The streamer discharge is the precursor of breakdown in non-uniform field configurations in air and other gases. Flashover under lightning impulse and direct voltages is determined by the potential gradient needed streamer propagation; under switching impulse voltage, the streamer usually transforms to a leader prior to breakdown. Earlier measurements have shown that the electric fields required for streamer propagation in an uniform field in air are quite accurately measurable and reproducible to within 1 per cent and their velocities of propagation, as a function of electric field, are quite precisely defined. Thus, where a perturbation to the propagation process is introduced, such as an insulating surface in close proximity, the streamers can be used as a 'probe' to discover more of what happens in the surface flashover process.
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