JOINT DEVELOPMENT AND NOISE FREQUENCY INDUCTION 



189 



reduced by interposing conducting paths which circulate current to 

 set up counter magnetic fields. The power and telephone cables in use 

 are probably the simplest examples of shielding. A cable sheath is 

 almost 100 per cent effective as a shield for electric induction, either on 

 a power cable or on a telephone cable. The sheath is less effective as a 

 shield for magnetic induction, because of its finite conductivity. It 

 does not seem feasible at this time to obtain anywhere near perfect 

 magnetic shielding. 



Factors Contributing to Noise Frequency Induction 

 Influence Factors 



There are two characteristics of a power system which are of primary 

 importance in determining its inductive influence upon neighboring 

 telephone systems, i.e., its wave-shape and its balance. The wave- 

 shape is determined by characteristics of apparatus associated with 

 the system. The balance is determined by the degree of symmetry 

 of the supply voltages, load impedances, and of the series impedances 

 and shunt admittances of the lines. While it is not practicable to 

 design rotating machinery or other apparatus using magnetic cores 

 entirely free from harmonics, or to realize ideally balanced three-phase 

 systems, it is practicable to control both these factors within limits 

 which, in conjunction with a similar degree of control on the coupling 

 and in the susceptiveness of the communication circuits, permit 

 satisfactory operation of both services without unduly burdening 

 either. 



The work on influence factors which has been conducted by the 

 Joint Subcommittee on Development and Research has, therefore, been 

 directed for the most part toward the study of the wave-shape charac- 

 teristics of power systems and apparatus and methods for their im- 

 provement and the investigation of factors affecting the balance of the 

 power systems and method for their control. 



Wave Shape. — In initiating its work on influence factors, the Joint 

 Subcommittee found little information available as to wave-shape 

 which might be expected on operating power systems equipped with 

 various types of apparatus. In order to obtain a broad picture of 

 wave-shape conditions as they exist in the field, the Subcommittee 

 conducted an extensive survey of wave-shape conditions on 34 operat- 

 ing power systems in the eastern half of the country. The program was 

 arranged to obtain information as to the average and range of magni- 

 tudes of harmonics present in various types of transmission and distri- 

 bution systems under normal operating conditions, to observe the 

 relation between the wave shape of generating machinery under open- 



