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Enhancement of the electric field at a solid insulator/electrode/gas triple junction and on the surface of an insulator, due to imperfect contacts, surface irregularities and contamination, reduces the withstand voltage of the gas-only system. The charges deposited on the surface and in the bulk of the insulator, when stressed under high voltage, further modify the electric field. In the paper, the influence of accumulated static charges on the electric field distribution associated with a cylindrical insulator held between plane electrodes in atmospheric air is investigated. The electric field along the solid dielectric/air interface is calculated for different surface and volume charge distributions by using the finite-difference method based on Gauss's law. Derivation of the iteration equations are relatively easy, and, unlike most methods, there is no need for external imposition of solid dielectric/gas boundary conditions. The results indicate that, for some surface and volume charge distributions, field enhancement occurs at the triple junction, and this could initiate a surface flashover.