External wiring

It is sometimes necessary to run cables from one building to another. This can be done
either with cables running under the ground or with overhead cables. Cables that are to be
run underground can be armoured paper, armoured PVC or MICC cables. All these are
strong enough to be laid directly in the ground and buried, but in order that they should
have reasonable protection against damage they should be laid at least 600mm below the
ground level. Since they all have metal armouring or sheathing, no protection in the
ground is really necessary, but it is very usual to provide a bed of sand, and cover the
cable loosely with tiles before the trench in which it has been laid is backfilled. If
someone later has occasion to dig the ground near the cable they will hit the tiles first and
be warned that there is something underneath them. Tiles are available for this use which
have lettering on them saying ‘Danger–Electric Cables’; they are known as electrical
tiles. A cheaper alternative is plastic tape with the same lettering. This has almost
universally superseded tiles although it may be questioned whether it will be noticed
before the digging tool has gone through it and the cable. Another method is to install
over ground markers showing the line of the run of the underground cable
It is also possible to bury conduit in the ground and pull cables through it in the
ordinary way. Because of the danger of corrosion in the soil this is not however a good
practice. It is better to use builder’s polythene pipe as ducts instead of conduit, and in fact
this is very often done. It is harder to provide frequent access to underground ducts than
to conduit in a building, and the lengths between draw-in points can become rather large.
At the same time there is plenty of room in the ground for large-diameter pipes, and it is a
sound precaution never to use anything smaller than a 100mm polythene or earthenware
duct.
In certain cases it is common practice to use polythene conduit underground. This
happens for example when the cable from a communal TV aerial has to cross from one
building to another on the same site. It is also the standard method of bringing telephone
cables into a building in urban areas where the main telephone cables are in the road
outside the new building. Telephone cables are quite small and can be easily pulled into
20mm conduit over considerable distances.
Buried cables rely on conduction of heat through the soil to dissipate the heat
generated by the current in the resistance of the cables. If there are other services which
heat the soil locally then the rate of dissipation of heat could be reduced, and the current
carrying capacity of the cables would then also be reduced. The obvious example of a
service which would have this effect is a district heating main. But in addition to this
consideration some thought must be given to what happens when maintenance work is
done on underground services. It is undesirable that workmen who may have to expose a
length of buried gas or water main should have to dig near a live electric cable. There are
thus two reasons why underground cables should be kept well away from other buried
services. A good practical rule is to have a minimum distance apart of 2m.
There may be cases where cables have to cross from one building to another overhead.
This situation will normally arise where cost is the overriding factor and neither
restrictions on headroom nor appearance are of great concern. For very short distances, as
for example from a house to a shed, a piece of conduit can support its own weight over
the gap to be spanned and an ordinary conduit system can be used. This method is not
suitable for distances of more than about 3m. For larger distances special overhead cables
must be used. Cables made for this purpose consist of solid drawn copper or aluminium
conductors covered with a PVC sheath and are suitable for spans of up to 30m. They can
traverse greater distances provided they are supported every 30m. If supports cannot be
found on walls or roofs then wooden or metal poles must be erected to carry cleats on
which the cable can be supported.
The limitation on span can be overcome by the use of a catenary cable, which can be
used in one of two ways. A separate catenary cable of adequate strength can be strung
Design of electrical services for buildings 66

between the two end supports and a sheathed cable can be suspended from the catenary at
regular intervals of about 2 or 3m.
Alternatively it is possible to obtain cable which incorporates a catenary cable within
the sheath. Such cable is specially made for overhead use, and the manufacturer’s
recommendations on spacing should be noted and adhered to.
Since air is a good insulator, overhead cables which are out of reach of people or
animals do not need further insulation. The cables we have just described are sheathed,
but overhead line cable is also made consisting of bare copper or aluminium conductors
of a size to have enough mechanical strength to support themselves. These cables can
span long distances and are used by electricity companies for their distribution systems
outside towns. They are used much more in public supply systems than in building
services. Whatever method of running overhead cables is used, at the end of the overhead
section the cables must be connected to the cables within the building in a terminal block
and in such a way that there is no mechanical strain on them.