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Transient Overvoltage Protection

IET Wiring regulations BS 7671 18th edition

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).

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.




Burnt out chip due to lightning surge
(photo credit:https://new.abb.com/)



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 article was extracted from "Transient Overvoltage Protection" by Furse/ABB

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