Effects of Glow Discharge Conditioning and Addition of Gases on Surface Flashover

The dc, ac (60 Hz), and lightning impulse (1.2/50 ¿s) flashover voltages of solid insulators in high vacuum at a pressure of about < 10-6 Pa have been measured before and subsequent to conditioning with an ac (60 Hz) glow discharge at a pressure of 130 Pa. Sulphur hexafluoride, air, nitrogen, and hydrogen have been used in the glow discharge conditioning. Quartz, Pyrex glass, and Teflon insulators in the form of right angle cylindrical spacers sandwiched between a pair of planar stainless steel electrodes are employed. It has been found that the dc, the ac, and the lightning impulse flashover voltages of the insulators in vacuum are higher after conditioning with a glow discharge. The flashover voltage is also measured in the pressure range 10-6 - 5xl0-4 Pa prior to and after glow discharge conditioning. The flashover voltage increases in the pressure range 10-2 - 10-1 Pa only when sulphur hexafluoride is used. No pressure dependence is found in the range 10-6 - 10-1 Pa in air, nitrogen, and hydrogen when using a pair of stainless steel electrodes bridged by any one of the three dielectric materials. SF6 gives the highest surface flashover voltage compared to air, N2, and H2.