Surge Protective Devices Section 443 — under BS 7671:2018+A4:2026

Section 443 of BS 7671:2018+A4:2026 sits inside Part 4 — Protection for Safety — and deals exclusively with one threat: transient overvoltages. A transient overvoltage is a short-duration spike, typically lasting microseconds, that travels along the supply cables or down the equipotential bonding system and lands on the terminals of equipment connected to the installation. The two sources Section 443 cares about are atmospheric (a lightning strike to the distribution network, the structure or nearby ground) and switching (high-current breaks elsewhere on the network, large inductive loads cycling, capacitor switching).

Regulation 443.4.1 says protection shall be provided where the consequence of an overvoltage could be serious injury or loss of life, or significant financial or data loss, and in all other cases unless the owner declares it is not required and accepts the risk. The mechanism that delivers that protection is then governed by Section 534 of the standard — how the SPD is selected, where it is installed, how it is connected, and how multiple SPDs are coordinated.

In BS 7671:2018+A4:2026 the requirement sits in Regulation 443.4.1 (transient overvoltages from indirect lightning) and 443.4.2 (switching overvoltages from equipment such as motors, transformers and capacitor banks), with the rated impulse voltages of equipment in Table 443.2. Regulation 443.5, which carried the old risk calculation, was deleted by Amendment 2 (2022). Amendment 4 deleted Annex B443 but did not reintroduce a calculation.

For a fuller picture of how A4:2026 affects the rest of the standard, see our BS 7671 A4:2026 summary. For the SPD device itself in isolation — what it is, how it clamps, and how it differs from an MCB — see our SPD guide and the broader power surge protection explainer.

BS 7671:2018 as first published let the designer decide by calculation. Regulation 443.5 set out a Calculated Risk Level (CRL), based on the supply length, the environment and the lightning flash density, and SPDs were only needed where the CRL fell below a set threshold. Many electricians used it to justify leaving SPDs out of domestic consumer units.

Amendment 2 (2022) deleted Regulation 443.5, its figures and Table 443.1. What replaced it is the rule now in Regulation 443.4.1, and Amendment 4:2026 keeps it unchanged:

1. Protection against transient overvoltages shall be provided where the consequence could result in (a) serious injury to, or loss of, human life, or (c) significant financial or data loss. Limb (b) was deleted by the A2:2022 Corrigendum (May 2023).
2. For all other cases, protection shall be provided unless the owner of the installation declares it is not required, because any loss or damage is tolerable and they accept the risk of damage to equipment and any consequential loss.
3. Separately, Regulation 443.4.2 asks for protection against switching overvoltages to be considered where equipment such as motors, transformers, capacitor banks, storage units or high current loads could produce disturbances above the rated impulse voltages in Table 443.2.

Putting Regulation 443.4.1 into plain English for working electricians: protection is mandatory where the consequence of an overvoltage could result in serious injury or loss of human life, or significant financial or data loss. For all other cases, SPDs must still be fitted unless the owner declares the risk tolerable in writing.

In domestic practice, the practical answer for most A4:2026 consumer unit replacements is: fit SPDs unless the client has been told what they protect against and has declined them in writing. That position lines up with what most insurers and CPS schemes are now expecting on a 2026-onwards installation, and it means the documentation trail at handover is clean.

BS 7671:2018+A4:2026 retains the three-tier classification of low-voltage SPDs, tested and declared to BS EN 61643-11. The Type number is not a quality grade — it describes the test waveform the device is qualified against, and therefore the kind of event the device is designed to handle and the position in the installation where it is meant to live.

1. Type 1 SPD — tested with a 10/350 µs current waveform that simulates a partial direct lightning current. Required as close as possible to the origin where the structure has an external lightning protection system or protection against the effects of direct lightning (Regulation 534.4.1.3). Typically declared with an Iimp (impulse current) rating in kA.
2. Type 2 SPD — tested with an 8/20 µs current waveform that simulates an induced transient, including the residual of a strike further away on the network. This is the standard origin-of-installation device for the vast majority of UK installations and is declared with an In (nominal discharge current) and Imax (maximum discharge current). Most consumer-unit-mounted SPDs are Type 2.
3. Type 3 SPD — tested with a combination waveform and intended for the finer level of protection close to sensitive equipment. Type 3 devices live downstream of the main Type 2 device — at a sub-board, in an equipment rack, or as a socket-outlet or plug-in unit serving an item of high-value electronics.
4. Combined-type devices — many modern SPDs are declared as Type 1+2 or Type 1+2+3. These are convenient at the origin of an installation that has an LPS overhead, because one chassis covers the impulse and the induced events.

Selecting the right Type starts with the question: is there a Lightning Protection System? If yes, you need Type 1 (or combined 1+2) at the origin. If no, you typically need Type 2 at the origin. Then, separately, decide whether you also need Type 3 close to sensitive equipment. The two decisions are independent — fitting Type 3 alone, without a Type 2 upstream, leaves the Type 3 device exposed to events well beyond its rating and will not deliver compliant protection.

For a deeper dive into device construction and how the metal-oxide varistor (MOV) or spark-gap technology actually clamps the transient, see our SPD surge protection article.

Table 443.2 sets out four overvoltage categories (OVC I to OVC IV) that describe the level of transient overvoltage a piece of equipment must withstand at its terminals before it is considered damaged. The categories are progressive: OVC IV equipment is built to withstand more than OVC III, which is built to withstand more than OVC II, and so on. Selecting the right SPD at the origin is then a matter of clamping the transient down to a residual voltage Up at the equipment terminals that is below the equipment's declared withstand category.

The practical relevance for SPD selection is that the residual voltage Up declared on the device must be lower than the impulse withstand voltage Uw of the equipment it protects, with an appropriate margin for the connecting conductors. For a 230/400 V installation, Table 443.2 gives 6 kV for category IV, 4 kV for III, 2.5 kV for II and 1.5 kV for I. An SPD with a declared Up of 1.5 kV at the origin, plus a well-controlled installation with short connection conductors, will sit comfortably below that limit.

Section 534 ("Devices for protection against transient overvoltages") is where the regulation gets unforgiving. A perfectly specified Section 443 SPD will deliver no protection at all if it is installed with the wrong conductor length, the wrong topology, or the wrong upstream device. The headline rule electricians need to remember:

1. Use parallel ("V-shape") connection at the busbar — the SPD is teed off the live conductor and bonded to the main earthing terminal with the shortest practicable conductors. Do not loop the live or neutral through the SPD; tee it.
2. Keep the total lead length preferably within 0.5 m and never over 1.0 m (Regulation 534.4.8). Earthing conductors must be at least 6 mm² copper for a Type 2 SPD at the origin, or 16 mm² for Type 1; line connections at least 2.5 mm² (Type 2) or 6 mm² (Type 1) (Regulation 534.4.10).
3. Provide a backup overcurrent device upstream of the SPD, selected according to the SPD manufacturer's declared maximum backup overcurrent rating. This protects the SPD against end-of-life short-circuit failure and ensures the installation's prospective fault current does not exceed the SPD's declared Iscr (short-circuit current rating).
4. Where the upstream device is shared with the installation supply, confirm that the discrimination between the backup OCD and downstream devices is acceptable — an SPD failure should not trip the entire supply.
5. Earth the SPD to the main earthing terminal of the installation, not to a separate rod or to a structural steel that has not been confirmed as bonded to the MET. The SPD only works if its earth reference is the same as the equipment it is protecting.
6. Coordinate the SPD with RCD protection: where the SPD is on the load side of an RCD, that RCD must have a surge current immunity of at least 3 kA 8/20 (Type S RCDs meet this), and Type 1 SPDs downstream of an RCD are not recommended (Regulation 534.4.7).

For installations that need protection at multiple levels — origin, sub-board and equipment — Section 534 requires that the SPDs be coordinated. Coordination means that when a transient arrives at the origin, the upstream device clamps the bulk of the current and the downstream device handles only the residual that gets past. The two devices must share the load in a way that is consistent with both manufacturers' declared performance.

In domestic and small-commercial practice, most installations need only a single Type 2 (or combined Type 1+2) at the origin. Cascade only becomes a routine design question on larger sites with multiple sub-boards, in installations that include particularly sensitive electronics, or where part of the building has been retrofitted with on-site generation.

Installations with solar PV, battery storage and bidirectional EV charging need surge protection thought through on both the AC and DC sides. The AC origin device is chosen under Regulation 443.4.1 and Section 534 like any other installation; the DC side follows Section 712 for PV and the equipment manufacturers' instructions.

One of the most frequently asked questions from inspectors since Amendment 2 introduced strengthened SPD requirements has been: how do I code an installation that does not have surge protection? The answer under A4:2026 is unchanged in principle but tighter in practice. The codes available are C1 (immediate danger), C2 (potentially dangerous), C3 (improvement recommended) and FI (further investigation required). The right code depends on what the installation actually contains and what the consequence of a transient event would be.

There is no fixed code for a missing SPD; the inspector judges the risk. Record why you chose the code, and note any owner declaration declining SPDs if one exists.

Pulling together the regulation-level changes A4:2026 introduces into Section 443 and Section 534:

1. Amendment 2 (2022) deleted the Calculated Risk Level calculation (Regulation 443.5 and Table 443.1). Since then Regulation 443.4.1 requires protection unless the owner declares it is not required; Amendment 4:2026 keeps that rule.
2. Amendment 4:2026 deleted Annex B443, the informative guidance that accompanied Section 443.
3. The Section 534 rules are unchanged in substance: lead length preferably within 0.5 m and never over 1.0 m (534.4.8), minimum conductor sizes (534.4.10) and RCD coordination (534.4.7).
4. The model forms have carried SPD fields since Amendment 2, so the presence, type and status of SPDs are recorded on EICs and EICRs.

Elsewhere, A4:2026 brings protective neutral bonding (PNB), Table 52.1 and a new Chapter 57 for batteries. The AFDD requirement in Regulation 421.1.7 dates from A2:2022, with A4:2026 only rewording the high rise premises entry. See our BS 7671 AFDD rules guide and the broader BS 7671 A4:2026 summary.

An SPD is not a tested device in the same sense as an RCD — there is no equivalent of an RCD trip test that an electrician performs on energisation. What there is, however, is a body of checks at commissioning that confirm the device is fitted correctly, that the upstream backup OCD is in place and the right size, and that the SPD's status indicator is reading healthy. Those checks belong on the Schedule of Inspections.

1. Visual inspection — confirm the SPD is the correct Type (1, 2, 1+2 or 3) for its position in the installation, that its Uc (maximum continuous operating voltage) is appropriate for the system voltage, and that its status window reads healthy.
2. Connection inspection — confirm the live and earth connecting conductors are as short as practicable, parallel-connected at the busbar, and that the total length is below 1 m. Photograph the installation for the records.
3. Backup OCD verification — confirm the upstream backup overcurrent device is in place and sized in accordance with the SPD manufacturer's declared maximum backup OCD. Record the type and rating on the schedule.
4. Earth path verification — confirm the SPD's earth lead lands on the main earthing terminal of the installation, not on a separate rod or a non-bonded structural item.
5. RCD coordination — verify the SPD is upstream of the RCD/RCBO protection wherever this is achievable. Where the SPD is downstream of an RCD, confirm the RCD is Type S or otherwise rated for the SPD's discharge current and will not nuisance-trip.
6. Documentation — record the SPD make, model, Type, Uc, Up, In, Iimp (where applicable), and the backup OCD type and rating on the EIC schedule of inspections. Hand the client the SPD manufacturer's data sheet alongside the certificate.

For consumer-unit-mounted SPDs, the device is part of the same enclosure as the rest of the assembly, and the new A4:2026 EIC model form makes provision in the schedule of inspections for the SPD entry to be ticked off. Make sure this is captured at energisation rather than being treated as an extra item the electrician scribbles on the back of the form.

Unlike most other parts of the installation, an SPD is consumable. Each transient event the device clamps degrades its internal varistors slightly, and large events can take the device out of service in a single hit. Section 534 therefore expects the device to have a clear status indicator visible without dismantling the enclosure, and Section 443 expects the installation's periodic inspection regime to check that indicator.

For most UK electricians, the Section 443 / Section 534 workflow now sits inside the standard consumer-unit-replacement and EICR routines. The Elec-Mate platform supports this in three places:

For the working electrician, the takeaway is simple: fit SPDs as standard, and only leave them out where the owner has declined them in writing and accepts the risk.