A circuit-breaker includes two contacts which are disposed in an interrupting space filled with a dielectric gas under pressure and between which an electric arc strikes during circuit-breaker opening, a thermal blast chamber communicating directly with the interrupting space, and a piston-driven compression chamber communicating with the thermal blast chamber. The piston-driven compression chamber communicates with the interrupting space via a discharge channel that is separate from the thermal blast chamber and that is closed by a discharge valve. The discharge valve is disposed between the thermal blast chamber and the piston-driven compression chamber in a manner such that the discharge valve opens to enable the gas under increased pressure in the piston-driven compression chamber to be discharged towards the interrupting space via the channel when the increase in the pressure of the gases in the thermal blast chamber is larger than the increase in the pressure of the gases in the piston-driven compression chamber.