Automatic Line Insulation Test Equipment 

 for Local Crossbar Systems 



By R. W. BURNS and J. W. DEHN 



(Manuscript received February 9, 1953) 



Moisture entering faults in the insulation of subscriber lines provides 

 so-called "leakage^^ paths which reduce the insulation resistance. Testing 

 the insulation resistance of all lines under the environmental conditions 

 which tend to produce these leakages is a maintenance technique, relatively 

 new, for detecting the insulation defects. The faults can then be corrected 

 before they become serious enough to affect the customers^ service. Subscriber 

 reports are thereby reduced and the correction of the faults on a preventive 

 maintenance basis tends toward a more uniform work load for the repair 

 personnel. Rapid testing of the lines is necessary, otherwise the environ- 

 mental conditions may change and the leakages will disappear without de- 

 tection. 



Rapid line insulation testing is practiced quite generally in all the 

 switching systems throughout the Bell System, but the testing arrangements 

 used are wholly or partially manually controlled in the testing and recording 

 operations. While the benefits derived from rapid line insulation testing 

 apply to all systems alike, this article is confined to a discussion of the 

 entirely automatic testing and recording arrangements which are now being 

 introduced in the No. 1 and No. 5 crossbar systems. 



INTRODUCTION 



The insulation resistance of subscriber lines is an important considera- 

 tion in the design and operation of central office switching circuits. If 

 the insulation resistance between the two conductors of the line, or the 

 insulation resistance from the "ring" or '^battery side" to ground, be- 

 comes low enough, the "leakage" current flowing produces the same effect 

 as lifting the handset and failing to dial or to pass a number to the opera- 

 tor. This condition is called a permanent signal and the line is said to be 

 "permanent." As long as the condition persists, the line is out of service 

 both to incoming and outgoing traffic. Insulation resistance of a slightly 



627 



