An SPD, or Surge Protective Device, is fitted to an electrical installation to limit short-duration voltage surges before they damage wiring or connected equipment. In UK homes it is commonly installed in or alongside the consumer unit and works alongside circuit breakers, RCDs and RCBOs rather than replacing them.
What Does an SPD Actually Do?
Electrical equipment is designed to operate within defined voltage limits. A transient overvoltage is a very brief voltage rise that can exceed those normal levels. The event may last only a tiny fraction of a second, but the voltage can still be high enough to damage electronic components or stress insulation within an electrical installation.
A Surge Protective Device responds when that abnormal voltage appears and limits the voltage passed further into the installation. It provides a controlled path for surge energy and reduces the level of transient overvoltage reaching downstream circuits and equipment.
The IET describes SPDs as protection for the electrical installation and connected equipment against transient overvoltages. They are commonly fitted within a consumer unit or distribution board, although additional protection can also be installed closer to particular equipment where the design requires it.
Where Do Electrical Surges Come From?
A surge does not necessarily mean that lightning has directly struck the property. Transient overvoltages can have natural or electrical causes.
Lightning-related surges
A lightning strike on or near electrical infrastructure can create a transient overvoltage that travels through connected conductors towards a property.
Electrical switching
Switching motors, transformers and other electrical loads can also create transient voltages within electrical systems.
Modern homes contain many devices with electronic controls, including televisions, computers, kitchen appliances, heating controls, chargers and smart equipment. Surge protection therefore concerns both the fixed electrical installation and equipment connected to it.
What Are Type 1, Type 2 and Type 3 SPDs?
SPD types describe where the device sits within the protection arrangement and the type of surge duty it is intended to handle. They are not simply three versions of the same product that can be chosen interchangeably.
Type 1 SPD
Type 1 protection is installed at the origin of an installation where protection against lightning current is required. Combined Type 1 and Type 2 devices are also available.
Type 2 SPD
Type 2 protection is commonly associated with distribution boards and consumer units. Its role is to limit transient overvoltages within the electrical installation before they reach downstream equipment.
Type 3 SPD
Type 3 protection is installed close to the equipment being protected. The IET states that Type 3 protection is supplementary and should be used in conjunction with upstream Type 2 protection rather than as a substitute for it.
Correct selection depends on the installation, incoming supply, earthing arrangement, other protective devices, cable routes and any lightning protection system. Multiple SPDs may also need to be coordinated with one another.
Is an SPD the Same as an RCD, RCBO or Circuit Breaker?
No. These devices deal with different electrical conditions and can appear alongside one another in the same consumer unit.
Swipe left or right to view all columns ↔
This distinction matters when looking at a modern consumer unit. Adding an SPD does not remove the need for the correct RCD, RCBO or overcurrent protection. If you are comparing those other protective devices, the separate guide on
RCBO and RCD protection
explains their different roles in more detail.
Are SPDs Required in UK Homes?
SPD requirements form part of BS 7671, the IET Wiring Regulations, but the correct answer depends on the installation and the edition of BS 7671 being applied to the work. Surge protection should therefore be considered as part of the electrical design rather than added or omitted simply because a property is domestic.
There is also an important standards transition in 2026. BS 7671:2018+A4:2026 was published on 15 April 2026 and may already be used. The IET confirms that the previous BS 7671:2018+A2:2022+A3:2024 remains valid during the six-month transition and is due to be withdrawn on 15 October 2026.
This means an electrician carrying out new installation work, additions or alterations during the transition needs to establish which valid edition is being used and apply its requirements correctly. It is not appropriate to judge an existing installation simply by assuming that every current requirement applied when that installation was originally built.
How Is an SPD Installed in a Consumer Unit?
SPD installation is part of the fixed electrical installation and involves more than placing a spare module into the board. The device has to be suitable for the consumer unit and installation, connected correctly and coordinated with the relevant protective arrangements.
Cable length and routing, earthing arrangements, the SPD type and the manufacturer's installation instructions all affect the design. The IET also confirms under BS 7671:2018+A4:2026 that an SPD must have appropriate short-circuit overcurrent protection. Depending on the SPD design, that protection may be internal or external and must follow the manufacturer's instructions.
For this reason, adding surge protection is not suitable as a homeowner DIY modification inside a live consumer unit.
Can an SPD Be Added to an Existing Consumer Unit?
Sometimes. An existing consumer unit may have suitable physical space and a compatible arrangement for surge protection. Where space is not available, IET guidance notes that an SPD may instead be installed in a suitable enclosure adjacent to the existing consumer unit.
That does not mean every old fuse box should simply have an external SPD added. The existing board, protective devices, earthing and bonding, available space and overall condition of the installation should be assessed first.
If an older board is already being reviewed because of planned electrical work, an electrician can consider surge protection as part of the wider
consumer unit assessment
rather than treating the SPD as an isolated accessory.
How Do You Know If an SPD Is Still Working?
Many consumer-unit SPDs include a visual status indicator. The exact colour or display arrangement is manufacturer-specific, so the marking on the device and the manufacturer's instructions should be followed rather than relying on a universal colour rule.
SPDs are designed to deal with surge events and can eventually reach the end of their serviceable condition. If the device indicates that protection is no longer available, it should be assessed and the relevant replaceable module or SPD replaced in accordance with the manufacturer's instructions.
A homeowner should not remove covers or replace modules inside a consumer unit. The status can normally be observed from the front where the design permits it, while electrical replacement work should be carried out safely with the installation properly isolated.
Is a Surge-Protected Extension Lead the Same Thing?
No. Plug-in surge protection is local protection for connected equipment. It does not perform the same installation-wide role as an SPD correctly selected and fitted at the consumer unit or distribution board.
Local Type 3 protection can form part of a coordinated surge-protection arrangement, but the IET specifically describes Type 3 devices as supplementary to upstream Type 2 protection.
References & Standards
Current technical and standards information used for this article:
Need Your Consumer Unit Assessed?
If you are planning electrical work or reviewing an older consumer unit, its existing protection and suitability for surge protection can be checked before any upgrade is decided.
Book a Consumer Unit Assessment