A Study of Telephone Line Insulators * 



By L. T. WILSON 



This paper discusses the major factors contributing to the (total) leakage 

 conductance of telephone line insulators, especially at carrier frequencies up 

 to 50,000 cycles. The influence of both the design and material of the in- 

 sulators and pins on each factor is discussed and illustrated by test data. 



The electrical performance of three different designs is analyzed to illus- 

 trate, in a general way, the relative importance of the several factors. 



TO the layman, the long strings of large power insulators suspended 

 from tall steel towers naturally present a more imposing picture 

 than do the small telephone insulators mounted on the crossarms of 

 relatively short wooden poles. 



To the engineer, however, these little insulators present problems 

 quite as stimulating and interesting in the telephone field as do the 

 large insulators in the power field. 



It is the purpose of this paper first to discuss briefly how some of 

 these interesting problems arose and then to cover in more detail a 

 study of the major phenomena involved in insulator leakage and finally 

 to show how the knowledge gained from that study has helped bring 

 about improved telephone insulators, some of which will be described. 



Origin of Problem 



For many years the requirements of telephone insulators were rela- 

 tively easy to meet because the frequency of the currents transmitted 

 did not exceed about 3 kc. and because the leakage of insulators is 

 generally low at such frequencies. 



Therefore, the familiar glass insulators such as are shown in Figs. 

 1 and 2 sufficed, the former design (D. P. type) being employed on the 

 longer circuits and the latter (toll type) on the shorter ones. Indeed 

 they still suffice very generally, especially where only currents of voice 

 frequencies or less are transmitted. 



The advent of carrier systems employing higher frequencies ranging 

 from about 3 to 30 kc. changed the insulator requirements substan- 

 tially. At first these systems were few in number and relatively short 

 in length and the insulator problem accordingly less important. 



* Presented at the Summer Convention of the A. I. E. E., Toronto, Out., Canada, 

 June 23-27, 1930. 



697 



