Mutual Impedances of Ground-Return Circuits

T HE magnitude of the inductive coupling between power and telephone 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 Research of the National Electric Light Association and the Bell Telephone 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 groundreturn circuits. The mutual impedance of two ground-return circuits is determined by measuring the ground-return current in one circuit (the "disturbing" 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, 628