The invention concerns a drive unit (10) for a high and medium voltage circuit breaker comprising : - a main shaft (12) being rotatable around its main axis (A) between a first position corresponding to a closed state of the circuit breaker and a second position corresponding to an opened state of the circuit breaker, - elastic means for driving the main shaft (12) in rotation towards the second position, and - releasable means (16) for preventing the rotation of the main shaft (12) from the second position towards the first position, wherein the releasable means (16) comprise retention pawl (18) which removably engages the main shaft (12) and which, when engaged, enables the main shaft (12) to rotate towards the second position but prevents its rotation from the second position towards the first position.
