Handbook of Electrical Installation Practice - BeKnowledge

involves civil works. The supply authority will normally require information about
any new installation but apart from checking the busbars immediately after their
own protection the supply authority does not have a responsibility for the client’s
installation.
As stated, the arrangement of supply to consumers’ premises depends primarily
on the nature of the load. For loads from say 100 to 300kVA, a 400/230V threephase
supply from an Electricity Distributor’s substation is normally satisfactory. A
load of between, say, 200 and 500kVA may have its own Electricity Distributor’s
transformer.Above 500kVA one might expect an 11kV supply and above 1000kVA
it would normally be split into two or more 11kV circuits. Where a duplicate supply
is provided the incomers are generally arranged to have manual changeover facilities
or to be fully automatic with feeder protection depending on whether or not a
firm supply is required. Firm supplies are normally provided for all loads above
5MVA.
Internal to the factory or other industrial complexes single radial feeders are generally
acceptable provided that there is no safety hazard or process which is unduly
sensitive to the loss of supply. A smelter or furnace could be badly damaged by the
solidification of molten metal caused by a supply failure. In such circumstances a
ring main may be installed or duplication of connections to the sensitive load. It is
also important to remember that electrical equipment including busbars, transformers
and circuit-breakers needs maintenance, and therefore bus-section
switches, duplicate transformers and interconnecting cabling may be inescapable for
a continuous process.
An alternative supply may be better obtained from an in-house standby generator
than a duplicate feed. This is particularly so in remote areas where it is recognised
that there is only a single overhead line, or in overseas territories with
unreliable power supply systems.
It is simpler and there is less risk of damage to equipment if the standby supply
is not synchronised with the public supply.This is generally satisfactory since a small
diesel generator can pick up full load in about a minute. Where there are installations
such as computers for which a continuous supply is essential, a batteryoperated
inverter may be used. Emergency lighting is traditionally energised by
direct current (d.c.) and it is wise to consider whether circuit-breaker closing and
tripping supplies should be d.c. together with supplies to contactors and electrically
operated valves.
SUBSTATIONS
Substations and Control Rooms 37
Early consultation with the local Electricity Distributor is essential for agreement
on a mutually approved substation to act as the intake point for a particular site.
This consultation is usually before detailed knowledge of the plant or project is
known but it is essential that a fairly accurate load requirement be determined. Plant
manufacturers must be approached to provide information relative to the equipment
they are supplying and this, together with experience, enables a reasonably
reliable load demand to be ascertained. Having this knowledge enables the rating
of the power transformers and of associated switchgear to be decided.