Combined switchgear elements

From Electrical Installation Guide

Single units of switchgear do not, in general, fulfil all the requirements of the three basic functions, viz: Protection, control and isolation.

Where the installation of a circuit-breaker is not appropriate (notably where the switching rate is high, over extended periods) combinations of units specifically designed for such a performance are employed. The most commonly-used combinations are described below.

Switch and fuse combinations

Two cases are distinguished:

  • The type in which the operation of one (or more) fuse(s) causes the switch to open.
This is achieved by the use of fuses fitted with striker pins, and a system of switch tripping springs and toggle mechanisms (see Fig. H14)
Fig. H14 – Symbol for an automatic tripping switch-fuse
  • The type in which a non-automatic switch is associated with a set of fuses in a common enclosure.
In some countries, and in IEC 60947-3, the terms “switch-fuse” and “fuse-switch” have specific meanings, viz:
  • A switch-fuse comprises a switch (generally 2 breaks per pole) on the upstream side of three fixed fuse-bases, into which the fuse carriers are inserted (see Fig. H15)
Fig. H15 – Symbol for a non-automatic fuse-switch
  • A fuse-switch consists of three switch blades each constituting a double-break per phase.
These blades are not continuous throughout their length, but each has a gap in the centre which is bridged by the fuse cartridge. Some designs have only a single break per phase, as shown in Figure H16.

The current range for these devices is limited to 100 A maximum at 400 V 3-phase, while their principal use is in domestic and similar installations. To avoid confusion between the first group (i.e. automatic tripping) and the second group, the term “switch-fuse” should be qualified by the adjectives “automatic” or “non-automatic”.

Fuse - disconnector + discontactor
Fuse - switch-disconnector + discontactor

As previously mentioned, a discontactor does not provide protection against short-circuit faults. It is necessary, therefore, to add fuses (generally of type aM) to perform this function. The combination is used mainly for motor control circuits, where the disconnector or switch-disconnector allows safe operations such as:

  • The changing of fuse links (with the circuit isolated)
  • Work on the circuit downstream of the discontactor (risk of remote closure of the discontactor)

The fuse-disconnector must be interlocked with the discontactor such that no opening or closing manœuvre of the fuse disconnector is possible unless the discontactor is open (Figure H17), since the fuse disconnector has no load-switching capability.

Fig. H17 – Symbol for a fuse disconnector + discontactor

A fuse-switch-disconnector (evidently) requires no interlocking (Figure H18).

The switch must be of class AC22 or AC23 if the circuit supplies a motor.

Fig. H18 – Symbol for a fuse-switch disconnector + discontactor

Circuit-breaker + contactor
Circuit-breaker + discontactor

These combinations are used in remotely controlled distribution systems in which the rate of switching is high, or for control and protection of a circuit supplying motors.