A low-voltage circuit breaker , comprising: - at least one fixed contact , for each pole , which is electrically connected to a terminal for connection to an electric circuit and a corresponding moving contact which is associable/separable with respect to said fixed contact by means of a rotation of said moving contact ; - an arc chamber positioned in correspondence of said fixed contact ; - a rotating contact supporting shaft common to all poles , 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 a quick acting trip device comprising a plunger inserted in a first channel connected to the arc chamber of the corresponding pole , 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 , for each pole , which is electrically connected to a terminal for connection to an electric circuit and a corresponding moving contact which is associable/separable with respect to said fixed contact by means of a rotation of said moving contact ; - an arc chamber positioned in correspondence of said fixed contact ; - a rotating contact supporting shaft common to all poles , 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 a quick acting trip device comprising a plunger inserted in a first channel connected to the arc chamber of the corresponding pole , 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. <EOS>