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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, Type 2 and Type 1+2 surge protection devices (SPDs) in main switchboards.

    Executive Summary

    or Type 1 and Type 1+2 combined SPDs at service entrance installations, backup protection ratings typically range from 63A to 250A depending on switchboard size and Lightning Protection Level (LPL), 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 and Type 1+2 Combined SPD Protection Ratings

    Type 1 SPDs, which handle direct lightning strikes with 10/350μs waveforms, require coordination with their protective devices [4] [5]. The following table provides practical default ratings:

    Switchboard RatingLPL I/II Backup ProtectionLPL III/IV Backup ProtectionAlternative gG FuseShort-Circuit Capacity
    250A80A B/C curve MCCB63A B/C curve MCCB63A gG / 80A gG10-25kA
    400A100A B/C curve MCCB80A B/C curve MCCB80A gG / 100A gG10-25kA
    630A125A B/C curve MCCB100A B/C curve MCCB100A gG / 125A gG15-36kA
    800A160A B/C curve MCCB125A B/C curve MCCB125A gG / 160A gG15-36kA
    1000A200A B/C curve MCCB160A B/C curve MCCB160A gG / 200A gG25-36kA
    1200A200A B/C curve MCCB160A B/C curve MCCB160A gG / 200A gG25-36kA
    1600A250A B/C curve MCCB200A B/C curve MCCB200A gG / 250A gG25-50kA
    2000A315A B/C curve MCCB250A B/C curve MCCB250A gG / 315A gG25-50kA
    2500A400A B/C curve MCCB315A B/C curve MCCB315A gG / 400A gG25-50kA
    3200A500A B/C curve MCCB400A B/C curve MCCB400A gG / 500A gG50-100kA

    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

    Type 2 SPDs, designed for 8/20μs waveforms from indirect lightning and switching transients, typically require lower protection ratings [4] [8]:

    Switchboard RatingRecommended Protection RatingDevice TypeAlternative gG FuseShort-Circuit Capacity
    250A32AMCB/MCCB B/C32A gG6-10kA
    400A50AMCB/MCCB B/C50A gG6-10kA
    630A63AMCCB B/C63A gG10-15kA
    800A80AMCCB B/C80A gG10-15kA
    1000A100AMCCB B/C100A gG15-25kA
    1200A100AMCCB B/C100A gG15-25kA
    1600A125AMCCB B/C125A gG25-36kA
    2000A160AMCCB B/C160A gG25-36kA
    2500A200AMCCB B/C200A gG25-50kA
    3200A250AMCCB B/C250A gG50-100kA

    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].

    Lightning Protection Levels (LPL) per BS EN 62305-1

    Lightning Protection LevelMaximum Lightning Current (10/350 μs)Protection EffectivenessRequired SPD Capacity (per pole)
    LPL I200 kA98%25 kA
    LPL II150 kA95%18.75 kA
    LPL III100 kA88%12.5 kA
    LPL IV100 kA81%12.5 kA

    Selection Criteria and Application Guidelines

    Lightning Protection Level Selection

    LPL selection requires professional risk assessment per BS EN 62305-2 [*]. Key factors include:

    Building Type and Occupancy: Critical facilities require LPL I/II protection (98-95% effectiveness), while standard commercial and residential applications typically use LPL III/IV (88-81% effectiveness) [*].

    Geographic Location: Areas with high lightning density (>2 strikes/km²/year) typically require higher protection levels [*].

    Consequence of Failure: Life safety considerations, economic impact, and service continuity requirements influence LPL selection[*].

    Structural Lightning Protection: Buildings with external lightning protection systems require Type 1 or Type 1+2 SPDs coordinated with the LPS [*].

    When to Use Type 1+2 Combined SPDs

    Type 1+2 combined devices are preferred for [*]:

    • Service entrance installations with limited space

    • Buildings with external lightning protection systems

    • Installations fed by overhead power lines

    • Critical facilities requiring extra protection

    Critical Design Principles

    Manufacturer Coordination Tables

    Always consult manufacturer-specific coordination tables before final selection [1] [2]. Generic rules of thumb provide starting points, but actual coordination must be verified against manufacturer test data per BS EN 61643-11[1] [2].

    Short Circuit Current Considerations

    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

    SPDs can be installed upstream or downstream of RCDs, but downstream installation may cause nuisance tripping [14]. If installed downstream, use “S” type (selective) RCDs with immunity to 3kA (8/20μs) surge currents [4] [14].

    Practical Application Examples

    400A Three-Phase Installation (LPL III)

    Required Protection: 80A Type B MCCB
    SPD Rating: 50kA (8/20 μs) or 12.5kA (10/350 μs) per pole
    Cable Sizing: 6mm² live/neutral, 16mm² earth
    Connection Length: Maximum 1.0m total

    1200A Industrial Installation (LPL II)

    Required Protection: 200A Type B MCCB
    SPD Rating: 75kA (8/20 μs) or 18.75kA (10/350 μs) per pole
    Cable Sizing: 16mm² live/neutral, 25mm² earth
    Connection Length: Preferably ≤0.5m total

    63A Single-Phase Consumer Unit (LPL IV)

    Required Protection: 32A Type B MCB
    SPD Rating: 20kA (8/20 μs) or 12.5kA (10/350 μs)
    Cable Sizing: 2.5mm² live/neutral, 6mm² earth
    Connection Length: Maximum 1.0m total

    Coordination with Other Protection Devices

    Discrimination with Main Protection

    SPD backup protection must consider selectivity (discrimination) with upstream protection devices to prevent loss of supply during SPD operation [*]. This requires selectivity studies considering:

    • Time-current characteristics

    • Energy let-through values

    • Selective tripping under fault conditions

    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.

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