A high tension or medium tension circuit breaker having abutting arcing contacts. The circuit breaker includes a gastight insulating case filled with a gas having good dielectric properties. The case contains a stationary arcing contact; a moving arcing contact driven by a drive rod actuated by a mechanism outside the case. A blast chamber is defined by a piston integral with the moving arcing contact, slidably received in a fixed cylinder terminating in a blast nozzle, the cylinder and a ring. The circuit breaker further includes a lost motion mechanism to enable the drive rod to move over a given length in the circuit breaker opening direction without driving the moving arcing contact away from the fixed arcing contact.