6.2.2 - Diversity
A domestic ring circuit typically feeds
a large number of 13 A sockets hut is usually protected
by a fuse or circuit breaker rated at 30 A or 32 A. This
means that if sockets were feeding 13 A loads, more than
two of them in use at the same time would overload the circuit
and it would be disconnected by its protective device.
practice, the chances of all domestic ring sockets feeding
loads taking 13 A is small. Whilst there maybe a 3 kW washing
machine in the kitchen, a 3 kW heater in the living room
and another in the bedroom, the chance of all three being
in use at the same time is remote. If they are all connected
at the same time, this could be seen as a failure of the
designer when assessing the installation requirements; the
installation should have two ring circuits to feed the parts
of the house in question.
Most sockets, then, will feed smaller loads
such as table lamps, vacuum cleaner, television or audio
machines and so on. The chances of all the sockets being
used simultaneously is remote in the extreme provided that
the number of sockets (and ring circuits) installed is large
enough. The condition that only a few sockets will be in
use at the same time, and that the loads they feed will
be small is called diversity.
By making allowance for reasonable diversity,
the number of circuits and their rating can be reduced,
with a consequent financial saving, but without reducing
the effectiveness of the installation. However, if diversity
is over-estimated, the normal current demands will exceed
the ratings of the protective devices, which will disconnect
the circuits - not a welcome prospect for the user of the
installation! Overheating may also result from overloading
which exceeds the rating of the protective device, but does
not reach its operating current in a reasonably short time.
The Regulations require that circuit design should prevent
the occurrence of small overloads of long duration.
The sensible application of diversity to
the design of an installation calls for experience and a
detailed knowledge of the intended use of the installation.
Future possible increase in load should also be taken into
account. Diversity relies on a number of factors which can
only be properly assessed in the light of detailed knowledge
of the type of installation, the industrial process concerned
where this applies, and the habits and practices of the
users, Perhaps a glimpse into a crystal ball to foresee
the future could also be useful!