16th Edition (reference only) – NOW superseded by the 17th Edition IEE Regulations.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

3.2.1 - Switch positions

A switch is defined as a device which is capable of making or breaking a circuit under normal and under overload conditions. It can make, but will not necessarily break, a short circuit, which should be broken by the overload protecting fuse or circuit breaker. A switching device may be marked with ON and OFF positions, or increasingly, the numbers 1 for ON and 0 for OFF are being used.

A semiconductor device is often used for switching some lighting and heating circuits, but will not be suitable for disconnecting overloads; thus, it must be backed up by a mechanical switch. The semiconductor is a functional switch but must NOT be used as an isolator.

{Figure 3.1} shows which poles of the supply need to be broken by the controlling switches. For the TN-S system (earth terminal provided by the Electricity Company), the TNC-S system (protective multiple earthing) and the TT system (no earth provided at the supply), all phase conductors MUST be switched, but NOT the protective (earth) conductor.

The neutral conductor need not be broken except for:
1 - the main switch in a single-phase installation, or
2 - heating appliances where the element can be touched, or
3 - autotransformers (not exceeding 1.5 kV) feeding discharge lamps

The neutral will need to he disconnected for periodic testing, and provision must be made for this; it is important that the means of disconnection is accessible and can only be completed with the use of a tool.

The protective conductor should never be switched, except when the supply can he taken from either of two sources with earth Systems which must slot be connected together. In this case the switches needed in the protective conductors must be linked to the phase switches so that it is impossible for the supply to be provided unless the earthing connection is present.

Fig 3.1 Supply system broken by switches

(a)TN-C Systems (b) TN-S, TN-C-S (c) TT Systems

Every circuit must be provided with a switching system so that it can be interrupted on load. In practice, this does not mean a switch controlling each separate circuit; provided that loads are controlled by switches, a number of circuits may be under the overall control of one main switch. An example is the consumer unit used in the typical house, where there is usually only one main switch to control all the circuits, which are provided with individual switches to operate separate lights, heaters, and so on. If an installation is supplied from more than one source there must he a separate main switch for each source, and each must be clearly marked to warn the person switching off the supplies that more than one switch needs to he operated.

It should he noted that a residual current device (RCD) may be used as a switch provided that its rated breaking capacity is high enough.


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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