16th Edition (reference only) – NOW superseded by the 17th Edition IEE Regulations.

chapter 5

chapter 6

Installation control and protection
  3.1 - Introduction 3.5 - High temperature protection
  3.2 - Switching 3.6 - Overload currents
  3.3 - Isolation 3.7 - Protection from faults
3.4 - Electric shock protection

3.8 - Short circuit and overload
------- protection

3.3.1 Isolator definition

An isolator is not the same as a switch. It should only be opened when not carrying current, and has the purpose of ensuring that a circuit cannot become live whilst it is out of service for maintenance or cleaning. The isolator must break all live supply conductors; thus both phase and neutral conductors must be isolated. It must, however, be remembered that switching off for mechanical maintenance (see {3.2.3}) is likely to be carried out by non-electrically skilled persons and that they may therefore unwisely use isolators as on-load switches. To prevent an isolator, which is part of a circuit where a circuit breaker is used for switching, from being used to break load current, it must be interlocked to ensure operation only after the circuit breaker is already open. In many cases an isolator can be used to make safe a particular piece of apparatus whilst those around it are still operating normally.


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Extracted from The Electricians Guide Fifth Edition
by John Whitfield

Published by EPA Press Click Here to order your Copy.

Click here for list of abbreviations