Choice of Electric Cable

Choice of Electric Cable Points : Choice of Electric Cable, Basic Information on Electric Cables Choice of Electric Cable There is not much sense in planning a perfect installation on paper and then spoiling it all in practice by installing cheap low quality cables. Within a few months of installing inferior cables, consumers nearly always experience trouble with cables burning out, insulation breaking down, fuses blowing inexplicably, danger of shock ad inefficiency. This usually gets to such a state that a rewire of the whole installation has to be done. In other words, cheap cable is not so cheap after all. Surely the most sensible thing to do in the first place would be to install high quality cables thus saving the cost of a rewire in the near future, and giving the consumer complete confidence in the installation. Details of cables manufactured by Pakistan Cables Limited are given in this section together with advice for their particular use. Basic Information on Electric Cables The purpose of cable is to enable power to be transmitted from the supply to a load with maximum efficiency. General wiring cables are manufactured in two voltage grades — low and medium. For a given cable of the same conductor size the insulation for the medium voltage range is thicker than for the low voltage range, in- order to withstand the increased voltage. The two grades are termed 250/440 volt grade and 660/1100 volt grade.

250/440 volt grade denotes that the cable can be used as a conductor in a system where the voltage to earth is not greater than 250 volts, and the voltage between the conductors is not greater than 440 volts. Thus a 250/440 volt grade cable could be used in a 3 phase 440 volt system having its neutral point solidly earthed.

In the same way, 660/1100 volt grade denotes that the cable can be used as a conductor in a system where the voltage to earth is not greater than 660 volts, and the voltage between the conductors is not greater than 1100 volts. Copper is the most commonly used conductor in cables because fits low resistance to electric current and its cheapness compared with other good conductors of electricity.

A cable can have any number of cores, but the most common are single, twin, three and four core cables. If the size of the conductor in the core is over .044 ins. Diameter; then a number of stranded wires arc used. This is to make the cable more flexible and easier to handle.

For this reason, the process of “spurring off’ can be used, where lead are taken from the nearest socket outlet and run off to the desired position. The standard formations of stranded conductors comprise 3,7, 19, 37, 61 and 127 wires. The reason for these odd numbers .is that, when stranded, they form as near ,as possible a circular sectional shape. A cable can be specified by its nominal cross sectional area, as well as by its number and diameter of wires, e.g., a cable having 7/.036 or 0.007 sq. ins. The area quoted is the sum of the areas of each .036 in. dia. Wire.

Two types of insulation are in common use for general wiring installation. These are Vulcanized Rubber and Poly Vinyl Chloride (commonly known as V.R.I and P.V.C). Depending on the installation requirements, the cable can be just plain’ insulated; or insulation and protected with braid and compound; or insulated and sheathed over-all with vulcanized rubber.

Copper has a deleterious effect on rubber, due to the catalytic oxygen carrying properties of the copper. However, since tin does not react with rubber, this effect is completely overcome by tinning each wire of the conductor of a V.I.R. cable.

The copper conductors need not be tinned in P.V.C insulated cable, since there is not chemical reaction between the P.V.C. and the bare copper. In certain cases they can be supplied tinned to make soldering easier, but naturally are more expensive than the normal untinned type. An example is given below of how a typical cable is specified, e.g. 3 core, 7/.029, 250/440 volt grade, V.R.I., T.R.S. cable.

This means that the cable has three cores, each insulated with vulcanized rubber, and sheathed overall with tough rubber. There are 7 wires stranded in each core, each wire of 029 in: diameter. The cable can be used as a conductor in a system where the voltage to earth does not exceed 250 volts and the voltage between conductors does not exceed 440 volts.