6.1.1 - Basic requirements for circuits
The Regulations require that installations
should be divided into circuits, the purposes being:
1. - to prevent danger in the event
of a fault by ensuring that the fault current is no greater
than necessary to operate the protective system. For example,
a large three-phase motor must be connected to a single
circuit because the load cannot be subdivided. If, however,
a load consisted of three hundred lamps, each rated at 100
W, it would be foolish to consider putting all this load
onto a single circuit. In the event of a fault, the whole
of the lighting would be lost, and the fault current needed
to operate the protective device (single-phase circuit current
would be 125 A at 240 V) would be high enough to cause a
fire danger at the outlet where the fault occurred. The
correct approach would be to divide the load into smaller
circuits, each feeding, perhaps, ten lamps.
2. - to enable part of an installation
to be switched off for maintenance or for testing without
affecting the rest of the system.
3. - to prevent a fault on one circuit
from resulting in the loss of the complete installation
(see
{3.8.6} on the subject of discrimination).
The number of final circuits will depend
on the types of load supplied, and must be designed to comply
with the requirements for overcurrent protection, switching
and the current-carrying capacity of conductors. Every circuit
must be separate from others and must be connected to its
own overcurrent protective fuse or circuit breaker in a
switch fuse, distribution board, consumer's unit, etc. See
{Fig 6.1 and Fig 6.2}.
Fig 6.1 Typical arrangement
for feeding final circuits in a domestic installation
Fig 6.2 An arrangement
for main and final circuits in a large installation
A durable notice giving details of all
the circuits fed is required to be posted in or near each
distribution board. The data required is the equipment served
by each circuit, its rating, its design current and its
breaking capacity. When the occupancy of the premises changes,
the new occupier must be provided with full details of the
installation (see reference to the Operating Manual in {8.8.1}).
This data must always be kept up to date.
