The invention relates to a circuit breaker (10) for current at medium, high or very high voltage, comprising at least one cut-off chamber (12) as well as an outer envelope (14) in which is arranged the cut-off chamber (12). According to the invention, the circuit breaker comprises means (52) for sealing an opening (50) through an escape hood (40) of the cut-off chamber (12), the means (52) being displaceable between a sealed position and a withdrawn position in which they allow the passage of gas, the circuit breaker being configured in such a way that in its closure position, the sealing means (52) occupy their withdrawn position, and in such a way that the passage from the withdrawn position to the sealed position occurs in the course of an operation of opening of the circuit breaker.