High-voltage gas-blast circuit-breaker

A gas-blast gas-insulated high-voltage circuit-breaker comprising a fixed main contact, a fixed arc contact and a mobile assembly comprising a main contact, an arc contact and a compression volume cooperating with a fixed piston and communicating with a blast volume leading to a blast nozzle, said compression volume being provided with a valve preventing said gas passing from said blast volume to said compression volume, wherein said piston includes openings enabling said gas to be evacuated from said compression chamber during a tripping operation, said openings being associated with means for maintaining them closed between the start of said compression of said compression volume and the closure of the valve and opening them upon closure of said valve, said means comprising fixed rods sliding in said openings of said piston, said rods having longitudinal recesses or notches or grooves or splines over a portion of their length such that the recessed part of said rods is in said openings when said valve closes.