Abstract: An electrical circuit breaker of the gas blast type comprises a pair of contact members disposed within an arc quenching zone of a chambered part at which separation of the contact members takes place and an arc is established therebetween. An annular blast gas in-flow duct surrounds the arc quenching zone and through which the gas is forced by piston action in the direction of the quenching zone at the time of contact separation, and the discharge end of this in-flow duct includes a gas deflection zone for preventing blow-back of hot arc extinction gases formed in the quenching zone through the in-flow duct in the opposite direction thereby minimizing the chances of damage to the piston system.