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The electrical current in a conductor causes a voltage drop and power losses. The temperature rise in the cable caused by the losses must be kept within certain limits in order not to shorten the service life of the cable. The temperature limit varies with the type of cable, or more precisely the type of insulation. The best way to keep the temperature within limits specified for the type of cable, is to choose the conductor cross-section so that the cable and its surroundings with the actual continuous load, achieve thermal balance at a temperature below or equal to the temperature limit recommended.
Cable type and size should be selected keeping in view the following.
1. Application.
2. Working voltage, earthed or unearthed neutral system.
3. Load current, load factor, starting duty and frequency
4. Installation method.
5. The environment in which the cable has to operate.
6. Short circuit current and system protection.
7. Acceptable voltage drop.
8. Economics.
How the above factors influence on the choice of cable?
1. The application of the cable determines the basic factors for the choice of cable type and the rules according to which it must be manufactured.Following are the major factors for choice of cable type.
a. Conductor material: No doubt copper is better conductor material owing to its high electrical conductivity and other electrical/mechanical properties but due to its high cost and scarcity, researches are being carried out to find other cheaper and abundant metals which may be a close substitute to copper in electrical application. Aluminium conductors are being used in place of copper conductors for the past many years and have proved to be quit satisfactory.
The lower conductivity of a aluminium (61% of the annealed copper) results increased dimensions of the cable and ultimately of conduits and fitting fot the same current carrying capacity as that of a copper conductor cable. Aluminium conductor than the equivalent copper conductor cables, are still flexible enough to be installed where a copper conductor cable was formerly used. The advantages of the lighter weight offsets the disadvantages of larger sizes for a given capacity.
b. The use of aluminium conductor cables should be preferred as far as possible. The use of flexible copper conductor is recommended where very high degree of flexibility is required as in the case of mines etc.
c. Inner Sheath: in case of PVC & XLPE Cables the normal inner sheath is done by PVC tape, but in special case, a layer of PVC extruded inner sheath is also used.
d. Armouring: The purpose of armour is to provided mechanical protection to the cable and to facilitate earthing for safety requirements. This also carries phase to ground fault currents of the system safely.
Double steel tape provides good mechanical protecting but when in addition longitudinal stresses are encountered during the installation or in service, steel wore/stip armouring should be preferred. Double wire armouring is recommended for vertical runs, river crossing, cables laid on bridges mine shafts etc. Where longitudinal stresses are expected.`
2. The system voltage determines the voltages class of the cables.
3. The current rating is, in general, the decisive factor for fixing conductor cross section. But in certain applications where intermittent load is required, it is more relevant to use the squared average r.m.s. current with a reduced cross section.
4. The power cables must be capable of carrying, the required normal full load current continuously under the site conditions throughout the year. Therefore, the current ratings specified must be corrected to site conditions by applying suitable derating/uprating factors depending upon:-
5. A. Chemical substances in the environment might cause special stringent requirements on the outer covering.
B. If it is required or necessary to reduce the propagation of the fire along cable route combined with low corrosivity, toxicity and smoke generation characteristics for cables. "FRLS" cable (Flame Retardant Low Smoke) with thermoplastic or thermosetting material or fire survival cables with elastomeric material should be used. These types of cables are use in such critical fire risk installations.
6. Short circuit current together with duration of short circuit determines the short circuit energy the cable insulation has to withstand thermally. In certain cases a larger size of cable then the cable required for normal full load current may be needed to match system short circuit current levels.
7. Voltage drop is also a major factor in deciding the conductor size of the cable. The cross section of the cable should be chosen such that voltage drop of the cable for the given route length does not exceed the statutory requirement.
8. Naturally, the most economical construction and the size of the cable consistent with required current carrying capacity and laying condition has to be selected. Thus the selection of particula type of cable i.e. PVC or XLPE and the particular material for sheathing, bedding, armouring etc. out of many choices available depends upon the usages. laying and climatic conditions. The design of the cable for a particular application must be optimized taking into account all the above mentioned factors.