IET Wiring Regulations (BS 7671) | New Electrical System Designs & Installations

Published 22 Sep 2020

Furse BS 7671 Guide

BS 7671 18th Edition

IET Wiring Regulations

The IET Wiring Regulations (BS 7671) now require all new electrical system designs and installations, as well as alterations and additions to existing installations, to be assessed against transient overvoltage risk and, where necessary, protected using Surge Protection Devices (SPDs). Covered in the Furse BS 7671 Guide is the following:

Transient Overvoltage Protection

What are transient overvoltages/surges?
How are transient overvoltages created?
The problem transients overvoltages cause
Why surge protection is required and how to safeguard systems?

Section 443 & 534 of BS 7671 – Protection Against Overvoltage

Risk assessment against transient overvoltages
Compliance to BS EN 62305 & BS 7671
Selection and installation SPDs

The IET Wiring Regulations require all new electrical system designs and installations, as well as alterations and additions to existing installations, to be assessed against transient overvoltage risk and, where necessary, protected using appropriate surge protection measures (in the form of Surge Protection Devices SPDs).

Transient overvoltage protection

Introduction

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, and also to additions and alterations to existing installations. Existing installations that have been installed in accordance with earlier editions of BS 7671 may not comply with the 18th edition in every respect.

This does not necessarily mean that they are unsafe for continued use or require upgrading.

A key update in the 18th Edition relates to Sections 443 and 534, which concern protection of electrical and electronic systems against transient overvoltages, either as a result of atmospheric origin (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 transient overvoltage risk and, where necessary, protected using appropriate protection measures (in the form of SPDs).

Within BS 7671

Section 443 defines the criteria for risk assessment against transient overvoltages, considering the supply to the structure, risk factors and rated impulse voltages of equipment.

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

Readers of this guide should be mindful of the need to protect all incoming metallic service lines against the risk of transient overvoltages. BS 7671 provides focussed guidance for the assessment and protection of electrical and electronic equipment intended to be installed on AC mains power supplies.

In order to observe the Ligntning Protection Zone LPZ concept within BS 7671 and BS EN 62305, all other incoming metallic service lines, such as data, signal and telecommunications lines, are also a potential route through which transient overvoltages to damage equipment.

As such all such lines will require appropriate SPDs. BS 7671 clearly points the reader back to BS EN 62305 and BS EN 61643 for specific guidance. This is covered extensively in the Furse guide to BS EN 62305 Protection Against Lightning.

Safeguarding your electrical systems

Why is Transient Overvoltage Protection so Important?

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

Atmospheric origin (lightning activity through resistive or inductive coupling (see Figures 02 & 03 in pdf), and/or
Electrical switching of inductive loads

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

Equipment damage leads to unexpected failures and expensive downtime, or risk of fire/electric shock due to arc flash or flashover, if insulation breaks down. Degradation of electronic systems, however, begins at much lower overvoltage levels and can cause data losses, intermittent outages and shorter equipment lifetimes (see Figure 01 see pdf).

Where continuous operation of electronic systems is critical, for example in hospitals, banking and most public services, degradation must be avoided by ensuring these transient overvoltages, which occur between L-N, are limited below the impulse immunity of equipment. This can be calculated as twice the peak operating voltage of the electrical system, if unknown (i.e. approximately 715 V for 230 V systems).

Protection against transient overvoltages can be achieved through installation of a coordinated set of SPDs at appropriate points in the electrical system, in line with BS 7671 Section 534 and the guidance provided in this publication. Selecting SPDs with lower (i.e. better) voltage protection levels (UP) is a critical factor, especially where continuous usage of electronic equipment is essential.

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