A single-stage circuit breaker, comprising a shell comprising a first housing, a second housing, and a mounting housing located therebetween, a first accommodation chamber being formed between the first housing and the mounting housing, and a second accommodation chamber being formed between the second housing and the mounting housing; an electromagnetic trip device arranged in the first accommodation chamber; a first contact assembly and a first execution assembly cooperating with the electromagnetic trip device and located at one side of the first accommodation chamber; an arc extinguishing device accommodated between the electromagnetic protection device and the first contact assembly; a thermal protection and arc fault detection device located near the arc extinguishing device and in the first accommodation chamber; an arc fault and leakage trip device arranged in the second accommodation chamber; a second contact assembly and a second execution assembly cooperating with the arc fault and leakage trip device and located at one side of the second accommodation chamber; and a circuit board assembly for arc fault handling and leakage protection, wherein an L-pole main switch is formed between a movable contact and a stationary contact in the first contact assembly, wherein an N-pole main switch is formed between a stationary contact and a movable contact in the second contact assembly, wherein the first execution assembly and the second execution assembly move simultaneously with an operating handle via a common driving member, so that the first execution assembly and the second execution assembly are respectively actuated by the action of the handle, and then the first contact assembly and the second contact assembly are respectively actuated, characterized in that the circuit board assembly comprises a first circuit board and a second circuit board, wherein the first circuit board is accommodated in the first accommodation chamber, and the second circuit board is accommodated in the second accommodation chamber, wherein an auxiliary contact assembly is arranged inside the second accommodation chamber, one end of the auxiliary contact assembly being electrically connected to the second circuit board and the other end of the auxiliary contact assembly being disconnectable from or connectable to the second contact assembly, the auxiliary contact assembly being equivalent to the addition of an auxiliary switch, wherein, when the single-stage circuit breaker is in an ON state, the L-pole main switch and the N-pole main switch are in a switching-on state, the auxiliary switch is also in a switching-on state, wherein, when the circuit breaker is in an OFF state, the L-pole main switch and the N-pole main switch are in a switching-off state, and the auxiliary switch is also in a switching-off state. a shell comprising a first housing, a second housing, and a mounting housing located therebetween, a first accommodation chamber being formed between the first housing and the mounting housing, and a second accommodation chamber being formed between the second housing and the mounting housing; an electromagnetic trip device arranged in the first accommodation chamber; a first contact assembly and a first execution assembly cooperating with the electromagnetic trip device and located at one side of the first accommodation chamber; an arc extinguishing device accommodated between the electromagnetic protection device and the first contact assembly; a thermal protection and arc fault detection device located near the arc extinguishing device and in the first accommodation chamber; an arc fault and leakage trip device arranged in the second accommodation chamber; a second contact assembly and a second execution assembly cooperating with the arc fault and leakage trip device and located at one side of the second accommodation chamber; and a circuit board assembly for arc fault handling and leakage protection, wherein an N-pole main switch is formed between a stationary contact and a movable contact in the second contact assembly, wherein the first execution assembly and the second execution assembly move simultaneously with an operating handle via a common driving member, so that the first execution assembly and the second execution assembly are respectively actuated by the action of the handle, and then the first contact assembly and the second contact assembly are respectively actuated, characterized in that the circuit board assembly comprises a first circuit board and a second circuit board, wherein the first circuit board is accommodated in the first accommodation chamber, and the second circuit board is accommodated in the second accommodation chamber, wherein an auxiliary contact assembly is arranged inside the second accommodation chamber, one end of the auxiliary contact assembly being electrically connected to the second circuit board and the other end of the auxiliary contact assembly being disconnectable from or connectable to the second contact assembly, the auxiliary contact assembly being equivalent to the addition of an auxiliary switch, wherein, when the single-stage circuit breaker is in an ON state, the L-pole main switch and the N-pole main switch are in a switching-on state, the auxiliary switch is also in a switching-on state, wherein, when the circuit breaker is in an OFF state, the L-pole main switch and the N-pole main switch are in a switching-off state, and the auxiliary switch is also in a switching-off state.