Abstract
Qualitative results of an investigation, in which high-voltage impulses were applied to an energized transmission line, indicate that the probability of a lightning flashover developing into a power arc is increased when current is flowing past the flashover point, and when the normal frequency voltage is increased or the insulation decreased. An oscillographic study of power arcs ranging in magnitude from 8 to 800 amperes indicated that the resistance of an arc varies over the cycle and over the duration of the arc. The effective arc resistance is approximated by the ratio of the peak voltage to the peak current. The apparent power factor of the arc current and voltage is practically unity in most cases. The tests indicated that arcs formed in a capacitive circuit are more persistent than those in inductive or resistive circuits, and that the average peak volts per foot of arc length, for currents above 100 amperes peak, is about 300. The maximum overvoltage recorded on an isolated neutral 75-kv. transmission system, 134 miles in length, as a result of 63 arcing ground tests was 3.0 times normal on a sound phase. The overvoltage is essentially a fundamental frequency phenomenon in which the sound phase voltage takes its new value without going through an oscillation of appreciable magnitude.
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