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Time-to-breakdown of flashover on a solid dielectric surface in transformer oil with a point-to-plane gap configuration is studied using a microsecond rectangular high-voltage pulse generator. The time was directly measured from prebreakdown current. These prebreakdown currents were measured with a photo-optical current-measuring technique. Time-to-breakdown of the negative point was greatly decreased by the presence of solid dielectrics, more so than that of a positive point. Positive streamers on a dielectric surface were found generally to develop faster than negative ones. Without a back electrode, the magnitude of the pulse voltage has less influence on the positive streamer development than on the negative one. No effects of dielectric material or their thickness on streamer development were observed. Based on these observations, a hypothesis that the initiation condition of the streamer determines it subsequent propagation is proposed.