Handbook of Electrical Installation Practice - BeKnowledge

36 Handbook of Electrical Installation Practice
served. If the load demand is of sufficient magnitude, the consumer will find it practical
and economic to take the supply at high voltage (h.v.). For the purpose of this
handbook the upper limit is 11kV, although for very large complexes the intake
might be at 33kV or even higher. It is the consumer’s responsibility to control and
distribute the electricity around his site or premises to suit his own requirements.
The main substation serving a network incorporates the supply intake point,
but there may be other substations located at strategic areas in a large complex
and connected to the main substation by a cable network. The manner in which
this interlinking occurs depends on a number of factors and is fully discussed in
Chapter 3.
If the supply is taken at 11kV the main substation will include 11kV switchgear
which may serve other substations at 11kV throughout the site. Stepdown transformers
at the site substations provide power at the consumer voltage. For very
small installations the intake may be a single cable providing a three-phase 400/
230V supply to a switch and fuse board. In between are a variety of arrangements.
Depending on the importance of the load served, there may be a standby generator(s)
linked to the system and arranged to supply essential loads. Some
manufacturing complexes may generate their own power, particularly where the
generation of process steam is related to the electrical requirements. Generally
speaking it is uneconomic to rely on self-generation totally because of the cost of
standby generators. Such sites would normally have a standby supply from the mains
network. In that case synchronising is required in the main substation to allow
paralleling of the two systems. Alternatively, the two systems are interlocked to
prevent accidental paralleling. Particular attention would be required to limit the
fault power, the effects of reverse power and for discrimination under fault conditions.
There would also be a problem in sharing responsibility. The magnitude and
the importance of the load dictate the nature of the supply and distribution network
and hence the design of the substations and control facilities.
Two points should be noted in respect of the demarcation between the supply
authority and the consumer’s equipment. The first is that there may be annual rates
levied on any transformer between the supply point and the consumer’s distribution
system. Secondly, while the supply authority may provide, install, operate and
own all of the equipment up to the point of supply, it may require the consumer to
make a contribution to the capital cost of making this supply available and will in
many cases ask for a site on the consumer’s premises to house it. It is common but
not invariable for the supply authority to insist on a separate room or locked-off
area for its apparatus.
There is therefore a wide variety of possible arrangements. The client’s contractor
is normally responsible for ensuring that the whole scheme is installed in a safe
and secure manner with due regard to all health and safety regulations in accordance
with the Health and Safety at Work etc. Act and in compliance with the
client’s specifications. The site engineer therefore has a heavy responsibility. If a
design drawing shows an unsafe arrangement or one which conflicts with good practice,
he must draw this to the attention of his employer so that it may be remedied.
Electrical power systems can cause death and injury if not properly installed. The
local authority and the fire authority inspect any new properties to check that they
are safe. If they are not, the remedial work could be expensive, especially if it