Comparison of long sparks in air and over an insulator surface

The breakdown characteristics of a spark gap can be influenced by the presence of an insulator between the electrodes. This experiment was conducted on a 250 mm rod-plane gap, in the presence and in the absence of a rod insulator, with the applications of both positive and negative lightning impulse voltages on the rod electrode. The affinity of the breakdown channel whether it follows the insulator surface or is repelled from it was investigated. It was observed that for negative lightning impulses, the breakdown channel followed the insulator surface in most of the cases while for the positive lightning impulses, the breakdown channel was always repelled from it. Differences were found in waveforms of breakdown voltages when the breakdown channel is following or being repelled from the insulator surface