Flashover as an illustration of the charge trapping and detrapping phenomena

Numerous works have been achieved to sort out the flashover origin. Recent investigations have shown the importance of charge injection at the insulator and metal interface and of the surface treatment on the flashover characteristics. High voltage tests show that at a given voltage the flashover is a quasi periodic phenomenon. By measuring the variation of the current as function of time and observing the insulator surface with a video camera allow to decompose the components of a flashover cycle. First, the current slowly increases. In the steady state the I(V) curve is of Space Charge Limited Current type. Then the current increases, the vacuum may significantly degrade whereas light emitting precursors appear. The emitters can be located anywhere on the surface even they are often observed at the cathode insulator interface. At the last step the discharge occurs over the surface where trapped charges where located. It is proposed that the flashover is due to charge trapping followed by the propagation of a collective detrapping wave that occurs when the density condition for a self sustained detrapping wave propagation is reached, the precursors located at some surface defects being the trigger of the wave