Abstract: In or for a fluid-filled metal-clad switchgear an automatically-operating device for short-circuiting an internal arc in the earthed metal enclosure comprises a piston movable in a cylinder mounted on the enclosure wall, with a rod projecting towards the interior space filled with insulating medium. A pipe or other means connects the space to the cylinder at the remote side of the piston, so that on occurrence of an internal arc in the enclosure the rapid pressure rise creates a pressure differential across the piston to drive it and the rod inwardly and cause the rod to engage the high-voltage conductor and short-circuit the arc to the earthed enclosure. The piston is held normally by a spring or by breakable pegs.