Optimum design of local twin telephone cables with aluminium conductors

Abstract

A method is described of determining, for multipair, twin telephone cables, the relationship between conductor diameter and insulant thickness which will give the best value, in terms of transmission performance, for the cost of the materials used. The method takes account of the value of the space that cables may occupy in ducts. The cost penalties for departures from optimum designs are evaluated. Data are given for optimum-design cables with copper and aluminium conductors and polythene insulation. It is found that cables with aluminium conductors, which have hitherto been designed to reproduce the electrical characteristics of cables with copper conductors, should have a higher capacitance and, paradoxically, a lower resistance than the equivalent cables with copper conductors. This change in electrical characteristics results in cables of reduced diameter, which are a better match to the limiting parameters of the British Post Office (BPO) local-line network. Cables with 0.5mm aluminium conductors, with a capacitance of approximately 108nF/mile, appear very attractive for use by the BPO. They could replace cables with 2½ and 4lb/mile copper conductors in most applications, with significant cost reductions. A new relationship between cable dimensions and capacitance is derived in an Appendix.