Handbook of Electrical Installation Practice - BeKnowledge

586 Handbook of Electrical Installation Practice
An isolator, also known as a disconnecter, is a device which disconnects the supply
to the system, or parts of the system, in the off-load condition. Generally, this should
be accessible to skilled persons only.
An isolating switch, or switch disconnecter, should be capable of disconnecting
the supply to the system under all possible conditions, i.e. operating, overload and
fault conditions. This is intended for use by unskilled persons.
Isolators are often confused with isolating switches so when selecting isolation
devices it is essential that the correct application is known and the appropriate
device is selected. Isolators should be situated in a readily accessible position having
regard to the use of the installation and premises, the capabilities of personnel and
the nature of the activity.
An isolating device should comply with BS EN 60974–3 which sets out the creepage
and clearance requirements when in the open condition. It should have a positive
indication of the ‘off’ or ‘open’ position which should not be displayed until
the gap between the contacts has been achieved. The purpose of the device, and if
necessary the parts of the installation it controls, need clearly marking on or adjacent
to it so that there is no possibility of confusion. Access to the contacts and
terminals should not be possible whilst the device contacts are closed. Provision for
securing in the open position is required wherever it is likely that inadvertent or
unintentional operation could cause danger.
Switching off for mechanical maintenance
This term is used to indicate the need to be able to disconnect equipment from the
supply to enable non-electrical maintenance, repairs or replacements to equipment
to be carried out safely by persons who may not necessarily be electrically skilled.
The intention is to provide a means of local control for the person doing maintenance
work so as to ensure that the equipment being worked on cannot start up
while this work is in progress.
The switch does not necessarily have to have the same characteristics required of
the isolator and does not have to be connected to the loaded conductors. However,
if connected into the load conductors it must be able to switch off the load current
under full operating conditions. Particular regard should be given to inductive and
capacitive loads. Where the switch is connected to a control circuit it should be so
arranged that a fault on the circuit will not have the effect of bypassing the control
switch and thus allowing the equipment to operate.
Where switches provide for mechanical maintenance work, these should be positioned
so as to be under the complete control of the person(s) carrying out the work.
They should be sited away from the equipment but must be positioned so that they
can either be secured in the ‘off’ or ‘open’ position, or be in a lockable enclosure.
In the latter situation, the equipment must be clearly labelled to indicate the position
of the disconnecting switch.
Emergency switching
The definition of emergency switching is ‘Rapid cutting-off of electrical energy to
remove any unexpected hazard to persons, livestock or property. The hazard may
be either electrical or mechanical’.