Gas Pressure for Telephone Cables

Communication cables consist of a number of electric conductors insulated from one another and encased in a metal sheath. This encasement is subjected to numerous hazards, such as those caused by electrolysis, crystallization, various kinds of mechanical damage, and lightning burns. Any damage to the sheath which will permit water enter the cable will decrease the effectiveness of the insulation material and thus cause an impairment or an interruption to the service. The entrance of moisture through small openings in the sheath can be materially retarded when the space inside the cable, not occupied by the conductors or insulation, is filled with a gas maintained under controlled conditions. Nitrogen is the gas usually used for this purpose because it is inert and does not combine chemically with the conductors or insulation. In addition the use of the gas provides a method of locating openings in the sheath by means of a pressure gradient, which is a material aid in cable maintenance.