This paper describes the fifty percent flashover voltage and the V-t curve for a rod-plane gap applied bipolar impulse voltage, and the behaviour of the space charge in the gap. In the distribution line, the bipolar impulse voltage had been observed frequently when lightning hit near it. Then, the bipolar impulse wave used in the experiment was decided as the composite voltage wave made from the preceding wave of positive (or negative) polarity, 20/500 μsec. and the following wave of negative (or positive) polarity, 2/60 μsec. The connecting time of the abovementioned two pulses was decided as 20±5 μsec.. The test gap was the rod-plane gap with a gap length of 6.0cm, and the rod diameter of 1.0cm as a non-uniform field, mainly. As for the experimental results, when the applied preceding impulse voltage was positive polarity and a corona occurred, the fifty percent flashover voltage was up by 15%. And, when the preceding impulse voltage was negative polarity and with a corona, the fifty percent flashover voltage was down by 10%. The space charge phenomena as the cause given by the abovementioned results were observed by using an image converter camera
