A magnetic actuator unit is disclosed for a circuit-breaker arrangement, having an armature arranged to be movable between first and second, closed and opened end switching positions of the circuit-breaker, a single electrical coil for moving the armature to the second position due to electrical current feed, a permanent magnet for loading the armature in the direction of the second position, an outer ferromagnetic yoke at least partly surrounding the single electrical coil and the ferromagnetic core for directing magnetic flux to the armature, and an opening spring for permanent loading of the armature in the direction of the first position, which is coaxially arranged between the armature and the front side of the electrical coil. The opening spring can be at least partly accommodated inside an armature groove.