33,000-volt cables with metal-sheathed cores, with special reference to the S.L. type

The troubles that are beingexperienced in the use of plain 3-core cables for working at 33,000 volts are referred to and the theories advanced in explanation discussed. The various types of multi-core cable with metal sheaths over the individual cores are then enumerated and their advantages over the plain 3-core type detailed. The claims of the S.L. type in which each core is separately lead-covered, over the other metal-sheathed-core types in which a common lead sheath covers all three cores, are explained. The paper gives practical formulae for calculating sheath effects in singlecore cables, together with the results of a series of tests carried out to determine the extent of these effects in the S.L. type. Consideration is given to the temperature-rise in the various types, along with the results of a series of heating tests. As the employment of the S.L. type of cable is a move in the direction of employing single-core instead of 3-core cable, this question is also discussed and results of loading tests on both plain lead and armoured single-core cables are given. The mechanical properties of the S.L. type are also investigated and in the conclusions at the end of the paper it is shown that the presence of the sheaths, in addition to having well-known electrical advantages, produces a very distinct improvement in the thermal properties of the cable and results in a construction which is very much sounder, both mechanically and electrically, than other systems of multi-core super-tension cable.