Handbook of Electrical Installation Practice - BeKnowledge

Do not confuse emergency switching with emergency stopping which provides a
means of stopping any dangerous movement.
Emergency switching devices should act directly on loaded conductors to disconnect
the supply as quickly as possible. This device may be connected to the load
conductors although it is more commonly connected to the control circuit. The
switch again should be positioned as close to the equipment operating position or
within easy reach and line-of-sight of the operator. Where this is not possible
or practical, the position of the device should be clearly indicated and in sight of
the operator.
Some installations, such as machine workshops, may require emergency switching
to disconnect supplies to all equipment simultaneously. In this case the device(s)
should be sited in a prominent position near to walkways and access points. BS 7671
states the particular requirements for fireman’s emergency switching for high
voltage discharge lighting.
Functional switching
A functional switch is a device that controls the operation of the equipment in its
normal working mode. The requirements for the device are usually contained in the
British Standards relating to the equipment.
Applications
A single switching device complying with BS EN60947–3 may be able to perform
all of the required switching functions provided it is capable of interrupting the
supply on load. Other devices may be used for individual or multiple functions but
may not necessarily be able to perform all functions, in which event more devices
should be used. Table 21.1 indicates the applications for some devices; the list is not
exhaustive.
Detailed requirements
The detailed requirements for isolation and switching are contained in BS 7671 (the
IEE Wiring Regulations) and the associated IEE Guidance Note No. 2. The installation
designer needs to consider all of the relevant Regulations in formulating its
design. In particular, consideration should be given to:
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Control and Protection of Low Voltage Installations 587
The type of building, i.e. domestic, commercial, industrial, etc.
The type of user, i.e. adult, children, elderly, disabled
The environment, i.e. inside, outside, wet, dry, explosive, etc.
Isolation
It is necessary to provide for the isolation of all circuits from all supplies at the origin
of an installation. The means of isolation must be such that it interrupts all live conductors
of the system, except in three-phase supplies connected to TN earthing