OPEN-WIRE TELEPHONE LINES 741 



power transmitted over the pair except the normal PR loss in the wires 

 themselves. This inclusion of numerous little-understood losses in the 

 general term leakage has at times served to insulate the individual losses 

 from analysis. Methods of determining the value of the "equivalent 

 leakage conductance" and of analyzing its component losses are avail- 

 able, however.^ 



The nature and magnitude of the different losses which occur at the 

 insulators are being discussed in detail in a parallel paper. ^ Accord- 

 ingly, only a brief mention will be made in this paper of the types of 

 insulators which are now in use on the open-wire lines of the Bell Sys- 

 tem, and of the values of leakage conductance experienced with these 

 different types. 



The DP or double-petticoat glass insulator illustrated in Fig. 7 is 

 now standard for use on all important toll circuits, except those 

 equipped with the special carrier insulators discussed below. On a 

 number of older circuits single-petticoat glass insulators, known as 

 toll insulators (see Fig. 7) and double-petticoat porcelain insulators are 

 still in place. 



In view of the numerous and complex sources of leakage loss, it is not 

 surprizing that the leakage conductance for a given pair at a particular 

 frequency varies with changing weather conditions and with the age of 

 the insulators over a very wide range of values. Because of this wide 

 range of variation it is possible to give here only selected leakage values 

 which serve for engineering purposes. Values of the total leakage 

 conductance for open-wire pairs equipped with DP insulators are 

 plotted in Fig. 8. These values are intended to represent the highest 

 values ordinarily obtained on an old circuit which is in a good condition 

 of maintenance. The wet weather values have been so chosen that 

 they should be exceeded on only a few days of the year, while the 

 dry weather values represent the performance that should be expected 

 from any circuit on a clear, dry day. 



Particular difficulty is experienced in selecting standard values of d.-c. 

 leakage owing to the fact that the range of values encountered in prac- 

 tise is exceedingly great. The measured values depend to a great ex- 

 tent on the degree to which the line is kept free from tree branches, 

 foliage, moss, broken insulators, and other possible sources of leakage. 

 These special sources of loss, of course, represent a much smaller part 

 of the total leakage losses at carrier frequencies. 



The standard values of leakage conductance are derived for a line 

 having 40 pairs of insulators per mile. Where the number of insulators 



* See 

 quencies 



"Methods of Measuring the Insulation of Telephone Lines at High Fre- 

 ," by E. I. Green, A. I. E. E. Trans., Vol. 46, 1927, pp. 514-519. 



