Handbook of Electrical Installation Practice - BeKnowledge

CABLES INSTALLATION
Since this chapter deals with site distribution it is not intended to describe here cable
installations within buildings, although it should be recognised that some parts of
the site distribution network may well be within buildings either in ducts or cable
trenches or on cable tray or ladder systems.
The major proportion of route cable length is however likely to be laid underground.
Where the ground may be easily opened up for subsequent access to the
cables they may be buried directly in the soil. In locations where cables are to pass
beneath roads, railways or large areas of concrete or tarmac, the cost and inconvenience
of subsequent excavation (and reinstatement) to gain access to the cables
makes it desirable to run them in suitable ducts, at least beneath these areas. Where
ducts are employed it is necessary to provide draw-pits at all bends and at approximately
every 50m on straight and level runs. Closer intervals may be required in
adverse circumstances. Figure 3.10 illustrates a typical draw-pit. Careful design of
the draw-pit is necessary to ensure that cables can be pulled into it and then drawn
back down into the outgoing duct for the next leg of the run. Any joints that are
required should be positioned in a draw-pit and its dimensions should allow for this.
Suitable access covers should be provided and must be capable of withstanding the
maximum traffic loading to which they will be subjected.
Where cables are buried directly in the ground they should be approximately
750mm deep for l.v. cables and preferably 1000mm deep in the case of h.v. The
presence of underground cables, even in ducts, is best indicated by laying proprietary
plastic marker tape above the cable and about 150mm below the final
surface level. Such tape is usually yellow and bears a repeated inscription such as
‘ELECTRICITY CABLES BELOW’. These tapes are reasonably likely to be
observed at an early stage by people digging on the route of the cable, whereas
since the use of mechanical excavators became widespread, the employment of
concrete cable-tiles has been found in practice to offer little protection. Figure 3.11
illustrates a cross-section through a suitable completed cable trench.
PROVISION FOR MAINTENANCE
Site Distribution Systems 77
Modern transformers and switchgear of reputable manufacture are generally
very reliable and maintenance requirements have been steadily reduced over the
years. Nevertheless, all such apparatus at some time, however infrequently, will need
to be isolated for inspection, testing, maintenance or repair. The configuration of
the system and the type of equipment installed will determine how much operational
disruption will be caused while such work is undertaken. For example, a
conventional non-extensible RMU has a relatively low first cost but in order to
carry out maintenance involving any of its h.v. components the whole RMU must
be isolated from all sources of supply so that the entire substation is out of service.
On the other hand a withdrawable circuit-breaker can be isolated and maintained
while both its busbar and circuit spouts remain live. It may even be replaced by a
spare circuit-breaker while maintenance is carried out. It does, however, have a
high first cost.