TECHNICAL
snags and solutions
A practical guide to everyday electrical problems ‘Snags & Solutions’, NICEIC’s problem solving book, is now available in three parts, which cover many commonly-encountered electrical installation problems. All parts take account of the requirements of BS 7671: 2008 (17th Edition of the IEE Wiring Regulations), where appropriate. Part 1 addresses 53 problems relating to earthing and bonding, Part 2 covers 55 problems relating to wiring systems, and Part 3 covers 52 problems relating to inspection and testing. Each book is available from NICEIC Direct at £15. To give an indication of the value of these books, a snag and solution is being covered in each issue of Connections. This issue addresses a snag from Part 3 relating to assessing existing main protective conductors.
50 Summer 2010 NICEIC Connections
Assessing existing main protective bonding conductors Main bonding conductors in older installations may be found to have a crosssectional area (csa) less than that required by BS 7671.
Snag 14 Difficulty arises in assessing the suitability of a main bonding conductor where its csa is less than required by BS 7671. Where PME conditions apply, main bonding conductors having too small a csa could result in the maximum permitted temperature of the conductor to be exceeded, due to diverted neutral currents that can flow continuously or for long periods from the installation concerned and/or from other installations. Solution Main bonding conductors have been required since 1966 in installations where protection against electric shock is provided by Automatic Disconnection of Supply (ADS), as is usually the case. The csa of such conductors should be not less than the applicable value given in Regulation 544.1.1. In an existing installation where the main protective bonding conductors have a csa less than that required by BS 7671, the inspector may, having carefully considered all the circumstances conclude that the departure from the requirements of BS 7671 does not pose a serious risk to the users of the installation. For an installation served by a PME supply, careful scrutiny is required of existing main bonding conductors having a csa less than required by Table 54.8 of BS 7671, to look for
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