Discharge characteristics in short rod-plane gaps under lightning impulse voltages of both polarities

Results are presented concerning prebreakdown phenomena and breakdown in short rod-plane gaps in air under lightning impulse voltages of both polarities. Breakdown probability distributions were obtained and the discharge parameters such as first and second corona inception time and voltage, dark period, time and voltage required for the discharge to arrive at the earthed plane and for breakdown were measured as a function of gap length and impulse voltage polarity. Breakdown under negative impulse voltages is closely associated with the prebreakdown phenomena since there is a close correlation between coronas inception and breakdown voltages; this is not always the case under positive impulse voltages. For both polarities there is a tight correlation between the breakdown and discharge arrival voltages. At 50% breakdown probability, the mean electric field required for the discharge to arrive at the plane is roughly two times higher under negative than positive impulse voltages and for both polarities it decreases with gap length according to a power law. These explain the similar dependence of the 50% breakdown voltage on polarity and gap length.