An add-on module adapted to be attached to the basic mechanical structure of a multi-pole circuit breaker includes multiple extended terminal plates each of which is adapted to replace one of the input and output terminals for one of the poles, multiple electromechanical transducers each of which is coupled to one of the extended terminal plates for producing a mechanical movement in response to a predetermined magnitude of electrical current in the extended terminal plate to which that transducer is coupled, a mechanical actuator coupled to the electromechanical transducers and to the movable contacts for operating the trip mechanism in response to a predetermined movement of any of the transducers, and a calibration element for adjusting mechanical movement of at least one of said multiple electromechanical transducers so as to control an aspect of trip actuation.