A low-voltage circuit breaker, comprising: - at least one fixed contact (20, 30), for each pole (3, 4, 5), which is electrically connected to a terminal for connection to an electric circuit and a corresponding moving contact (21, 31) which is associable/separable with respect to said fixed contact (20, 30) by means of a rotation of said moving contact (21, 31); - an arc chamber (40, 41) positioned in correspondence of said fixed contact (20, 30); - a rotating contact supporting shaft common to all poles (3, 4, 5), which is functionally connected to an actuation mechanism of the circuit breaker, said actuation mechanism comprising a kinematic system operatively connected to an actuation lever for opening/closing operations and provided with opening springs and a tripping shaft for releasing said kinematic system and allowing its movement from a closed to an open position, wherein the low-voltage circuit breaker comprises for each pole (3, 4, 5) a quick acting trip device comprising a plunger inserted in a first channel connected to the arc chamber of the corresponding pole (3, 4, 5), said plunger having a first operative surface subjected to pressure of said arc chamber and a second operative surface, said quick acting trip device further comprises a trip lever having a first portion cooperating with said tripping shaft and a second portion cooperating with the second operative surface of said plunger, characterized in that said plunger has a substantially cylindrical body and is slidingly movable in said first channel under the action of an overpressure within said arc chamber between a first resting position and a second operating position in which it urges against the second portion of said trip lever determining the movement thereof from a normal-operating position to a tripping position in which said trip lever acts on said tripping shaft determining the release of said kinematic system. - at least one fixed contact (20, 30), for each pole (3, 4, 5), which is electrically connected to a terminal for connection to an electric circuit and a corresponding moving contact (21, 31) which is associable/separable with respect to said fixed contact (20, 30) by means of a rotation of said moving contact (21, 31); - an arc chamber (40, 41) positioned in correspondence of said fixed contact (20, 30); - a rotating contact supporting shaft common to all poles (3, 4, 5), which is functionally connected to an actuation mechanism of the circuit breaker, said actuation mechanism comprising a kinematic system operatively connected to an actuation lever for opening/closing operations and provided with opening springs and a tripping shaft for releasing said kinematic system and allowing its movement from a closed to an open position, wherein the low-voltage circuit breaker comprises for each pole (3, 4, 5) a quick acting trip device comprising a plunger inserted in a first channel connected to the arc chamber of the corresponding pole (3, 4, 5), said plunger having a first operative surface subjected to pressure of said arc chamber and a second operative surface, said quick acting trip device further comprises a trip lever having a first portion cooperating with said tripping shaft and a second portion cooperating with the second operative surface of said plunger, characterized in that said plunger has a substantially cylindrical body and is slidingly movable in said first channel under the action of an overpressure within said arc chamber between a first resting position and a second operating position in which it urges against the second portion of said trip lever determining the movement thereof from a normal-operating position to a tripping position in which said trip lever acts on said tripping shaft determining the release of said kinematic system.