Under BS 7671:2018+A2:2022 (as amended by the May 2023 Corrigendum), Section 443 and Section 534 govern protection against transient overvoltages. The complex calculated risk level (CRL) equation of previous editions has been deleted. Instead, Regulation 443.4.1 establishes explicit mandatory triggers and a general requirement for overvoltage protection across all installations.
When BS 7671 Requires SPDs (Regulation 443.4.1)
Following the IET Corrigendum of May 2023, Regulation 443.4.1 establishes a clear two-tier structure for transient overvoltage protection:
BS 7671:2018+A2:2022 Regulation 443.4.1 (as amended):
Protection against transient overvoltage shall be provided where the consequence could result in:
(i) Serious injury to, or loss of, human life;
(ii) [Deleted by May 2023 Corrigendum];
(iii) Significant financial or data loss.
For all other cases, protection against transient overvoltages shall be provided unless the owner of the installation declares that protection is not required because loss or damage is tolerable.
Key Statutory Distinction: The owner opt-out declaration is not limited to domestic dwellings; it applies to any installation that falls outside criteria (i) and (iii). For example, a small agricultural barn or storage lock-up where equipment damage is easily tolerable may opt out. Conversely, a domestic dwelling housing vital life-support equipment or home dialysers falls under (i) and must have SPD protection, with no opt-out permitted.
SPD Types: 1, 2, and 3 Explained
Surge Protective Devices are categorized by their impulse withstand testing and physical location in the electrical installation:
| SPD Type | Standard Test Waveform | Primary Origin Location | Design Duty / Purpose |
|---|---|---|---|
| Type 1 | 10/350 μs impulse current (Iimp) | Main incoming intake board | Direct lightning current diversion. Mandatory where building has an external Lightning Protection System (LPS) to BS EN 62305 or overhead line supply. |
| Type 2 | 8/20 μs nominal discharge current (In) | Main intake or sub-distribution boards | Protection against indirect lightning strikes and utility network switching transients. The standard device in commercial boards and domestic consumer units. |
| Type 3 | Combination wave: 1.2/50 μs voltage & 8/20 μs current (Uoc) | Point of use (≤ 10 m from load) | Supplementary fine protection for sensitive electronic devices (servers, fire alarm panels, scientific instruments). Must be coordinated with upstream Type 2 (Regulation 534.4.4.5). |
Connection Types: CT1 vs CT2 (Regulation 534.4.3)
BS 7671 recognises two distinct connection configurations for SPDs:
- Connection Type 1 (CT1 , "4+0" or "2+0"):SPDs are connected directly between each live conductor (Line and Neutral) and the Main Protective Conductor (PE). Common on TN-C-S systems where Neutral and Earth are bonded at the supply origin.
- Connection Type 2 (CT2 , "3+1" or "1+1"):Metal Oxide Varistors (MOVs) are connected between Lines and Neutral, with a heavy-duty Gas Discharge Tube (GDT) / encapsulated spark-gap connected between Neutral and PE.Mandatory on TT earthing systems and strongly preferred on TN-S / TN-C-S installations. In a CT2 arrangement, a failed or degraded MOV cannot clamp line potential onto the earthing conductor, eliminating electric shock risks and preventing unwanted tripping of upstream RCDs. Under Table 534.3, the N to PE spark gap at origin must have In ≥ 10 kA (Type 2) or Iimp ≥ 50 kA (Type 1).
Critical Lead Length Rules: Regulation 534.4.8
During high-frequency surge discharge (8/20 μs), the rate of current change (di/dt) generates substantial inductive voltage drop along the connecting cables: approximately 1,000 V per metre of conductor.
Connecting Conductor Length Limit (Regulation 534.4.8):
The total connecting conductor length (a + b + c) in Figure 534.8 (from busbar to OCPD, OCPD to SPD, and SPD to Main Earthing Terminal) preferably shall not exceed 0.5 m, and shall in no case exceed 1.0 m.
If total lead length exceeds 0.5 m, the total effective protection level (Up/eff) increases significantly. In such cases, designers must select an SPD with a lower voltage protection level (Up), install a coordinated secondary SPD downstream, or implement the V-type connection configuration of Figure 534.9.
Earthing Conductor Cross-Sectional Area (Regulation 534.4.10)
Connecting conductors between the SPD and the earthing terminal must withstand high discharge currents without thermal failure or excessive impedance:
- Type 1 SPDs: Minimum 16 mm² copper conductor (or equivalent mechanical withstand).
- Type 2 SPDs: Minimum 6 mm² copper conductor.