Summary

Surge protection devices have been a requirement — or at minimum a mandatory risk assessment — in BS 7671 since the 18th Edition (2018). The 2022 amendment (BS 7671:2018+A2:2022) strengthened the requirements further, particularly for TT-earthed installations (very common in rural areas with overhead services) and for buildings exposed to direct lightning risk.

Amendment 4:2026 (BS 7671:2018+A4:2026, published 15 April 2026) went further and changed the underlying approach for domestic installations. The old risk-assessment method — Regulation 443.5, which scored the installation against a table of factors — has been deleted. Regulation 443.4.1 now requires SPD protection where an overvoltage could cause (a) serious injury to or loss of human life, or (c) significant financial or data loss; the previous indent (b), which triggered SPD provision automatically for "failure of a safety service," has been removed, and examples such as alarm panels, computers, and home electronics were deleted from the accompanying table. In practice this makes little difference to outcomes for ordinary domestic work, because the amendment's real effect is procedural: a Type 2 SPD is now the default expectation at the origin of most new domestic installations, and it is the installation owner — not a calculated risk score — who must formally declare that protection is not required and accept the risk of damage to equipment. Designers should treat SPD installation as the default and document a positive decision to omit it, rather than the reverse.

The impetus for mandatory SPD consideration in domestic installations came from the increasing volume of sensitive electronics in modern homes — LED drivers, smart home devices, inverters, EV chargers, heat pump controls — all of which can be damaged or destroyed by transient overvoltages that a traditional MCB/RCD will not protect against. A 6kV surge lasts microseconds — far too fast for a circuit breaker to respond — but can destroy semiconductor components instantly.

For electrical contractors, SPD specification is now a routine part of the consumer unit design process. Getting the risk assessment right (and documenting it) is important: if an SPD is required by the assessment and not installed, the installation is non-compliant with BS 7671, and any subsequent surge damage to the client's equipment creates liability for the contractor.

Key Facts

Quick Reference Table

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SPD Type Installation Point Technology Protection Against Typical Imax
Type 1 Service entrance (before main switch) Spark gap or gas discharge tube Direct lightning strike conducted surge 12.5–25 kA (10/350μs)
Type 2 Consumer unit / sub-board MOV (Metal Oxide Varistor) Conducted surges, switching transients 20–40 kA (8/20μs)
Type 3 Equipment socket or din-rail MOV + TVS diode combination End-point protection 1–5 kA (8/20μs)
Combined Type 1+2 Service entrance Spark gap + MOV Both lightning and switching Per component spec

Detailed Guidance

BS 7671:2018+A4:2026 — From Risk Assessment to Presumed Requirement

Prior to Amendment 4:2026, Regulation 443.4 (supported by the now-deleted Regulation 443.5) required the designer to carry out a scored risk assessment, weighing factors such as:

against factors that reduced risk (underground service entry, TNC-S/PME earthing, an existing lightning protection system, urban location). This produced a risk score that determined whether an SPD was required.

A4:2026 has removed this scored methodology. Regulation 443.4.1 now requires SPD protection where an overvoltage could cause serious injury or loss of life, or significant financial/data loss — but for practical purposes, the amendment's real effect on domestic work is procedural: a Type 2 SPD is now the default expectation at the origin of most new domestic installations. To omit one, the installation owner must make a positive declaration accepting the risk of damage to equipment and any consequential loss; the designer is no longer expected to run a scored assessment to justify installing one.

The factors listed above (exposed location, overhead service, TT earthing, high-value equipment, etc.) remain useful for judging severity of consequences and for the conversation with the client about whether to accept the risk of omission — but they are no longer a formal scoring mechanism under BS 7671.

For domestic TT earthing with overhead service: This combination presents an obvious case for installing Type 2 SPD as standard — TT + overhead = high conducted surge risk. Document the specification and any client discussion regardless.

For domestic TNC-S (PME) with underground service (typical new-build suburban house): Under A4:2026 the default position is still to install Type 2 SPD unless the client formally declares they accept the risk of omission. Most contractors install Type 2 as standard practice to protect client electronics and to align with the amended regulation's presumption.

SPD at Consumer Unit — Installation

A Type 2 SPD at the consumer unit is the standard residential installation. Most modern consumer unit manufacturers (Hager, Wylex, MK, Schneider) offer either:

Connection method: The SPD connects between live (L) and earth (PE), and between neutral (N) and earth. Some devices are 3-wire (L-PE, N-PE); others are 4-wire (L-PE, N-PE, and L-N). Connection is directly to the busbar or to the supply side of the main switch.

MCB protection for SPD: A Type 2 SPD is typically connected through a 63A MCB (or as specified by manufacturer) — this MCB protects the wiring to the SPD and provides disconnection in the event of SPD failure. Many modern SPD units incorporate an internal disconnector/fuse — check the manufacturer instructions.

Earth connection:

Lead length rule: The total length of SPD connecting leads (L + PE, or N + PE) must be minimised. BS EN 61643-11 and SPD manufacturers typically state:

Type 1 SPD — When Required

Type 1 SPDs are specified for buildings with an external lightning protection system (LPS per BS EN 62305) or in locations with very high ground flash density. A building with air terminals (lightning rods), down conductors, and earth electrodes as a complete LPS requires Type 1 protection at the service entrance to prevent lightning current that has been partially conducted into the LPS from coupling into the electrical installation.

Type 1 SPDs use spark gap or gas discharge tube technology, capable of handling the high-energy 10/350μs lightning impulse current waveform. Type 2 MOV devices cannot handle this waveform without destruction.

In domestic premises without an LPS, Type 1 is not normally required. The exception is agricultural buildings with overhead services in exposed locations — these should be assessed individually.

Coordination Between SPD Types

Where both Type 2 (at consumer unit) and Type 3 (at equipment) SPDs are installed, they must be coordinated:

SPD Monitoring and Maintenance

SPDs have a limited life measured in impulse current events absorbed. After absorbing their rated energy, MOV-based SPDs degrade silently — they may appear functional but provide no protection.

Most modern Type 2 SPDs include a status indicator (window indicator or LED): green = operational; red = SPD degraded, requires replacement. Some premium units include remote monitoring output.

Maintenance recommendation: inspect SPD status indicator annually during electrical inspection. Replace after any confirmed major surge event (e.g., nearby lightning strike that damaged other equipment).

Frequently Asked Questions

Do I need to install an SPD on every consumer unit upgrade?

Under BS 7671:2018+A4:2026, the default position for most new domestic installations is that a Type 2 SPD is required. To omit one, the client/installation owner must make a positive, documented declaration that they accept the risk of damage to equipment and any consequential loss — the burden has shifted from justifying inclusion to justifying omission. In practice, the cost of a Type 2 SPD (£50–150 for the device) is low relative to the cost of replacing damaged electronics, so most contractors install as standard and rarely seek a declaration of omission.

Will an SPD protect against a direct lightning strike on the building?

A Type 2 SPD at the consumer unit protects against conducted surges from lightning entering through the service cable — but not from a direct lightning strike to the building. A direct strike can deliver tens of kiloamperes of current; a Type 2 SPD rated at 20kA (8/20μs) will be destroyed. Full protection from direct lightning requires an external lightning protection system (BS EN 62305) with Type 1 SPD at the service entrance.

The SPD indicator has turned red — does the whole board need replacing?

No. The SPD itself needs replacing (the MOV has been depleted). The consumer unit remains functional — the SPD failure is a discrete event. Isolate the consumer unit, remove the SPD module, replace with an identical or equivalent rated unit, restore supply. The replacement is Part P notifiable work; ensure the new SPD is BS EN 61643-11 compliant and the installation is recorded on the Electrical Installation Certificate.

Regulations & Standards