Railway cable with high screening properties

Telecommunications cables installed in close proximity to power transmission lines or a.c. electrified railways, are subject to induced voltages resulting from mutual inductive coupling between the circuits. The magnitude of these induced voltages, which can assume dangerous values, are dependent upon the separation between the circuits concerned, the inducing current and frequency, the electrical parameters of the cable protective sheathing, and the earth conductivity. In Norway long parallelisms between power and telephone cable systems exist and earth conductivity is low. This presented serious problems with respect to railway communications cables and also the national telephone cables which frequently follow the same route, or even occupy the same trenches. In this paper the authors describe the theoretical and experimental work carried out to design communications cables having protective sheathing which would effectively screen the included circuits from induced voltages. It was concluded that in certain cases the problem was only soluble economically by the employment of aluminium sheathing with an overall steel tape armouring. Technical information is given of the Oslo-Bergen electrified line and details of the estimated induced voltages in the track communications cables. Induced voltage and screening factor curves are given for the types of sheathing con-considered.