Positive lightning breakdown characteristics of short rod-plane air gaps

Breakdown characteristics of short rod-plane air gaps under positive standard lightning impulse voltages are experimentally investigated, as affected by the amplitude of the applied voltage and the gap length. Discerned discharge phases including leader inception and growth, final jump, and breakdown of 0.15,0.3 and 0.6 m long air gaps are studied based on recordings of the applied impulse voltage and discharge current. The effect of the applied impulse voltage amplitude on discharge characteristics is assessed by overstressing the gap. The breakdown mechanism is streamer dominated; however, the inception and growth of a small leader is a necessary condition for breakdown to occur. A close relationship is shown to exist between leader and breakdown characteristics. At breakdown the leader-streamer system progressing in the gap is characterized by average potential gradients and velocities comparable to those commonly reported for positive streamers; threshold average breakdown fields vary between $520 \mathrm{kV} / \mathrm{m}$ and $680 \mathrm{kV} / \mathrm{m}$ and velocities between $1 \times 10^{5} \mathrm{~m} / \mathrm{s}$ and $1 \times 10^{6} \mathrm{~m} / \mathrm{s}$.