Handbook of Electrical Installation Practice - BeKnowledge

588 Handbook of Electrical Installation Practice
Table 21.1 Applications for switching devices.
* Provided it is capable of breaking the full load current.
systems where the neutral conductor need not be isolated if it can be assured that
it is connected to earth.
Notwithstanding the above condition, provision must be made on all installations
for disconnection of the neutral conductor from earth for testing purposes. So, in
practice, it is normally necessary to provide facilities for isolating all live conductors
regardless of the earthing system.
Installations with supplies from more than one source must have isolating facilities
provided for all supply sources. This may be by means of separate, linked or
interlocked devices. Isolating devices must be clearly marked indicating the supply
source so that there can be no possibility of confusion.
Domestic types of installation, or installations controlled by a single distribution
board, may only require a single isolating device incorporated either in the consumer
unit or distribution board. In larger and more complex installations a single isolating
device will prove to be very inconvenient for the user. It is, therefore, necessary
to provide a number of isolators to control different sections of the premises and
different operating systems. In such installations the designer should consider the
various aspects of control, use and system maintenance; and, in addition to the main
isolator, provide for isolation of small circuit groups and even individual circuits.The
minimum standard should be for every distributor and submain to be controlled by
an isolator. If the isolation requirements are not properly considered and thus not
properly planned, switching off live parts for maintenance or repair is likely to cause
considerable disruption and inconvenience and can result in persons taking chances
and working on live equipment without first isolating the supply – a dangerous practice
which is virtually prohibited under the Electricity at Work Regulations.
Where an earthing system is either TN-S or TN-C-S, isolators fitted downstream
from the main isolator need only interrupt phase conductors. Where the earthing
system is TT or IT, all isolators must interrupt all live conductors.