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The dc, ac (60 Hz) and ac superimposed on dc surface flashover of TeflonÂ® (Â¿r = 2.2), quartz-silica (Â¿r = 3.8) and glass-ceramic (Â¿r = 3.8) is investigated in vacuum prior to and following the application of a dc pre-stress. The surface flashover is measured at ~ 300 K, at a residual gas pressure of <10-8 Torr (< 10-6 Pa) after the solid insulators have been thoroughly degassed at elevated temperatures (~ 200Â° C). The ac surface flashover voltage is unaffected by the dc pre-stress. The combined dc and ac flashover voltage exceeds the withstand voltage of the ac by itself. Depending on the level of the applied dc pre-stress, the value of the combined dc and ac flashover is between 1.5 to 1.8 of the ac flashover voltage. The dc flashover voltage measured subsequent to applying a pre-stress with an opposite polarity is found to be considerably lower than the dc flashover voltage without a pre-stress. Applying a dc pre-stress with the same polarity improves only slightly the subsequent dc flashover strength. The observed phenomena are attributed to the changes in the electric fields at both electrode junctions due to the creation of charges on the surface of the solid insulator upon the application of the voltage stress.