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.
