Recently，much attention has been paid to polymeric materials used for housing of electric power apparatus. However，because polymeric material is made by organic matters，the aged deterioration is anxious. The lifetime of polymeric material is influenced by environmental conditions such as ultraviolet，acid rain，and polluted deposits. The change of the surface condition of polymeric material causes dry band arc discharges. To understand the long-term reliability of the insulation performance of polymeric materials，a composite pollution test of a liquid silicone rubber (LSR) was performed with dc or ac voltage application. After the test，heavy erosion was confirmed around a cathode electrode when dc voltage was applied. In the case of ac voltage application，heavy erosion was confirmed around a grounding electrode. It was also found that the initiation time of leakage current and cumulative charge depended on the kind of polluted deposits and that the difference of cumulative charge among polluted deposits in the case of dc voltage application was larger than that in the case of ac voltage application.

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