An arc-suppressing apparatus for a circuit breaker has a pair of side plates (1A,1 B) having a plurality of first and second grooves (2, 22) formed therein, the second grooves (22) being shallower than the first grooves (2) and extending linearly from the ends of the first grooves (2) to an end of each of the side plates. The side plates (1T, 1B) are preferably separated by a spacing strut (10) and loosely connected by a bolt (5A) and nut so that a space (B) exists between the plates which is large enough to allow a plurality of grids (4) to slide along the second grooves (22) into the first grooves (2). Thereafter, the nut and bolt (5A) are tightened to firmly secure the grids (4) in the first grooves (2).