Handbook of Electrical Installation Practice - BeKnowledge

Generally, the main switchboard is a factory-built assembly often of composite
design, incorporating circuit-breakers, fuse-switches and circuit-breaker panels.
Motor starters may also be included or may form a separate mcc board.
The rating of circuits is discussed in detail elsewhere, in Chapter 3, Site distribution
systems, but there are a number of general points which are often overlooked
in respect of substation design, which make it easier to understand the ratings commonly
utilised.
A widespread site such as a dockyard or a large petrochemical plant may have
a total load measured in megawatts. It is necessary therefore to have a number of
substations.
Circuits supplying current-using equipment should not have a voltage drop
exceeding 4.0% of the nominal voltage at the design current. However, it may be
necessary to use a conductor larger than that required for the voltage drop to satisfy
the motor starting conditions. In addition the cables and protective gear must be
designed to match the prospective fault current.
To size a cable therefore requires consideration of:
(1) Full load continuous rating
(2) Voltage drop under full load conditions
(3) Motor starting voltage drop
(4) Prospective fault current short time rating.
Substations and Control Rooms 41
Each of these conditions is subject to additional constraints. For example, the full
load rating must take into account the effect of a low voltage and a low power factor.
These conditions will also apply to the voltage drop. In addition, continuous full
load rating must be available despite proximity to other cables and high ambient
temperature. The motor starting situation will also be made more difficult by low
mains voltage and power factor. The fault current is obviously a function of the
supply characteristics. It is wise to allow for some strengthening of the supply system
as time passes.
Fault clearance
The circuit-breakers and fusegear must be able to clear faults before cables are overheated.
They must also themselves be capable of accepting the mechanical, thermal
and electrical stresses imposed by faults. Transformers, busbars, cable boxes and
insulators must also be suitable for the fault level.
To assist in the correct selection of fuses, manufacturers offer a variety of fuse
characteristics. These cover variations in current, voltage, time/current, Joule integral,
cut-offs, power dissipation and frequency including direct current.
The contractor must ensure that he is installing fuses which are appropriate to
the duty. A fuse failure may result in an explosion or the emission of flame. Comprehensive
guides are available from such organisations as ERA (Report No.
87–0186R) and the manufacturers on the selection of fuses. It is particularly important
to ensure that fuses in substations (which are subject to the highest fault levels
on the system) have adequate short-circuit capacity and are fitted with the correct
fuse link to protect the outgoing cables.