The possibility of preventing high‐voltage surface flashover of insulators in vacuum by means of a strong magnetic field perpendicular to the electric field and parallel to the insulator surface is investigated theoretically. A simple model predicts that with the right choice of insulating material one can design diodes and transmission lines so that the magnetic field from the line current inhibits the secondary‐electron‐emission avalanche which is believed to play an important role in the flashover process.