Since the introduction of Amendment 2 to BS 7671:2018, the rules governing Surge Protection Devices (SPDs) have shifted dramatically. The old calculated risk assessment formula has been removed, making protection against transient overvoltages mandatory across most installations.

When SPDs Are Legally Mandatory (Regulation 443.4.1)

Protection against transient overvoltage must be provided where the consequences caused by overvoltage could result in:

  • Serious injury to, or loss of, human life: Hospitals, care homes, medical locations, emergency lighting systems.
  • Interruption of public services or commercial/industrial activity: Telecoms, water treatment facilities, data centres, manufacturing lines.
  • Damage to cultural heritage: Museums, listed historical buildings, galleries.

The Single-Dwelling Exception

For all other installations (including standard private domestic dwellings):

Protection against overvoltage shall be provided unless the owner of the installation declares that protection is not required and accepts the risk of economic loss and equipment damage.

Practical Field Advice:

Given the widespread prevalence of expensive smart appliances, EV chargers, heat pumps, and home office electronics, fitting a Type 2 SPD during consumer unit upgrades is best practice. If a domestic customer refuses on cost grounds, obtain a signed written waiver before signing off the installation certificate.

SPD Types: 1, 2, and 3 Explained

Type 1: Direct Lightning Surges (10/350 μs waveform)

Designed to divert high-energy currents from direct lightning strikes to the building or adjacent overhead supply lines. Installed at the origin of installations fitted with external Lightning Protection Systems (LPS) or supplied via overhead cables.

Type 2: Switching Transients & Indirect Surges (8/20 μs waveform)

The standard main incoming device fitted inside the main distribution board or consumer unit. Protects against grid switching transients, inductive switching spikes, and indirect lightning strikes.

Type 3: Point-of-Use Fine Protection (1.2/50 μs waveform)

Supplementary low-voltage protection fitted close to sensitive terminal equipment (e.g. surge-protected socket strips, server racks, and fire alarm panels). Must only be used in conjunction with upstream Type 2 protection.

Crucial Installation Rule: Maximum Lead Length

Regulation 534.4.4.2 mandates that the total connecting lead length of an SPD (connecting phase conductor length L1 plus earth lead length L2) should preferably not exceed 0.5 metres, and must never exceed 1.0 metre.

Excessive lead lengths add inductive impedance, creating massive voltage spikes during rapid transients that bypass the protection of the device.