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The surface flashover of a Macor glass‐ceramic conical insulator in ultrahigh vacuum is investigated using direct current (dc), alternating current (ac at 60 Hz), standard lightning impulse (1.2/50 μs), and combinations of dc+1.2/50 μs and dc+ac. The dc and the 1.2/50‐μs flashover voltages are found to be at about the same level when the cone angle θ, which the surface makes with the applied field, is varied from -55° to +55°. At a fixed positive value of θ and a fixed thickness of the insulator, the lightning impulse, and the dc flashover voltages are higher than the ac. The level and the polarity of the dc prestress influence the combined dc+impulse flashover voltage in conical insulators. The ac+dc flashover depends on the dc prestress level and increases with increasing the prestress until a saturation is reached which is at a slightly higher voltage than the dc flashover. The electric field and the potential distributions at the interface of the cone insulator are calculated for different cone angles in the range 5° to 55°.