628 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



higher value mil not cause a permanent signal, but its presence may- 

 interfere with other circuit functions, for example, dial pulses are dis- 

 torted and a Avrong 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 con- 

 sists of some underground cable, many miles of aerial cable to distribute 

 the line conductors throughout the area and a small amount of open ^vire 

 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. Then repairs must be made on 

 an emergency basis to guard against a complete failure of all line cir- 

 cuits in the cable. One of the common causes of sheath breaks in some 

 residential areas is gnawing by squirrels — squirrel bites. 



Cable terminals are located on the poles or on the cable for making 

 drop wire connections to the customers' premises. Binding posts mounted 

 in a face plate within the terminal are connected to some of the cable 

 pairs through a terminal cable stub joined to the aerial cable. Each 

 cable pair is thus connected to binding posts in about four or five ter- 

 minals on the average. When water or condensation wets the face 

 plate, leakage currents will flow between the binding posts. If dirt 

 and dust have accumulated on the face plates, the combined resistance 

 of all leakage paths in parallel across the pair or to ground may become 

 relatively low. 



While open wire makes up only a small part of the outside plant 

 circuits most areas have some lines containing from a few spans up to 

 several miles of open-wire conductors. It is difficult to keep the open-wire 

 plant in a condition so that it will be free from leakages under wet 



