A Medium Voltage circuit breaker which comprises an insulating base frame supporting: a pole assembly having, for each phase, an interruption unit housing a fixed contact and a movable contact reciprocally couplable/uncouplable between an open and a closed position; an actuator to actuate the opening and closing operation of said circuit breaker; and, a control and diagnostic unit comprising a control box having a casing housing a plurality of accessory devices of said circuit breaker.