The factors influencing the use of pneumatic methods for the location of sheath faults in pressurized telephone cables

An expression, based on the classical Poiseuille equation for gas flow through capillary tubes, has been obtained to describe the passage of gas through non-coaxial telephone cables under steady-state conditions, and, for cables in this category, the factor governing the flow has been shown to depend upon the number, gauge and arrangement of the conductors. An approximation of the pressure gradient associated with a small rate of gas flow has been derived from this expression, and consideration has been given to the degree to which this limits the successful location of sheath faults by methods involving the direct establishment of the pressure gradient, the installation of pressure-sensitive contactors, rate-of-flow measurements and the use of flow-direction indicators.