IET Wiring Regulations BS 7671 18th Edition: Transient Overvoltage Protection

Introduction to Transient Overvoltage Protection

Based on the IEC 60364 series, the 18th Edition of BS 7671 Wiring regulations covers the electrical installation of buildings, including the use of surge protection.

The 18th Edition of BS 7671 applies to the design, erection, and verification of electrical installations, as well as additions and alterations to existing installations. Existing installations may not comply with the 18th edition in every respect, but this does not necessarily mean they are unsafe or require upgrading.

A key update in the 18th Edition relates to Sections 443 and 534, which concern the protection of electrical and electronic systems against transient overvoltages. These overvoltages can originate from atmospheric events (lightning) or electrical switching events.

Essentially, the 18th Edition requires all new electrical system designs and installations, as well as alterations and additions to existing installations, to be assessed against key consequences caused by transient overvoltages. Where necessary, protection must be provided using appropriate surge protection measures, in the form of Surge Protection Devices (SPDs).

Within BS 7671:

• Section 443 now requires protection against transient overvoltages where the consequence could result in: (i) serious injury to, or loss of, human life; (ii) failure of a safety service*; or (iii) significant financial or data loss.
• For all other cases, protection is required unless the owner declares it is not required due to tolerable loss or damage.

The benefits of SPDs and their relatively small cost far outweigh the potential hardware and consequential losses, especially for modern homes reliant on electronics.

Note: A safety service, as defined by BS 7671 Part 2, is an electrical system for electrical equipment provided to protect or warn persons in the event of a hazard, or essential to their evacuation from a location.

It should be noted that for Section 443, the BS EN 62305-2 risk assessment method must be used for high-risk installations (e.g., nuclear or chemical sites). Outside of such installations, if there is a risk of a direct lightning strike to the structure or overhead lines, SPDs are required per BS EN 62305.

• Section 534 details the selection and installation of SPDs for effective transient overvoltage protection, including SPD Type, performance, and co-ordination.

Readers should be mindful of protecting all incoming metallic service lines against transient overvoltages. BS 7671 provides guidance for electrical and electronic equipment on AC mains power supplies.

To observe the Lightning Protection Zone (LPZ) concept within BS 7671 and BS EN 62305, all other incoming metallic service lines (data, signal, telecommunications) are also potential routes for transient overvoltages and require appropriate SPDs.

BS 7671 refers to BS EN 62305 and BS EN 61643 for specific guidance, which is covered in the Furse guide to BS EN 62305 Protection Against Lightning.

IMPORTANT: Equipment is ONLY protected against transient overvoltages if all incoming/outgoing mains and data lines have protection fitted. [info-symbol]

Why Transient Overvoltage Protection is So Important

Transient overvoltages are short-duration surges in voltage between conductors (L-PE, L-N, or N-PE), which can reach up to 6 kV on 230 Vac power lines. They generally result from:

• Atmospheric origin (lightning activity through resistive or inductive coupling).
• Electrical switching of inductive loads.

Transient overvoltages significantly damage and degrade electronic systems. Outright damage to sensitive systems, such as computers, occurs when transient overvoltages between L-PE or N-PE exceed the withstand voltage of the equipment (e.g., above 1.5 kV for Category I equipment per BS 7671 Table 443.2).

Equipment damage leads to unexpected failures, expensive downtime, or risks of fire/electric shock due to flashover if insulation breaks down.

Degradation of electronic systems begins at much lower overvoltage levels, causing data losses, intermittent outages, and shorter equipment lifetimes.

For critical systems (e.g., hospitals, banking), degradation must be avoided by limiting transient overvoltages (between L-N) below the equipment's impulse immunity. This is approximately twice the peak operating voltage (e.g., 715 V for 230 V systems).

Protection is achieved through a coordinated set of SPDs at appropriate points in the electrical system, per BS 7671 Section 534.

Selecting SPDs with lower voltage protection levels (Up) is critical, especially for continuous usage.

Diagram Description: Voltage vs. Time

The diagram illustrates voltage levels over time. It shows a "Safe Operating Area" for the nominal system voltage (e.g., 230 V). Above this is a "Degradation" zone, and further up is "DAMAGE". Transient overvoltages are depicted exceeding 1.5 kV (L-PE/N-PE) and also exceeding twice the peak operating voltage (e.g., 715 V L-N).

Diagram Descriptions: Coupling Types

• 01 Equipment Risk: Degradation of electronic systems begins at lower transient overvoltage levels, affecting critical systems when impulse immunity is compromised.
• 02 Resistive Coupling: Transients caused by potential differences between two connected earths.
• 03 Inductive Coupling: Transients caused by electromagnetic pick-up.

Transient Overvoltage Protection: Selection of SPDs to BS 7671

Section 534 of BS 7671 aims to achieve overvoltage limitation within AC power systems for insulation co-ordination, in line with Section 443 and BS EN 62305-4.

Overvoltage limitation is achieved by installing SPDs as recommended in Section 534 (for AC power systems) and BS EN 62305-4 (for other power, data, signal, or telecommunications lines).

SPD selection should aim to limit transient overvoltages from atmospheric origin and those caused by direct lightning strikes or nearby strikes protected by a structural Lightning Protection System (LPS).

SPD Selection Criteria:

• Voltage protection level (Up)
• Continuous operating voltage (Uc)
• Temporary overvoltages (UTOV)
• Nominal discharge current (In) and impulse current (Iimp)
• Prospective fault current and follow current interrupt rating

The most critical aspect is the voltage protection level (Up). The SPD's Up must be lower than the rated impulse voltage (Uw) of the protected electrical equipment (Table 443.2) or its impulse immunity for continuous operation.

If unknown, impulse immunity can be calculated as twice the peak operating voltage (approx. 715 V for 230 V systems). Non-critical equipment may use SPDs with Up lower than Category II (2.5 kV), while sensitive equipment (laptops, PCs) requires Category I (1.5 kV).

Lower Up levels offer better protection by reducing risks from additive inductive voltages, voltage oscillations, and keeping equipment stress to a minimum, thereby improving operating lifetime.

Enhanced SPDs (per BS EN 62305) offer superior protection by providing Up levels considerably lower than equipment damage thresholds.

All SPDs installed to meet BS 7671 must conform to product and testing standards (BS EN 61643 series).

Diagram Description: Electronic Components

A photograph displays various electronic components, likely Surge Protection Devices (SPDs), with visible internal circuitry and connectors.

Typical Installation and SPD Types

Diagram Description: Installation Layout

The diagram illustrates a typical installation for a 230/400 V TN-C-S/TN-S system using Furse SPDs, compliant with BS 7671. It shows:

• Service Entrance/Main Distribution Board: An SPD (e.g., Type 1+2) is installed here to divert high-energy lightning currents to earth and prevent flashover.
• Sub-distribution Board: Located more than 10m from the MDB, an SPD (e.g., Type 2+3) protects fixed equipment against transient overvoltages.
• Terminal Equipment: At local level, a plug-in SPD (e.g., Type 3) protects critical equipment against switching transients.

The diagram also depicts Overcurrent Protective Devices (OCPDs) at various points and indicates the risk of switching transients and critical equipment.

Advantages of Enhanced SPDs:

• Combined equipotential bonding and transient overvoltage protection (Type 1+2 & Type 1+2+3).
• Full mode (common and differential mode) protection for sensitive equipment.
• Effective SPD co-ordination within a single unit.

Compliance to BS EN 62305/BS 7671:

BS 7671 Section 534 guides SPD selection and installation for AC power supplies. Section 443 requires SPDs at key locations:

• As close as practicable to the origin of the installation (main distribution board).
• As close as practicable to sensitive equipment (sub-distribution level) and local to critical equipment.

Where a building has a structural LPS or overhead metallic services at risk from lightning, equipotential bonding SPDs (Type 1 or Combined Type 1+2) must be installed at the service entrance.

Type 1 SPDs alone do not protect electronic systems. Transient overvoltage SPDs (Type 2 and Type 3, or Combined Types) must be installed downstream. Combined Type SPDs simplify the selection process.

ABB Furse ESP Range of SPDs

The Furse ESP range of SPDs (power, data, and telecom) is specified for ensuring the continuous operation of critical electronic systems and forms part of a complete lightning protection solution to BS EN 62305.

Furse ESP M and ESP D power SPD products are Type 1+2+3 devices, suitable for service entrance installation, offering superior voltage protection levels.

Active Status Indication:

The SPD's active status indication informs the user of:

• Loss of power
• Loss of phase
• Excessive N-E voltage
• Reduced protection

Status can also be monitored remotely via a volt-free contact.

SPD Selection for 230/400 V TN-S or TN-C-S Supplies:

The document provides examples for different supply types and installation scenarios, detailing SPD placement at the Main Distribution Board (MDB), Sub-distribution Board (SDB), and for final circuit equipment.

Main Distribution Board (MDB) - Type 1+2+3 (3 Phase 400 V):

For service entrance, after the electricity meter. SPDs like ESP M and D series are used where the MDB directly feeds critical electronics.

Sub-distribution Board (SDB) - Type 1+2+3 - 3 Phase:

Located >10 m from MDB, feeding electronic equipment.

Sub-distribution Board (SDB) - Type 1+2+3 - 1 Phase:

For 1 Phase 230 V or 3 Phase 400 V systems.

Final Circuit Equipment:

For 13 A sockets (e.g., servers) and equipment up to 32 A.

Fused spurs or single phase sockets:

For single phase spurs/socket outlets up to 16 A, and for residential consumer units.

Protection for Data, Signal, and Telecoms Applications:

Specific SPDs are available for protecting twisted pair signalling applications, local area networks (Cat 6, PoE), and equipment connected to BT telephone sockets.

Installation of SPDs (Section 534, BS 7671)

Table 1: Compatible Overcurrent Protection - Product Selection Guide

Key: l Suitable / Not suitable. Maximum OCPD ratings must be in accordance with the installation to follow co-ordination rules with main or upstream short-circuit protection.

* For ESP 240 M1 and ESP 415 M1R, use same overcurrent protection selection as ESP 415 M1.

** For ESP 415 M2R, use same overcurrent protection selection as ESP 415 M2.

*** For ESP 415 M4R, use same overcurrent protection selection as ESP 415 M4.

Critical Length of Connecting Conductors

An installed SPD presents a higher let-through voltage due to additive inductive voltage drops across its connecting leads. For maximum protection, these leads must be kept as short as possible.

BS 7671 defines that for SPDs installed in parallel (shunt), the total lead length between line conductors, protective conductor, and SPD should preferably not exceed 0.5 m and never exceed 1 m.

For SPDs installed in-line (series), the lead length between the protective conductor and SPD should preferably not exceed 0.5 m and never exceed 1 m.

Best Practice for Installation:

Poor installation significantly reduces SPD effectiveness. Keeping connecting leads short is vital. Binding connecting leads together (using cable ties or spiral wrap) is highly effective in cancelling inductance. Combining a low Up SPD with short, tightly bound leads ensures optimized installation per BS 7671.

Cross-Sectional Area of Connecting Conductors:

For SPDs connected at the origin of the installation (service entrance), BS 7671 requires the minimum cross-sectional area of SPDs connecting leads (copper or equivalent) to PE/live conductors respectively to be:

• 16 mm²/6 mm² for Type 1 SPDs
• 6 mm²/2.5 mm² for Type 2 SPDs

These cross-sectional area values are based on surge current handling, not supply current. However, in case of a short circuit (e.g., due to SPD end-of-life), connecting leads need protection by a suitable Overcurrent Protective Device (OCPD).

Fault Protection:

BS 7671 requires fault protection to remain effective even if SPDs fail. An SPD needs protection against short circuits via an appropriate OCPD installed in-line on its connecting leads, which must discriminate with the upstream OCPD of the main supply.

SPD manufacturers should provide clear guidance for selecting correct OCPD ratings in their installation instructions.

Diagram Description: SPD Installation with OCPD

The diagram shows an SPD installed in parallel with an OCPD, connected to the main earthing terminal or conductor bar. It emphasizes keeping SPD connections short, ideally below 0.25 m between the SPD, live conductors, and earth, but not more than 0.5 m, to reduce additive inductive voltage drops.

Diagram Description: SPD Unit

A photograph shows an ESP 415 D1/LCD SPD unit with status indicators (Green for full protection, Red for reduced protection, Red flashing for no protection, with a warning to check Neutral to Earth voltage).

Other Products to Consider

ESP SL Series - for protection of twisted pair signalling applications

Product range: ESP SL Series

Description: Two-stage removable protection module with a quick-release mechanism for easy commissioning, maintenance, and replacement.

Features: Available in various voltage variants (6V to 180V) and analogue telephone versions. Earthed and isolated screen versions available. Optional LED status indication.

ESP Cat-5e / 6 Series - for protection of local area networks

Product range: ESP Cat-5e/6 Series

Features: Models available to protect Cat-5e/Cat-6 and PoE. Protects all PoE powering modes (A and B). Suitable for shielded or unshielded twisted pair installations. Does not impair system's normal operation.

ESP TN/JP - for protection of equipment connected to BT telephone (BS 6312) socket

Part No.: ESP TN/JP

Features: Comes with BT (BS 6312) jack-plug for ease of installation. Also available with RJ11 connectors. RJ11 and JP versions are suitable for lines with a maximum ringing voltage of 296 V. ISDN suitable models with RJ45 connectors are available.