The invention relates to a medium-, high-, or very high-voltage circuit breaker, comprising at least one arc-control chamber and an outer casing in which the arc-control chamber is arranged. The circuit breaker includes a discharge cap (40) forming a portion of the outer wall external of the arc-control chamber (12), the discharge cap being situated inside the outer casing and internally defining a gas-flow chamber (31). In the invention, the discharge cap (40) includes at least one portion (40′) that is movable under the effect of the gas pressure in the gas-flow chamber (31), in such a manner that its volume is extensible.