Handbook of Electrical Installation Practice - BeKnowledge

Electrical Safety 291
poultry stunning cabinets, particularly the wet variety, have to be accessible for
cleaning. If the cabinet is within an enclosure, the access door can be interlocked
with the isolator in the same way as the switchgear cubicle, thus making it impossible
for operatives to gain access when the stunner is energised.
If it is necessary to ‘lock off’ a number of isolators to make a system safe to work
on, this should be done by an authorised competent person. He then puts the keys
in the key safe, locks it and retains its key or lodges it in a secure place, accessible
only to him or other responsible persons. None of the isolators can then be unlocked
and operated until a responsible person has satisfied himself that it is safe to do so
and has unlocked the key safe and made the isolators’ padlock keys available.
Multiple padlocking is appropriate when there is only one isolator controlling a
scattered system which is made ‘dead’ so that a number of electricians can work on
it simultaneously. The isolator is secured in the ‘off’ position by a caliper-shaped
device with a number of holes in it, for the insertion of a padlock by each operative.
Each operative has to unlock and remove his own padlock before the device
can be withdrawn and the isolator reclosed.
Production line testing of domestic, electrical product, part-assemblies is a
common application of interlocking using interlock switches. In these cases, the test
personnel are likely to be relatively unskilled and are not competent persons and
so they must not be able to touch exposed parts when ‘live’ or have access to ‘live’
terminals, yet the testing must be simple and quick to avoid disrupting production
efficiency. If the test piece has exposed ‘live’ conductors, the testing should take
place inside an interlocked box or enclosure. The operator inserts the test piece
which is automatically connected or is connected by the operator who then
closes the door or lid and, in so doing, automatically operates the test supply switch
which is interlocked with it. The switch is ‘on’ only when the door or lid is closed.
The interlock switch must be of the ‘fail safe’ type, as shown in Fig. 11.10, where
the circuit is made by the operation of the spring but is positively broken by the
Fig. 11.10 Interlocked enclosure for production line testing.