Materials for Wiring Installations

Wiring: general

Wiring may be carried out in several ways, which are conveniently discussed under four headings: the conductor the insulation the sheath the enclosure.

The conductor

In the majority of cases the conductors used in domestic or small commercial wiring installations are made of high conductivity copper, sometimes covered with a coating of tin to assist in preventing corrosion. In some cases, however, aluminium conductors are used.

Often, a single strand of conductor is used but to give more flexibility to heavier duty cables a number of small wires are stranded together to make a larger conductor.

The important aspect of the conductor is of course the total cross-sectional area of the metal, since the larger the metal area, the greater the current-carrying capacity. For example, a wire size very commonly used is known as 2.5 mm2. This means that the conductor has a cross-section area of 2.5 mm2.

The insulation

To insulate the live conductors, various methods are used.

Plastic insulation

There are several forms of plastic materials that are commonly used to insulate wiring conductors. First there is polyvinyl chloride which is extremely tough, will not support combustion, and is not affected by water, oil, or most chemicals. It has however a definite temperature limit, above which it will melt and leave the conductor bare.

Then there is polythene insulation, made from a different type of plastic, with a lower safe temperature rating.

PVC/SWA cable uses PVC insulation, but is enclosed in a protective wrapping of steel wires, to form an ‘armouring’ or protective covering. This type of cable is normally used where a high degree of mechanical protection is required, such as underground between buildings.

Rubber insulation

This is the oldest form of insulation, and is still used, but is largely superseded by plastic materials for all but special applications.

Rubber is very flexible, but has a number of disadvantages. It burns easily, and it can be attacked by chemicals and by oil. It is to some extent absorbent to water. Direct sunlight soon causes rubber to deteriorate, so unprotected rubber cables should never be used outdoors. It is also attacked by certain insects, and in any case it ages and tends to become brittle.

The sheath

The insulated conductor is more often than not grouped with others within an insulated sheath, to form a cable. A cable may have two or more cores – each core being a separately insulated conductor, except where the earth connection is also carried within the sheath. A common form of cable consists of two cores and earth – a red-coloured insulated conductor for connection to the phase wire, and a black-coloured insulated conductor for connection to the neutral wire, together with an uninsulated protective conductor.

These three cores are usually laid in a flat formation surrounded by a sheath or outer casing, for protection.

The cable may be made up with a single core, which still has an outer sheath for protection against mechanical damage, or with any number of cores. For 3-phase circuits there will be three phase conductors, each coloured brown, and a neutral conductor, coloured blue, all within the sheath.

Sheaths may be made up as follows:

Plastic sheathing

For PVC-insulated cores a PVC sheath is most often used. A variant is a polythene-insulated core or cores covered with a PVC sheath.

Tough rubber

Older cables are known as TRS.

Rubber sheathing is usually employed with rubber-insulated cores.

Mineral-insulated cables

These cables, though more expensive than the flexible sheathed type mentioned above, are capable of being used in situations where no other cable could be employed.

Single-strand copper wires are embedded in a tightly compressed white powder insulation within a copper or aluminium sheath. The powder is made of magnesium oxide.

The MI cable can be subjected to the full heat of a blowlamp without damage. With an overall covering of PVC the cable may even be buried direct in the ground.

The only disadvantage of the MI cable is that great care must be taken when it is cut and made off into a terminal. This is because the magnesium oxide is hygroscopic -in effect, it attracts moisture, which can reduce the insulation value. Therefore the ends of any MI cable must at all times be kept properly sealed, using the special sealing equipment supplied by the manufacturers. When the cable is cut and made off, the simple instructions supplied by the makers must always be carefully followed.

The enclosure

Electric wiring is best protected against mechanical damage by being inserted in a suitable enclosure.

Heavy gauge, steel screwed conduit

In this system, a complete enclosure made of screwed piping, and including all termination boxes and joint boxes, is provided. The steel conduit is cut to length, bent if necessary, and screwed so that it provides a perfect seal where it enters a termination box or where it is jointed.

The conduit system means the use of a considerable number of special tools such as dies, bending machines, pipe vices, and saws. It is costly, and although it should always be designed in accordance with the Regulations so that the conduits are not tightly packed with wire, and draw-in points should be accessible, it is nevertheless not always easy to alter or extend a conduit system. A certain degree of skill and experience is needed before a conduit system is embarked upon, although there are no insuperable difficulties that could not be overcome by anyone used to handling metal-working tools.

Other types of conduit

Aluminium alloy conduits are sometimes used, and these must be screwed at joints and installed in the same way as steel conduits. They are subject to corrosion when embedded in cement and plaster, but may be protected by bitumastic paint. Non-metallic conduit systems are also widely employed. These are made from a plastic tube which can be bent round corners. Some systems use screwed ends to fit into joints and terminal boxes, other systems employ cemented joints. This system is particularly suitable for installation where there is a strong possibility of corrosion, but where mechanical damage is not likely. Each circuit wired via this conduit must be connected to earth by a separate wire, with a covering of green/yellow PVC pulled in with the current carrying cables.

Protective channelling

While the conduit systems mentioned previously are used for the highest class of work and enclose single-core PVC cables to the required number, many domestic installations employ PVC sheathed cable, run on the surface where it is safe from damage, but protected when immersed in plaster by means of shaped metal channelling, or by short lengths of round or oval plastic tubing.

Steel and plastic trunking

Where a large number of cables have to be run in one direction, steel or plastic trunking, often of square section with a screwed-down or clipped removable lid is used. Steel trunking must be electrically continuous and bonded securely to earth. Plastic trunking must have an insulated protective conductor coloured green and yellow between all apparatus connected to the system.

Trunking is a very flexible system of installation. Suppose, for instance, a small business were started in a workshop, where initially only a few lights and one or two machines were required.

This installation could be completed using PVC cables in conduit, at moderate cost. If trunking were used, run at a high level on the walls around the room, the positions of lights, switches and machines could be fed using conduits, connected to the trunking and run down the walls to each position.

Though much of the installation would be superfluous at first, there would be plenty of room in the trunking for future additional wiring in the case of business expansion, requiring extra lights and machines, sockets, etc.

PVC steel wire armoured PVC

This cable has stranded copper conductors with PVC insulation, enclosed in galvanised steel wire armouring and an outer covering of PVC. It is often used for underground supplies to outbuildings, laid in fine earth or sand and covered with protective tiles. The cable may be fixed onto most surfaces using plastic clips; terminations require glands which are simple and quickly made off to the cable ends