Surge Protective Devices (SPDs) are now required in the majority of UK domestic installations following the strengthened requirements in BS 7671. This guide covers everything an electrician needs to know — what SPDs are, the risk assessment under Regulation 443.4, Type 1, 2, and 3 devices, earthing considerations, and installation at the consumer unit.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
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A Surge Protective Device (SPD) is a device designed to limit transient overvoltages and divert surge currents safely to earth, protecting the electrical installation and the equipment connected to it. Transient overvoltages are very short-duration voltage spikes — typically lasting microseconds to milliseconds — that can reach thousands of volts and are caused by lightning strikes (direct or nearby) and switching events on the supply network.
Without SPD protection, a transient overvoltage propagating through the supply cable into the installation can damage or destroy any connected electronic equipment. Modern homes contain thousands of pounds worth of electronic devices — smart TVs, computers, broadband routers, smart home controllers, LED lighting drivers, heating controls, washing machine and dishwasher control boards, and EV charger electronics. A single surge event can damage multiple items simultaneously.
SPDs work by providing a low-impedance path to earth for the transient overvoltage. Under normal operating conditions, the SPD has very high impedance and draws no current. When a transient overvoltage exceeds the SPD's clamping voltage (typically 1.5 to 2.5 kV), the internal components switch to a low-impedance state, diverting the surge energy harmlessly to earth. Once the transient has passed, the SPD returns to its high-impedance standby state.
The internal components vary by SPD type but typically include Metal Oxide Varistors (MOVs) and/or Gas Discharge Tubes (GDTs). MOVs respond rapidly (nanoseconds) and clamp the voltage to a safe level. GDTs can handle very high energy surges (such as direct lightning strikes) but have a slightly slower response time. Many SPDs combine both technologies for comprehensive protection.
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BS 7671 Section 443 covers protection against transient overvoltages of atmospheric origin (lightning) and due to switching. Regulation 443.4.1 requires the designer to carry out a risk assessment to determine whether SPD protection is necessary for the installation.
The risk assessment considers the consequences of a transient overvoltage event. Regulation 443.4.1 requires protection against transient overvoltages to be provided where the consequence caused by the overvoltage could result in:
Regulation 443.4.1 originally listed a third consequence, but limb (b) was removed by the BS 7671:2018+A2:2022 Corrigendum (May 2023), leaving the two consequences above.
For all other cases, Regulation 443.4.1 requires protection against transient overvoltages to be provided unless the owner of the installation declares it is not required because any loss or damage would be tolerable, and they accept the risk of damage to equipment and any consequential loss.
In practice, the risk assessment almost always concludes that SPDs should be installed in modern domestic installations. The average UK home now contains electronic equipment worth several thousand pounds, and much of it is essential for daily life (broadband router, heating controls, phone chargers). The cost of a Type 2 SPD — typically £30 to £60 for the device plus £15 to £30 for the back-up MCB — is a fraction of the potential damage cost from a single surge event.
Where the risk assessment determines that SPDs are not required — for example, in a simple installation with no valuable electronic equipment and no safety-critical systems — the designer must document this decision on the Electrical Installation Certificate.
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SPDs are classified into three types based on their location in the installation and their ability to handle different levels of surge energy. The types correspond to specific test categories defined in BS EN 61643-11.
Designed to handle the highest energy surges — direct or nearby lightning strikes. Type 1 SPDs are installed at the origin of the installation, between the supply intake and the main distribution board. They use Gas Discharge Tube (GDT) technology or a combination of GDT and MOV to dissipate very high surge currents (up to 25 kA or more per pole).
When required: Where the building has a lightning protection system (LPS) fitted, where the building has an overhead electricity supply (exposed to direct lightning strikes on the supply line), or where a Type 1 SPD is specified by the risk assessment due to the building's location or use.
The standard SPD for domestic and light commercial installations. Installed at the main distribution board (consumer unit) to protect the entire installation against transient overvoltages induced on the supply by distant lightning strikes and switching events. Uses Metal Oxide Varistor (MOV) technology with typical discharge capacities of 10 to 20 kA per pole.
When required: In the vast majority of installations where the risk assessment identifies the need for SPD protection. This is the default SPD type for domestic consumer units. Many modern consumer units include a dedicated SPD module position, and some come with a built-in Type 2 SPD.
Provides fine protection at the point of use — installed at or near specific equipment that requires additional protection beyond what the Type 2 SPD at the origin provides. Type 3 SPDs have lower energy handling capacity but provide tighter voltage clamping. Common forms include plug-in surge protectors, socket outlet strips with surge protection, and DIN-rail devices in sub-distribution boards.
When required: For equipment that is particularly sensitive to transient overvoltages — medical equipment, IT servers, laboratory instruments, telecommunications equipment. A Type 3 SPD should always be used in conjunction with a Type 2 (or Type 1 + Type 2) at the origin — it is not a standalone solution.
For most domestic installations, a single Type 2 SPD installed at the consumer unit provides adequate protection. Where the building has an overhead supply or a lightning protection system, a Type 1 + Type 2 combination is required. Where specific sensitive equipment needs additional protection, a Type 3 SPD is added at the equipment location.
The earthing arrangement of the installation has a significant impact on SPD selection and installation. SPDs divert surge energy to earth, so the quality and type of earth connection directly affects their performance.
The most common domestic earthing arrangement. SPDs are connected between line and earth, and between neutral and earth. The low impedance of the PME earth provides an effective surge diversion path. However, the SPD must be compatible with the PME system — some SPD configurations can draw neutral current to earth, which may affect RCD operation. Use SPDs specifically designed for TN-C-S systems.
Separate neutral and earth throughout. SPD installation is straightforward — the dedicated earth conductor provides a reliable, separate path for surge diversion. SPDs are connected between line and earth, and between neutral and earth. TN-S systems generally provide the best SPD performance because the earth path has low impedance and is independent of the neutral.
Earth via an electrode in the ground. TT systems present specific challenges for SPD installation because the earth electrode resistance is typically much higher than in TN systems. When the SPD operates and diverts surge current to earth through the electrode, the voltage rise at the electrode can be significant. This can cause RCDs to trip. SPDs for TT systems must be carefully coordinated with RCD protection — see the RCD coordination section below.
The earthing arrangement must be verified on site before specifying the SPD. The SPD manufacturer's installation instructions will specify the connection requirements for each earthing arrangement. Getting this wrong can result in the SPD not operating correctly, or causing nuisance tripping of protective devices during surge events.
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Start free trialThe SPD must be installed as close to the origin of the installation as possible — at or within the main consumer unit or distribution board. This minimises the length of the connection between the SPD and the main earthing terminal, which is critical because the inductance of long connecting leads reduces the effectiveness of the SPD.
BS 7671 Regulation 534.4.4 requires that the total length of the connecting conductors (from the point of connection on the line/neutral bus-bars to the SPD, and from the SPD to the earth bus-bar) should not exceed 0.5 metres. This is known as the "0.5m rule" and is one of the most commonly overlooked installation requirements. If the connecting leads are too long, the voltage drop across them during a surge event means the voltage at the equipment is higher than the SPD's clamping voltage — defeating the purpose of the protection.
Many modern consumer units from manufacturers such as Hager, Schneider, and Siemens include a dedicated SPD module position with direct bus-bar connections, eliminating the lead length issue entirely. When specifying a new consumer unit, choosing one with an integrated SPD position simplifies installation and ensures compliance with the 0.5m rule.
Every SPD installation requires a dedicated back-up protective device — a separate MCB or fuse — on the supply to the SPD. This back-up device serves two important functions.
First, it provides overcurrent protection for the SPD. If the SPD fails short-circuit (which MOV-based SPDs can do at end of life), the back-up device disconnects the SPD from the supply, preventing a sustained short circuit at the origin of the installation. Without back-up protection, a failed SPD could cause the main switch or the supply fuse to trip, disconnecting the entire installation.
Second, it provides a means of isolation for the SPD, allowing it to be replaced without disconnecting the entire installation.
The back-up MCB or fuse must be rated according to the SPD manufacturer's instructions — not arbitrarily chosen. Most domestic Type 2 SPDs specify a 32A or 40A back-up MCB. If the back-up device is rated too low, it may trip during a surge event before the SPD has finished diverting the surge energy, leaving the installation unprotected. If rated too high, it may not disconnect a failed SPD quickly enough.
Some consumer unit manufacturers integrate the SPD and its back-up protection into a single plug-in module, eliminating the need for a separate MCB. This is the simplest and most reliable approach when available.
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Try it free for 7 daysSPD operation can cause RCDs to trip, particularly on TT earthing systems. Understanding why this happens and how to prevent it is essential for reliable SPD installations.
When a transient overvoltage occurs and the SPD operates, it diverts a large surge current to earth in a very short time. On TN systems, this current flows through the neutral-earth bond and does not create an imbalance that the RCD would detect. However, on TT systems, the surge current flows through the earth electrode, and the transient nature of the current can create a momentary imbalance that causes the RCD to trip.
Additionally, some SPD technologies (particularly MOV-based devices) can produce a brief follow-through current after the surge has passed — this is mains-frequency current that flows through the SPD for a few milliseconds while the MOV returns to its high-impedance state. If this follow-through current flows to earth, it creates exactly the type of imbalance that an RCD is designed to detect.
The SPD manufacturer's installation manual will specify the recommended RCD coordination method for each earthing arrangement. Always follow the manufacturer's guidance — installing an SPD without considering RCD coordination can result in the main RCD tripping every time there is a storm, disconnecting the entire installation and leaving the occupants without power.
Elec-Mate's EIC and EICR forms include dedicated fields for SPD type, manufacturer, connection configuration, back-up protection rating…
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