16th Edition (reference only) – NOW superseded by the 17th Edition IEE Regulations.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

3.3.4   Semiconductor isolators

Semiconductors are very widely used in electrical installations, from simple domestic light dimmers (usually using triacs) to complex speed controllers for three phase motors (using thyristors). Whilst the semiconductors themselves are functional switches, operating very rapidly to control the circuit voltage, they must NOT be used as isolators.

This is because when not conducting (in the OFF position) they still allow a very small leakage current to flow, and have not totally isolated the circuit they control. {Figure 3.5} shows semiconductors in a typical speed control circuit.

Fig 3.5 - Speed control for a dc motor fed from a three-phase supply



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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