A miniature circuit breaker (1, 101) having: a pair of openable contacts (8, 10) located in a main current path between a line terminal and a load terminal; a trip mechanism for opening the contacts if an overcurrent condition occurs; wherein the trip mechanism includes: a contact closing mechanism, the contact closing mechanism comprising a slider movable between a retracted position and an extended position; a mechanical energy store (44, 46) arranged to accumulate mechanical energy and subsequently to release accumulated mechanical energy; a contact opening mechanism, wherein the contact closing mechanism and the contact opening mechanism include a common trip lever; and an electric motor (25, 125) operable to accumulate mechanical energy into the mechanical energy store and release the mechanical energy to close the contacts via the contact closing mechanism; a trigger mechanism, wherein the trigger mechanism being arranged to trigger the contact opening mechanism into opening the contacts if a trip signal is produced; a current sensor arranged to detect current through the main current path; and a control unit; wherein the control unit is: arranged to produce the trip signal to operate the trigger mechanism to trigger the trip mechanism into opening the contacts if it determines that a short circuit condition occurs based on an output of the current sensor; and is further arranged to operate the electric motor to move the slider towards its retracted position thereby causing the slider to rotate the trip lever to trigger the contact opening mechanism into opening the contacts independently of the trigger mechanism if it determines that an overload condition occurs. a pair of openable contacts (8, 10) located in a main current path between a line terminal and a load terminal; a trip mechanism for opening the contacts if an overcurrent condition occurs; wherein the trip mechanism includes: a contact closing mechanism, the contact closing mechanism comprising a slider movable between a retracted position and an extended position; a mechanical energy store (44, 46) arranged to accumulate mechanical energy and subsequently to release accumulated mechanical energy; a contact closing mechanism, the contact closing mechanism comprising a slider movable between a retracted position and an extended position; a mechanical energy store (44, 46) arranged to accumulate mechanical energy and subsequently to release accumulated mechanical energy; a contact opening mechanism, wherein the contact closing mechanism and the contact opening mechanism include a common trip lever; and an electric motor (25, 125) operable to accumulate mechanical energy into the mechanical energy store and release the mechanical energy to close the contacts via the contact closing mechanism; a trigger mechanism, wherein the trigger mechanism being arranged to trigger the contact opening mechanism into opening the contacts if a trip signal is produced; a current sensor arranged to detect current through the main current path; and a control unit; arranged to produce the trip signal to operate the trigger mechanism to trigger the trip mechanism into opening the contacts if it determines that a short circuit condition occurs based on an output of the current sensor; and is further arranged to operate the electric motor to move the slider towards its retracted position thereby causing the slider to rotate the trip lever to trigger the contact opening mechanism into opening the contacts independently of the trigger mechanism if it determines that an overload condition occurs.