A residual current operated circuit breaker for protection against a ground leakage current that is higher than a preset threshold value in a low-voltage electric circuit, said circuit being connected to a ground electrode and supplying at least one load connected to said ground electrode by means of a protection conductor, comprising: -- active conductors (8,9), each suitable to be connected, in input to said circuit breaker, to a corresponding conductor of the power supply circuit and to be connected, in output from said circuit breaker, to said load; -- an interruption unit, which has fixed and movable contact means (3, 4) that can be mutually separated in order to interrupt the flow of current in said active conductors, and a kinematic mechanisms suitable to move said moving contact means; -- a release unit, which is suitable to interact functionally with said kinematic mechanism so as to determine the separation of the contact means upon receiving an actuation signal that indicates the presence of a ground leakage current that exceeds the threshold value; characterized in that it comprises: -- a supplemental conductor, suitable to be functionally connected, in input to said circuit breaker, to the ground electrode and, in output from the circuit breaker, to a load protection conductor of the load; -- sensing means, which are functionally associated with said supplemental conductor and said active conductors and are suitable to detect the presence of a ground leakage current in said active conductors and/or in said supplemental conductor and to provide the actuation signal that causes the tripping of the release unit and the separation of the contacts when the value of the leakage current in the active conductors, reduced by a predefined quantity that is proportional to the leakage current flowing if any in said supplemental conductor, exceeds said threshold value. -- active conductors (8,9), each suitable to be connected, in input to said circuit breaker, to a corresponding conductor of the power supply circuit and to be connected, in output from said circuit breaker, to said load; -- an interruption unit, which has fixed and movable contact means (3, 4) that can be mutually separated in order to interrupt the flow of current in said active conductors, and a kinematic mechanisms suitable to move said moving contact means; -- a release unit, which is suitable to interact functionally with said kinematic mechanism so as to determine the separation of the contact means upon receiving an actuation signal that indicates the presence of a ground leakage current that exceeds the threshold value; characterized in that it comprises: -- a supplemental conductor, suitable to be functionally connected, in input to said circuit breaker, to the ground electrode and, in output from the circuit breaker, to a load protection conductor of the load; -- sensing means, which are functionally associated with said supplemental conductor and said active conductors and are suitable to detect the presence of a ground leakage current in said active conductors and/or in said supplemental conductor and to provide the actuation signal that causes the tripping of the release unit and the separation of the contacts when the value of the leakage current in the active conductors, reduced by a predefined quantity that is proportional to the leakage current flowing if any in said supplemental conductor, exceeds said threshold value.