2013 Volume 133 Issue 12 Pages 947-953
The impulse characteristics and breakdown mechanisms of soil are the fundament of grounding design for lightning protection. Photographic investigation using X-ray films is a useful means for studying the impulse characteristics and breakdown mechanisms of soil. Through a large number of tests, three types of streamer discharges, positive streamer, negative streamer, and negative feather and positive retrograde streamer, were found in sandy soil under negative impulse voltage in this paper. Basing on the streamer discharge theory of gas, the developing processes of streamer discharges in sandy soil are preliminarily described. These three types of streamer discharges in soil are very similar to those in gas, thus it strongly supports the electrical breakdown mechanism of soil. On the other hand, using X-ray film imaging technology we observed the discontinuous ionization phenomenon in the soil around the grounding electrode on which a surge voltage was applied. This phenomenon is formed due to the non-uniformities of soil dielectric properties and the shapes and sizes of air apertures. The soil breakdown under lightning impulse is mainly caused by the electrical breakdown of the air among the soil particles.