Patent ID: 7138597

Claim:
A circuit breaker comprising: a housing defining a contact chamber; a current interruption mechanism comprising: separable contacts comprising a fixed contact and a moving contact positioned in the contact chamber; a moving support with a piston by which the moving contact is carried for movement in the contact chamber along a first axis between a closed position in which the separable contacts are closed and an open position in which the separable contacts are open, the piston being driven toward the open position from the closed position by forces comprising: initially a repulsion force generated by an overcurrent through the separable contacts and then also by arc gas produced by an arc generated as the separable contacts separate; bias means biasing the piston to the closed position; and wherein the current interruption mechanism further comprises at least one shield member slidable across the contact chamber between a first position in which a first opening in the at least one shield member is aligned with the contact chamber to define a through opening through which the moving support extends in the closed position, and a second position in which the first opening does not align with the contact chamber to cut off the arc after the piston has begun movement toward the open position.