In order to study the DC flashover characteristics in SF

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of alumina filled epoxy resin

which is one of the important insulating materials

DC flashover voltage test system is established. The short-term and long-term withstand voltages of epoxy resin samples with various alumina contents（mass ratio range of epoxy resin and alumina is from 2.5 to 4.0） in 0.4 MPa SF

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as well as the voltage application times were measured

and then the

U–t characteristics were compared accordingly. The damages of the discharge process to alumina filled epoxy resin materials were also studied through the flashover traces and the microcosmic structure analysis. The "wormholes" formation in the discharge and its effect in the flashover process were analyzed and discussed. It is found that the withstand voltage of the epoxy resin with certain alumina content decreases exponentially with time

and the long-term withstand voltages is about 20%

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0% lower than the short-term withstand voltage. The epoxy resin sample with mass ratio of 3.5 has the lowest short-term and long-term withstand voltages and most serious carbonizations according to the experimental results. Wormholes are obviously found in the surface flashover path

and the superficial zone wormholes gradually transform into deep layer wormholes

which indicate that the flashover paths shift from creeping-discharge-based to bulk-breakdown-based due to wormholes effect with the increase of alumina content and the critical mass ratio of epoxy resin and alumina is about 3.0

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.5.