JOINT DEVELOPMENT AND LOW-FREQUENCY INDUCTION 207 



to telephone employees or interruption to service. Although such ab- 

 normal conditions occur infrequently and usually last for only the very 

 short period required to interrupt or clear the power circuit, the effects 

 which may be produced are so serious that protection against this type 

 of induction is an outstanding problem in the coordination of power and 

 telephone systems. A large part of the subcommittee's work has for its 

 object the development of means for controlling and minimizing such 

 induced voltages and their effects. 



While low-frequency induction is not usually severe except under 

 abnormal conditions, power circuits which operate at any time on an 

 unbalanced basis, or which are closely coupled to grounded wires 

 capable of carrying large currents may, even under normal operating 

 conditions, create a problem of low-frequency induction in paralleling 

 telephone circuits in addition to setting up high-frequency disturbances 

 as explained in the Blackwell-Wills paper. This is particularly true 

 where the exposed telephone circuits are used for special services such 

 as the transmission of radio broadcasting programs. Grounded types 

 of telegraph and other signal circuits also are sensitive to low-frequency 

 induction. 



Classification of Factors Responsible for Inductive 



Effects 



The same three class of factors which combine to underlie the noise- 

 frequency problem appear also in the low-frequency problem. As 

 they appear in the latter, these are: 



1. "Influence factors" in the power system, which are concerned 

 with the magnitude, duration, and frequency of occurrence, of unbal- 

 anced voltages and currents. 



2. "Susceptiveness factors" in the communication system, which 

 are concerned with the nature and seriousness of the effects produced 

 by the induced voltages. 



3. "Coupling factors" which determine the magnitude of the vol- 

 tages induced in the communication system, per unit unbalanced 

 voltage or current of the power system. 



In the low-frequency induction problem, the coupling factors are 

 largely dependent upon the characteristics of the earth and the relations 

 of power and telephone systems to the earth. If the earth were an 

 insulator instead of a conductor there could, of course, be no such 

 thing as fault current in the earth and the coupling between power and 

 communication circuits would be much less. It would not then be 

 hazardous for a lineman when in contact with earth to touch a charged 

 wire. Or if the lines were not in proximity to the earth, there would be 



