Breaker unit and air circuit breaker comprising same

An air circuit breaker, comprising:

• an arc extinguishing unit comprising a plurality of side plates and a grid coupled between the plurality of side plates;
• and
• a circuit breaking unit disposed adjacent to the arc extinguishing unit,
• wherein the circuit breaking unit comprises:
• a fixed contact terminal having a fixed contact disposed at a first end and extending in a first direction away from the fixed contact toward a second end;
• a movable contact, configured to move toward or away from the fixed contact;
• a runner disposed extending in the first direction from the fixed contact, wherein a first end of the runner is coupled to the fixed contact terminal, and a second end of the runner, opposite to the first end, is spaced apart from the fixed contact terminal;
• and
• a magnet unit disposed between the runner and the fixed contact terminal and forming a magnetic field between the fixed contact and the movable contact to guide a path of an arc generated when the fixed contact and the movable contact are separated,
• wherein the runner is shaped to define a gradually expanding space for the magnet unit between the runner and the fixed contact terminal, such that a distance away between the runner the fixed contact terminal increases in the first direction.