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SPD Protection Device Ratings for UK Main Switchboards: A Practical Guide
Based on extensive research of UK standards, manufacturer specifications, and industry practices, here are practical “rule of thumb” protection device ratings for Type 1 and Type 2 surge protection devices (SPDs) in main switchboards.
Executive Summary
For Type 1 SPDs (installed at the origin/main distribution board), typical thermal-magnetic MCCB protection ratings range from 63A to 200A depending on switchboard size, representing approximately 15-25% of the switchboard rating[1] [2]. For Type 2 SPDs, protection ratings are generally lower, ranging from 32A to 100A[1] [3].
Type 1 SPD Protection Device Ratings
| Switchboard Rating | Recommended MCCB Rating | Alternative gG Fuse | Short Circuit Rating |
|---|---|---|---|
| 250A | 63A Thermal-Magnetic | 63A gG | 10-25kA |
| 400A | 80A Thermal-Magnetic | 80A gG | 10-25kA |
| 630A | 100A Thermal-Magnetic | 100A gG | 15-36kA |
| 1200A | 160A Thermal-Magnetic | 160A gG | 25-36kA |
| 2500A | 200A Thermal-Magnetic | 200A gG | 25-50kA |
Key Considerations for Type 1 SPDs
BS 7671 Compliance: Every SPD must have its own coordinated overcurrent protective device per Regulation 534.4.1.5 [4] [5]. The main fuse or switch-fuse alone is not sufficient protection.
Coordination Requirements: The protective device must withstand 15 successive impulse currents at the SPD’s nominal discharge current (In) without tripping, while providing effective protection against short circuits and overload conditions [1] [2].
MCCB Characteristics: Type B or C curve thermal-magnetic MCCBs are preferred for coordination. Type B curve (3-5×In magnetic trip) provides better coordination for most applications [6] [7].
Type 2 SPD Protection Device Ratings
| Switchboard Rating | Recommended Protection Rating | Device Type | Alternative gG Fuse |
|---|---|---|---|
| 250A | 32A | MCB Type B/C | 32A gG |
| 400A | 63A | MCB/MCCB Type B/C | 63A gG |
| 630A | 63A | MCCB Type B/C | 63A gG |
| 1200A | 80A | MCCB Type B/C | 80A gG |
| 2500A | 100A | MCCB Type B/C | 100A gG |
Type 2 SPD Installation Notes
Domestic Applications: For single-phase consumer units, 32A MCB Type B is most common [9] [3]. Many manufacturers supply Type 2 SPDs with integrated 32A MCBs for convenience.
Commercial Applications: Three-phase Type 2 SPDs typically use 63A protection for most commercial installations [8] [10].
TN-C-S Earthing: Most UK installations use TN-C-S earthing systems, which affect SPD connection configuration. SPDs should be connected in “3+0” or “4+0” mode for optimal protection [11].
Critical Design Principles
Manufacturer Coordination Tables
The protective device’s breaking capacity must equal or exceed the prospective short circuit current at the installation point[1] [2]. This typically requires:
10-25kA rating for most commercial applications
25-50kA rating for large industrial installations with high fault levels
Cable Sizing Requirements
Per BS 7671 Section 534.4.8, SPD connecting conductors must be [13]:
Minimum 4mm² copper if line conductors ≥4mm²
Minimum 16mm² copper for Type 1 SPDs where structural lightning protection is installed
Keep connections as short as possible to minimize inductive voltage drop
Integration with RCDs
Practical Application Guidelines
250A Switchboards
Type 1: 63A MCCB or 63A gG fuse
Type 2: 32A MCB (single-phase) or 63A MCCB (three-phase)
Suitable for small commercial buildings and large domestic installations
400A Switchboards
Type 1: 80A thermal-magnetic MCCB
Type 2: 63A MCB/MCCB
Most common commercial application size
630A Switchboards
Type 1: 100A thermal-magnetic MCCB
Type 2: 63A MCCB
Medium industrial and large commercial applications
1200A Switchboards
Type 1: 160A thermal-magnetic MCCB
Type 2: 80A MCCB
Large commercial and industrial installations
2500A Switchboards
Type 1: 200A thermal-magnetic MCCB
Type 2: 100A MCCB
Major industrial installations with high lightning exposure risk
Important Limitations and Warnings
Risk Assessment Required: These are general guidelines only. Proper risk assessment per BS EN 62305-2 may require different ratings based on specific exposure levels, equipment sensitivity, and consequence of failure [15].
No Substitute for Professional Design: These rules of thumb are for concept design only. Final SPD selection and coordination must be performed by qualified electrical engineers using manufacturer data and proper coordination studies [1].
Standards Evolution: SPD standards continue to evolve. BS EN IEC 61643-01:2025 introduced new requirements that may affect future coordination practices [16]. Always verify compliance with current standards.
Installation Quality Critical: Even properly rated SPDs will fail to provide adequate protection if installation quality is poor. Short, direct connections and proper earthing are essential for effective surge protection [13] [14].
These practical guidelines provide electrical engineers with reliable starting points for SPD protection device selection in UK TN-C-S installations, while emphasising the critical importance of manufacturer verification and professional engineering judgment in final design decisions.
