Study of the electric fields required for streamer propagation in humid air

The electric field required to sustain streamer propagation is important to the interpretation of breakdown in nonuniform field geometries. Basic measurements of the minimum field for propagation have been carried out in uniform fields. These have shown that above a ´threshold´ value of field, there is an increasing probability of propagation up to a ´stability´ field at which propagation always occurs. Comparison is made with earlier work in both uniform and nonuniform fields, and with a survey of the mean stresses required for breakdown under lightning impulse and steady voltages. This leads to the conclusions that the stability field is a better parameter to use than the threshold field in making comparisons with lightning impulse breakdown and that the threshold field corresponds more closely with the stress required for steady voltage breakdown. Humidity effects are discussed.