A low-voltage circuit breaker includes a retrofittable motor reset mechanism. Its output shaft can be coupled to a charging shaft, mounted in a bearing block, of the spring energy store of the low-voltage circuit breaker. Centering elements are provided on a flange of the bearing block of the charging shaft of the spring energy store so as to engage in mating elements on the gearbox output of the motor reset mechanism, in such a way that the motor reset mechanism can be pushed axially into these centering elements with little movement in the direction of the charging shaft. In order to lock the motor reset mechanism in the pivoting direction, a fixing element is provided at a point fitted at a specific distance from the centering elements. The motor reset mechanism is fixed axially in the centering elements of the flange of the bearing block of the charging shaft of the spring reset mechanism, and in the coupling of the charging shaft.