Surface flashover of solid insulators in atmospheric air and in vacuum

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Abstract
The surface flashover of Teflon, plexiglass, quartz, Pyrex glass, Macor glass-ceramic, and sapphire solid insulators has been measured in vacuum (∼10−8 Torr, ∼10−6 Pa) and in atmospheric air using dc, ac (60 Hz), and 1.2/50-μsec lightning impulse voltages. The dependence of the flashover voltage on the following parameters is investigated: (1) spacer material, (2) diameter of the spacer, (3) spacer length, (4) number of spacers stacked in series, (5) air pressure in the range 10−6–105 Pa, (6) electrode material, (7) spark conditioning, and (8) the external resistance in series with the gap. At a fixed insulator length the flashover voltage decreases with increasing spacer diameter. The withstand voltage of spacers stacked in series increases with increasing the number of spacers. The dc flashover voltage of different insulating materials is theoretically calculated in vacuum as a function of the length of the insulator and compared with the experimentally obtained results. Good agreement is obtained.