A magnetic actuator unit is provided for a circuit breaker, such as a medium voltage vacuum circuit breaker. The magnetic actuator unit includes a core, a coil, an actuating shaft, a first movable plate, a second movable plate, and a non-magnetic flat insert arranged between the core and the second movable plate. The magnetic actuator unit configured to switch the circuit breaker ON and OFF by moving the first movable plate between an ON position and an OFF position. The non-magnetic flat insert and the second movable plate are configured to adjust a holding force of the magnetic actuator unit provided by the second movable plate at the OFF position. The holding force is sufficient for holding the second movable plate at the OFF position against outer forces that are acting on the magnetic actuator unit.