Abstract: The water and microwave permeable dielectric insulation in an electrotechnical high voltage equipment is dried by means of microwaves. The electrotechnical equipment has a central conductor on which the insulation is wrapped, and it is disposed within an electrically conducting hollow cylinder with its central conductor coaxial with the latter cylinder to form a coaxial, microwave transmission line. Microwaves are propagated through the coaxial transmission line to heat the water in the dielectric insulation, to transform it into water vapor, and to thereby dry the insulation. The microwave transmission line is air-tight and the water vapor is evacuated through a vacuum pump or cold trap.