Handbook of Electrical Installation Practice - BeKnowledge

Switching off for mechanical maintenance
This function of switching off for mechanical maintenance is restricted to maintenance
which does not involve access to live parts.
Where mechanical maintenance may involve a risk of burns or injury from
mechanical movement, provision must be made for the equipment to be switched
off while the maintenance is carried out. This type of maintenance may be done by
a person who is not electrically skilled and who has no understanding of the control
systems for the equipment. It is, therefore, essential that the switch is positioned
within easy access of and as close as possible to the equipment.
Care must be taken to ensure that the switch cannot be unintentionally or inadvertently
operated and it must be under the control or supervision of the person
doing the work, at all times. If it is not possible for the switch to be under such
control then facilities must be provided for securing the switch in the ‘off’ or ‘open’
position. This is normally effected by a locking device fitted to the switch but may
also be effected by siting the switch in a locked room where access is restricted to
authorised personnel only.
The switch should directly interrupt the load conductors if and where practical.
When this is not practical, as in the situation of a motor controlled by a star/delta
starter, the switch may operate on the control circuit of the starter unit. In this case
the switch must be of the stop/lock type and the control circuit wiring must be
designed and arranged to ensure that the switch cannot be overridden, either unintentionally
or by a fault occurring in the control circuit.
When designing an installation it may be advantageous to the user to select a
single device that will perform the functions of both isolation and switching off for
mechanical maintenance.
Emergency switching
Emergency switching is necessary where a possible danger may arise from equipment
while it is performing its normal function. Such equipment may be heating,
cooling or rotating machinery and machinery where there may be of necessity
exposed live parts constituting a shock risk.
The emergency switch must act directly on the supply conductors by a single
operation of the device. It must be readily accessible, clearly visible, have its
purpose marked on or adjacent to it and, for additional identification, be coloured
red.
Switching devices may be:
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Control and Protection of Low Voltage Installations 589
Inserted directly into the supply circuit
A push switch in the control circuit
An electro-sensitive safety control system incorporated into a machine.
Where a push switch in the control circuit is used it should be capable of being
latched or locked in the ‘open’ position. A plug and socket arrangement should not
be selected as an emergency switching device.