Automatic Line Insulation Test Equipment for Local Crossbar Systems

63-t THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 higher value will not cause a permanent signal, but its presence may interfere with other circuit functions, for example, dial pulses are distorted and a wrong number may be reached, the ringing signal may be tripped before the called party answers, or the switching circuits may fail to restore on hang-up of the receiver. If the insulation resistance is at least as high as the design limit, failures of the kind described above will not occur. The design limit for some switching systems used in the Bell System is 10,000 ohms; for others 15,000 ohms. WHERE LINE LEAKAGES OCCUR The outside plant distribution system for exchange lines usually consists of some underground cable, many miles of aerial cable to distribute the line conductors throughout the area and a small amount of open wire on the fringes. The insulation resistance of the cable conductors is normally quite high. If, however, a break in the cable sheath occurs moisture may enter and be absorbed by the paper insulation of the conductors near the break. This reduces the insulation resistance of the affected cable pairs. Sheath breaks may exist for a considerable length of time without reducing the insulation resistance sufficiently to cause any reaction on customer service. Eventually these sheath breaks will, if not detected and repaired, permit the entrance of sufficient water during a rain to reduce the insulation resistance to the point where permanent signals occur on several pairs.