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.

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

  • BS 7671:2018+A2:2022: Regulation 443.4 requires risk assessment to determine need for SPD; Chapter 53 covers SPD specification
  • Risk assessment: evaluates building type, earthing system, lightning exposure, and consequences of surge damage
  • Type 1 SPD: installed at service entrance; protects against direct lightning strikes; requires spark gap or gas discharge tube technology
  • Type 2 SPD: installed at consumer unit or sub-distribution board; protects against conducted surges from grid; varistor (MOV) based
  • Type 3 SPD: installed at equipment level (socket, din-rail after Type 2); final protection; plug-in or socket-type
  • Typical TT system requirement: Type 2 SPD at consumer unit — TT earthing has high loop impedance, making it more vulnerable to conducted surges
  • TNC-S (PME) system: Type 2 SPD recommended; risk assessment will typically indicate SPD requirement for overhead service
  • SPD voltage protection level (Up): maximum residual voltage the SPD allows through; for domestic BS 7671, Up ≤ 2.5 kV for LV equipment
  • SPD Imax: maximum impulse current capacity; for residential Type 2, minimum 20 kA (8/20μs waveform per IEC 61643-11)
  • Disconnector: SPDs must include a disconnector (thermal or fuse) to isolate a failed SPD — MOV failure is typically short-circuit, which would trip the upstream MCB
  • BS EN 61643-11: the product standard for LV SPDs; all specified SPDs must comply
  • IEC 62305: lightning protection standard; risk assessment methodology referenced in BS 7671

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+A2 Risk Assessment Methodology

Regulation 443.4 requires the designer to carry out a risk assessment. The assessment considers:

Factors increasing risk (each assigned a value in the BS 7671 Annex A assessment):

  • Building in an exposed location (hill, open ground)
  • Overhead service (power or telecoms entering the building above ground)
  • TT or IT earthing system (higher impedance than TNC-S)
  • Building contains explosive materials or critical processes
  • High value of equipment protected (data centres, medical, sensitive electronics)
  • Building used by large numbers of people (consequence of surge disruption is severe)
  • Building with no lightning protection system (LPS per BS EN 62305)

Factors decreasing risk:

  • Underground cable service entry
  • TNC-S (PME) earthing with low loop impedance
  • Existing lightning protection system
  • Urban location (lower direct lightning strike density)

The assessment produces a Risk Level score. If Risk Level > threshold (L = 1000 per BS 7671 Table 44.1), SPD installation is required.

For domestic TT earthing with overhead service: In practice, the risk assessment almost always mandates Type 2 SPD at the consumer unit for this combination. TT + overhead = high conducted surge risk. Contractors should not omit the assessment even when the outcome is predictable — document it.

For domestic TNC-S (PME) with underground service (typical new-build suburban house): Risk assessment may indicate SPD as recommended but not mandatory, depending on location and use. Many contractors now install Type 2 as standard practice regardless, to protect client electronics.

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:

  • Integrated SPD consumer units: the SPD is built into the board as a factory-fitted module
  • DIN rail SPD add-on: a separate SPD device (typically 1–3 modules wide) installed inside the consumer unit on the DIN rail

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:

  • The earth conductor from the SPD to the main earthing terminal must be as short as possible — ideally <0.5m, maximum 1m
  • Each additional metre of earth conductor adds approximately 1μH inductance, which reduces SPD effectiveness at high frequencies
  • Use 4mm² minimum for the SPD earth connection (16mm² for Type 1)

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:

  • Total lead length (L wire + PE wire) ≤ 0.5m for optimum protection
  • Any length over 1m significantly degrades protection voltage level

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:

  • Type 3 must be installed at least 10m of cable from Type 2 (or an impedance decoupling inductor fitted), otherwise the two SPDs may interact and reduce overall protection
  • Check manufacturer's coordination requirements — some brands offer matched Type 2+3 sets

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?

You are required to carry out a risk assessment. If the assessment indicates SPD is required (which it will for TT earthing with overhead supply, and for many other scenarios), you must install it. If the assessment indicates SPD is not required but recommended, document your assessment and discuss with the client. In practice, the cost of a Type 2 SPD (£50–150 for the device) is low relative to the cost of replacing damaged electronics — many contractors install as standard.

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