Spark breakdown in air/PP-film composite with point-sphere electrode system

A description is given of the spark breakdown in a point-sphere air gap with polypropylene film inserted between the electrodes and subjected to a linearly rising DC voltage. No appreciable polarity effect on the breakdown voltage of the composite was observed at any position of the film. The spark breakdown voltage V_b increased with increasing distance x of the film from the sphere electrode, to a critical position x_c at a distance just less than half of the gap length d, at which point it dropped suddenly, and then increased again as x increased to d. In the region x<x_c , spark breakdown occurred following repeated discharges between the film and the sphere electrode, whereas apparently spontaneous spark breakdown occurred when x>x_c. The breakdown mechanism for both regions is discussed