Coupling Between Parallel Earth-Return Circuits Under D-C Transient Conditions

Widely different types of earth structure were indicated by the a-c coupling measurements at the 3 locations where these tests were performed, and by the behavior of the transient induced voltages. The departures of the measured transient voltages from the values which would have occurred with a uniform earth resistivity could generally be correlated satisfactorily with the measured a-c coupling. At Savannah, Ga., a value of 100 meter-ohms was found to give about the best over-all agreement between the measured transient voltages and the voltages computed on the basis of uniform earth resistivity, this value being a reasonable one to choose from the standpoint of the measured a-c coupling. The voltages computed for this uniform resistivity were considerably lower than the measured voltages at the closest separation and, with increasing separation, the computed voltages gradually increased in comparison with the measured voltages, until at the widest separation they were substantially higher than the latter. At Macon, Ga., the earth structure was apparently highly irregular, but with a reasonable value of uniform earth resistivity, 500 meter-ohms, as indicated by the a-c coupling measurements, the maximum values of the computed transient voltages agreed roughly with the maximum values of the measured voltages. However, these computed voltages increased more rapidly than did the measured voltages, subsequent to the beginning of the transient, and decreased more rapidly after the maximum values were attained.