A STUDY OF TELEPHONE LINE INSULATORS 699 



The rapid growth of carrier systems during the past decade/ the 

 longer circuits to which they have been appHed, and improved stand- 

 ards of long-distance transmission have all been factors in increasing the 

 requirements imposed upon the insulator and in correspondingly 

 augmenting the importance of the problem. 



It may now be remarked that the problem has been mainly one of 

 securing economical insulators giving improved performance at these 

 higher frequencies. In addition the low frequency performance of new 

 designs had to be maintained substantially as good as that of the old 

 designs. 



Leakage Phenomena 



This study has been confined almost entirely to the pin type of 

 insulator. 



When an alternating potential exists between a pair of wires at the 

 point where those wires are supported by insulators, a current flows 

 from the one wire to the other. This current may be resolved into two 

 components, one in phase with the potential and one in phase quadra- 

 ture leading the potential. 



This in-phase component which, of course, represents an energy loss 

 is the one of chief interest here and in using the word leakage we refer 

 to this component, or more accurately to its equivalent conductance. 



Of course, both components in flowing through the resistance of the 

 line conductors produce energy losses but these are so small as to be 

 omitted here. Except these, all other energy losses which occur due to 

 the presence of insulators whether they actually occur in the insulators 

 proper or elsewhere will be charged to insulator leakage. 



It is convenient to divide insulator leakage into several sources. 

 This division is an arbitrary one, because some of the sources are not 

 independent of each other and are, as will be seen later, difficult to 

 separate experimentally. 



The division follows: 



A -c 

 ■ J ■ f A. Leakage directly through insulator material to pin. 



J \ B. Leakage directly over insulator surfaces from line conductor to pin. 



C. Dielectric absorption in insulator material. 



D. Dielectric absorption in pins. 



E. Displacement current losses in crossarms and pins. 



F. Losses due to unbalanced displacement currents in external resistances 

 such as those of crossarms, poles, etc. 



G. Displacement currents flowing over insulator surfaces through high 

 resistance. 



It should be noted that while all the items play a part in a.-c. leakage 

 only the first two enter in the d.-c. case. 



^"Carrier Systems on Long Distance Telephone Lines," H. A. Affel, C. S. 

 Demarest, and C. W. Green, A. I. E. E. Trans., Vol. 47, 1928, pp. 1360-1386. 



A.-c. 

 only. 



