The installation of metal-sheathed cables on spaced supports

The installation of cables above ground, mainly out of doors on hooks on posts, is a construction which has been extensively employed with lead-sheathed power cables, and some of them have developed sheath fractures, shown by compound leakage, as a result of flexing in service caused by expansion and contraction with changes of temperature. While electrical failures due to this cause have been few, the desirability of eliminating or minimizing such sheath fractures, with the resultant loss of compound and eventual possibility of the entrance of moisture, needs no emphasis. The design of such installations, in order to achieve this purpose, has apparently never received the attention which it merits in view of the large capital value involved. The behaviour of cables installed in this way is examined theoretically and by reference to service installations, and the conclusion is reached that the length between supports is the most important factor; normal practice has been to make this too short. A method is developed for determining for each cable what the minimum length should be, and other features of installation design are dealt with. Aluminium-sheathed cables erected similarly are also discussed. The conclusions are the author´s responsibility, and intended to give a lead to the industry in establishing much-needed recognized standards.