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The importance of studying the influence of accumulated surface charge on surface flashover is demonstrated by on-site trapped charge measurements. Using a hemispherically tipped electrode on an epoxy resin plate, it has been shown that, at a critical surface energy density, a back flashover can be induced when the DC is suddenly removed. The back-flash threshold levels were measured at different pressures and for two electrode-insulator geometries. The concept of minimum surface energy density Em capable of sustaining surface flashover was formulated. Various experimental observations can be interpreted by the concept, such as the aforementioned back-flash phenomenon, the close proximity to the insulator surface that flashover path tends to seek etc. From the measured back-flash threshold voltage and field analysis of the two geometries, the minimum surface energy density Em necessary for sustaining continuous surface discharge has been calculated. The compilation of the results for both geometries at different pressures is presented.