old exception for some domestic dwellings, for example those with an underground supply, was removed. The IET says this applies to new builds and to properties being rewired.^{[1]IET, Surge Protective Devices (SPD)} A second IET page says protection must be provided within single dwellings unless the value of the installation and equipment does not justify it.^{[2]IET, BS 7671 FAQs: Consumer Units and Protective Devices}

The rules are not retrospective. The IET says the decision to fit SPDs is in the customer’s hands, and the customer should be given enough information to decide whether to omit them.^{[1]IET, Surge Protective Devices (SPD)} The IET says an SPD can go in an existing unit if space allows, or in an external enclosure beside it.^{[1]IET, Surge Protective Devices (SPD)} NICEIC lists SPDs among the safety devices found in a modern consumer unit.^{[4]NICEIC, Consumer units and fuse boxes: installation and replacement}

Amendment 4 of BS 7671 is now the current edition. The IET’s summary of what it changed covers batteries, functional earthing, power over ethernet and medical locations. It does not list a change to surge protection.^{[3]IET, Ensure you are up to date with BS 7671}

Example

Aisha in Chapel Allerton has her unit replaced. The electrician explains what an SPD does and what it protects, including her computers, television and boiler controls. She asks her insurer first, as the IET advises, and then decides whether to include it.

Another customer on the same street has a consumer unit with no spare space. The electrician explains that the SPD could go in a separate enclosure next to it, which is one option the IET describes.

What people get wrong

Many people think a surge only comes from a storm. It does not. Everyday switching of motors and transformers, and newer equipment like EV chargers and heat pumps, can cause transients too.

The IET lists those sources alongside indirect lightning strikes.^{[1]IET, Surge Protective Devices (SPD)} The second mistake is to treat an SPD as optional paperwork. The IET says the regulation applies to all types of premises, including dwellings, and that omitting it is a customer decision made with enough information, not a default.^{[1]IET, Surge Protective Devices (SPD)}

The bottom line

An SPD guards a home’s wiring and sensitive electronics from short voltage surges. The IET says BS 7671:2018 expects it unless a risk assessment says otherwise, and that fitting or omitting it is for the customer to decide once informed. It goes in the consumer unit if there is space, or in a box beside it. Fitting one is work for a registered electrician.