Mutual Impedances of Ground-Return Circuits 



Some Experimental Studies * 



By A. E. BOWEN and C. L. GILKESON 



This paper describes some of the results of the work of the Joint Develop- 

 ment and Research Subcommittee of the National Electric Light Association 

 and Bell Telephone System on the mutual impedances of ground-return 

 circuits. 



The first part of the paper deals with some experiments which were per- 

 formed to establish an experimental background for the testing of theoretical 

 ideas. Different theories, one involving an "equivalent ground-plane," a 

 second a d. c. distribution in the earth, and a third an a. c. distribution in the 

 earth, are discussed in the light of the experimental results. While none of 

 these is adequate to explain all the observed phenomena, each has a field in 

 which it can be made useful. 



The second part of the paper is devoted to a description of practical 

 means for predetermining the mutual impedances of power and telephone 

 lines. This involves an experimental determination of a curve of mutual 

 impedance as a function of separation in the region of the proposed exposure 

 and the calculation of the overall mutual impedance between the proposed 

 lines from this curve and the dimensions of the exposure. The results of 

 trials of this method in two locations are given which indicate that itshould 

 be of sufficient accuracy for engineering purposes. 



Introduction 



THE magnitude of the inductive coupling between power and tele- 

 phone lines is a factor of fundamental importance in problems 

 of coordination to prevent interference between these two classes of 

 lines. Accordingly, this is one of the subjects under investigation by 

 project committees of the Joint Committee on Development and Re- 

 search of the National Electric Light Association and the Bell Tele- 

 phone System. It is the purpose of this paper to present the results of 

 some work which has been done under the auspices of the Committee 

 on one phase of this problem, namely, the mutual impedance of ground- 

 return circuits. 



The mutual impedance of two ground-return circuits is determined 

 by measuring the ground-return current in one circuit (the "disturb- 

 ing" circuit) and the open-circuit voltage at the terminals of the second 

 circuit (the "disturbed" circuit). The vector ratio of the open-circuit 

 voltage to the ground-return current is then defined as the mutual 

 impedance of the two circuits. 



For any normal or abnormal operating condition of a power system, 



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

 Canada, June 23-27, 1930. 



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