The current rating of single-conductor, lead-covered, low-tension cables on single-phase alternating-current circuits

Single-core cables in single-phase systems are bonded to annul the high voltages that would otherwise exist between the lead sheaths when the latter are open-circuited, but bonding brings into being induced currents which may reach high percentages of the conductor current and reduce the safe current-carrying capacity. In this paper some theoretical considerations of the losses of energy occurring in single-core cables on alternating-current circuits are given, and their effect on the line characteristics are noted. In the first portion of the paper the results of measurements carried out on a single-phase system in air to determine the loadings for a given temperature-rise are tabulated and represented graphically. In a series of experiments the following factors were varied one at a time:- (1) Current flowing in the system. (2) Spacing of conductors in an iron-free neighbourhood. (3) Spacing of conductors with iron interposed. (4) Cable cross-section. It is noted that, taking into account the mutual heating effect of two cables in proximity and the heating due to alternating currents flowing in the system, there is an arrangement of the cables in a horizontal plane which gives the maximum value of permissible current loading.