Abstract: A high-voltage apparatus having a casing (1) filled with an insulating medium, an active part (6) connected to high voltage in the casing, and a pressure-relief valve (2) closing off the casing (1). The pressure-relief valve (2) includes a hinged cover (9) and a bending spring (12) acting on the cover (9). The pressure-relief valve (2) responds extremely quickly to changes in pressure of the insulating medium located in the casing (1). This is achieved partially by the fact that the bending spring (12) lies in the current path of the active part (6) connected to the high voltage and due to the fact that the bending spring (12) is formed with weakened regions which are heated faster by the current. As a result, the rigidity of this bending spring (12) suddenly drops as the current exceeds a predetermined value. This results in sudden relieving of the pressure in casing (1).