Electrical Engineering ⇒ Topic : Most Economical Conductor Size in a Cable
Most Economical Conductor Size in a Cable
It has already been shown that maximum stress in a cable occurs at the surface of the conductor.For safe working of the cable, dielectric strength of the insulation should be more than the maximum stress. Rewriting the expression for maximum stress, we get
The values of working voltage rand internal sheath diameter D have to be kept fixed at certain values due to design considerations. This leaves conductor diameter d to be the only variable in exp.(i). For given values of f 'and D, the most economical conductor diameter will be one for which gmax has a minimum value. The value of gmax. will be minimum when d loge D/d is maximum i.e.
For low and medium voltage cables, the value of conductor diameter arrived at by this method (i.e., d = V/gMAX) is often too small from the point of view of current density. Therefore, the conductor diameter of such cables is determined from the consideration of safe current density. For high voltage cables, designs based on this theory give a very high value of d, much too large from the point of view of current carrying capacity and it is, therefore, advantageous to increase the conductor diameter to this value. There are three ways of doing this without using excessive copper
(1) Using aluminium instead of copper because for the same current, diameter of aluminium will be more than that of copper.
(2) Using copper wires stranded around a central core of hemp.
(3) Using a central lead tube instead of hemp.
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