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The lightning impulse breakdown characteristics of various combinations of insulation material in silicone oil for use in electric power apparatus were investigated, with the aim of reducing the amount of oil required and thus the cost. Breakdown characteristics were investigated in a system in which an insulating filler was mixed with silicone oil. Tests were conducted on a sphere-plane electrode in a vertical arrangement and a sphere-sphere electrode in a horizontal arrangement. The relation between the breakdown strength and the electric field strength was clarified. Breakdown characteristics were also investigated in a system in which am insulator with low relative permittivity was inserted between a filler with high relative permittivity and the high-voltage electrode. Based on the findings, optimum conditions for the application of silicone oil to electric power equipment are proposed.