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In many high voltage gas-insulated switchgears (GIS), flashover voltage is decreased in case of including conducting particles. In particular, the decrease is very severe, when the particles move to and attach on an insulator in GIS. The reason is that surface flashover voltage is usually lower than gaseous flashover voltage in comparatively high pressurized SF 6 gas. Some methods for flashover voltage prediction, one of which was proposed by authors, were already proposed of gaseous flashover. However, concerning the surface flashover, only few methods have been proposed.In the present paper, the prediction method for gaseous flashover of SF 6 gas including conducting particles proposed by authors is extended to the prediction method for the surface flashover with particles attached on an epoxy insulator.The characteristic quantities at particle tip are reformed with two supplementary factors included in order to estimate the phenomena peculiar to the surface flashover. It is revealed through comparison with experimental results that the proposed method is useful to predict the flashover voltage along the epoxy insulator of GIS including particles.

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In an interruption chamber of Gas Circuit Breaker (GCB), a current is interrupted at current zero . SF6 gas is blown to an arc occurred between contacts. In this process, the gas blown from puffer chamber flows through a nozzle at high speed. For this reason, a supersonic region is formed and a pressure is decreased. It often causes a fault of interruption.In the downstream, a shock wave is often occurred. In recent years, these phenomena are investigated through experiments and numerical simulations. In order to design superior and reliable GCB's, it is very important to optimize the position of shock wave for an improvement of an interruption capability and stabilize the shock wave for stabilizing an interruption performance clarifying the uncertainty limit of supersonic region caused by uncertainty of manufacturing and using conditions. However, these techniques have not been developed yet.In the present paper, a new simulation method is proposed in order to design GCB's which have superior interruption capability. The method proposed by an author to include uncertainty of manufacturing and using conditions is combined with a usual code to solve Navier-Stokes equa tions. It is shown by using this method that the supersonic region is able to be controlled by optimizing the relationship at current zero between a fixed arc contact and a ditch designed in the nozzle, and that an uncertainty limit of the supersonic region is able to be clarified.