A medium-, high-, or very high-voltage circuit breaker (10), comprising at least one arc-control chamber (12) and an outer casing (14) in which the arc-control chamber (12) is arranged. In the invention, the circuit breaker includes sealing ring (52) for sealing an opening (50) in a discharge cap (40) of the arc-control chamber (12), the ring (52) being movable between a sealing position and a retracted position in which they allow the gas to pass, the circuit breaker being configured in such a manner that in the closed position, the sealing ring (52) are in their retracted position, and in such a manner that passage from the retracted position to the sealing position takes place during an operation of opening the circuit breaker.