The resistance and reactance of armored cables

It has been asserted frequently that no form of iron protective covering is permissible for single conductors carrying alternating currents. Experience and simple calculation show this assertion to be well founded with regard to even the thinnest walls, if such walls form a completely closed magnetic circuit about the conductor. In a recent single-phase railway installation it was necessary to carry the trolley circuit across a drawbridge and the nature of the traffic through the draw rendered it advisable to armor the cables, several of which were to be installed for reserve and emergency. Obviously, the use of single-conductor, steel-armored cables was permissible if the reactance and losses in the armor were not prohibitive. There being an apparent absence of data on the subject, and some opinion adverse to the use of such cables, the values of reactance were calculated as explained below. The results showed that even at 60 cycles, and with outgoing and return cables placed close together, the reactance would not be serious; and the subsequent measurements indicate that in cables as manufactured the effect is considerably less than the calculated value, so that for purposes similar to that mentioned the use of the cables is entirely practical.