A circuit breaker includes a housing, an operating mechanism, and a cassette. The cassette includes a separable current path having a movable contact arm, two arc receptors each disposed for receiving an electrical arc in response to a short circuit overcurrent condition, and two slot motors each disposed proximate the movable contact arm for magnetically driving the movable contact arm to a blown open position in response to a short circuit overcurrent condition. The slot motors each have two side legs opposing each other and defining a space therebetween for receiving the movable contact arm, each side leg having a magnetic core and an insulative layer and being biased toward the interior of the side walls of the cassette, wherein each side leg is restrained from being magnetically attracted toward the movable contact arm during a short circuit condition.