6
Protection
Against
Overcurrent
GUIDANCE NOTE 6
IEE Wiring Regulations
BS 7671 : 2001 Requirements for Electrical Installations
Including Amd No 1 : 2002._CD GN6 Protection Against Overcurrent, inc 16 th Edition 2001 Amd No 1 2
Published by: The Institution of Electrical Engineers, Savoy Place,
LONDON, United Kingdom. WC2R 0BL
©2003: The Institution of Electrical Engineers
Printed January 1993
Reprinted August 1993, with minor amendments
2nd edition July 1996, incorporating BS 7671 : 1992 inc Amd No 1
3rd edition Aug 1999, incorporating BS 7671 : 1992 inc Amd No 2
4th edition Sept 2003, incorporating BS 7671 : 2001 inc Amd No 1
Copies may be obtained from:
The IEE
PO Box 96, STEVENAGE,
United Kingdom. SG1 2SD
Tel: 44 (0)1438 767 328
Fax: 44 (0)1438 742 792
Email: sales@iee.org
http://www.iee.org/publish/books/WireAssoc/
All rights reserved. No part of this publication may be
reproduced, stored in a retrieval system or transmitted in any
form or by any means — electronic, mechanical, photocopying,
recording or otherwise — without the prior written permission
of the publisher.
While the author, publisher and contributors believe
that the information and guidance given in this work
is correct, all parties must rely upon their own skill
and judgement when making use of it. Neither the
author, the publisher nor any contributor assume any
liability to anyone for any loss or damage caused by
any error or omission in the work, whether such error
or omission is the result of negligence or any other
cause. Where reference is made to legislation it is not
to be considered as legal advice. Any and all such
liability is disclaimed.
ISBN 0 85296 994 5, 2003._CD GN6 Protection Against Overcurrent, inc 16 th Edition 2001 Amd No 1 3
ACKNOWLEDGEMENTS 6
PREFACE 7
INTRODUCTION 8
SECTION 1 THE REGULATIONS CONCERNING PROTECTION
AGAINST OVERCURRENT 10
SECTION 2 PROTECTION AGAINST OVERLOAD 20
SECTION 3 PROTECTION AGAINST FAULT CURRENT 42
SECTION 4 DETERMINATION OF FAULT CURRENT 57
SECTION 5 EQUATIONS FOR THE CALCULATION OF
SHORT-CIRCUIT CURRENT 64
SECTION 6 EQUATIONS FOR THE CALCULATION OF EARTH
FAULT CURRENT 76
SECTION 7 SELECTION OF CONDUCTOR SIZE 90
SECTION 8 NOTES ON FAULT CURRENT WITHSTAND OF
FLEXIBLE CORDS 95
APPENDIX 1 CALCULATION OF REACTANCE 102
APPENDIX 2 CALCULATION OF K FOR OTHER TEMPERATURES108
INDEX 109.
