Prediction of withstand voltages in air-insulated systems are made on the basis of empirical models that are not sufficiently accurate for complex geometries. Bet-ter understanding of the spatio temporal development of electrical discharges is necessary to improve the present models. Discharges in lightning impulse stressed 20–100 mmrod-plane gaps are examined using a high-speed camera, photo-multiplier tubes (PMTs) and a high-bandwidth current measurement system.The images and measurements of gaps larger than 20 mm show a fast initial streamer discharge with a current rise time of some tens of ns, followed by a dark period of a few μs and a propagation of a slower leader-type channel leading to breakdown. The breakdown mechanisms in the shortest gaps are faster and geometry dependent,probably occuring by heating of initial streamer channels. Different light filters used with the PMTs indicate that all parts of the leader-type discharge development emit light over a spectrum from UV to IR. The initial discharges emit low amounts of warm light and IR compared to the leader-type channel. Finally, it is suggested that empirical breakdown voltage prediction models should be interpreted in light of the leader-type breakdown mechanism.
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