Development of spark discharge in nonuniform field gaps under impulse voltages

Investigations have been made of the electrical characteristics of leader strokes preceding the impulse breakdown of sphere-plane and rod-rod gaps up to 50cm spacing in atmospheric air. Oscillograms of the discharge current show a marked difference in the leader propagation in sphere-plane gap under the two polarities. The breakdown of the gap when the sphere is positive takes place as a result of positive leader developing at the sphere for all gap lengths. When the sphere is negative, however, a change in the development of leader from negative leader to positive leader from plate has been observed at a certain gap length. This transition explains the well known inflection in the negative-breakdown characteristic of a sphere-plane gap. Average leader velocities have been computed from the current oscillograms, and the velocity of the negative leader has been found to be higher than that of the positive leader. Breakdown of rod-rod gap has been observed to be initiated by the development of the positive leader for all gap lengths irrespective of the polarity of the h.v. rod. Curves of breakdown-voltage/gap lengths have been presented and the characteristics are explained in terms of the leader development.