Applicability of simple expressions for electrical breakdown probability increase in vacuum and gas

Simple analytical expressions for the law of breakdown probability increase are suggested. They are tested by comparing with experimental data obtained by impulse and DC breakdown. The pressure 10/sup -4/ Pa and interelectrode gaps d<0.1 mm are used for vacuum, and the pressure 1 bar-5 bars and the interelectrode gaps 1 mm-50 mm are used for gas breakdown. The insulation gas was a SF/sub 6/, N/sub 2/ and gas-mixture: SF/sub 6/-N/sub 2/. It is concluded that such a simplified approach is not appropriate for vacuum. It is also concluded that, for gas, the BPI (breakdown probability increase) law, in the form of a simple expression, is valid in the considered range of relevant experimental parameters.