Abstract: A circuit breaker mechanism has a contact arm (56) forming a portion of an interruptable load current path through the circuit breaker and mounted with respect to a pivot (110) for swinging motion to break the current path. A spring-loaded, over-center operating mechanism includes a toggle mechanism having an upper toggle (74) and a lower toggle (122) coupled by a spring pin (128) to form a toggle joint about which the toggles can relatively pivot. The spring pin has a circular groove (128b) at which it operatively connects to the toggle joint, and a circular surface (128a) adjacent the groove is disposed in the path of travel of the contact arm as the contact arm swings to break the current path.