Abstract: A medium or high-voltage circuit breaker is provided. An arc-extinguishing fluid is stored in a pressure chamber, which is made up by an arcing chamber of the circuit breaker where an electrical arc is extinguished. No tubings or other conduction devices for pressurized fluids are necessary during current interruption. This speeds up contact separation and arc extinguishing. In a first, closed position of the circuit breaker, the pressure chamber is delimited by a sealing wall which is fixed to a movable first arcing contact. The sealing wall acts as a valve and as a piston at the same time, because it opens the outflow of pressurized arc extinguishing fluid and it concurrently actuates arcing contact separation. The arcing chamber enables the stored pressurized arc extinguishing fluid to perform fast contact separation within a few milliseconds and extinguishing a gaseous arc on the same timescale.