Circuit breaker (SG) for protecting an electrical low-voltage circuit, having: - a housing (GEH) having at least one grid-side terminal and one load-side terminal, - a mechanical break contact unit (MK) which is connected in series with an electronic interrupt unit (EU), wherein the mechanical break contact unit is assigned to the load-side terminal and the electronic interrupt unit (EU) is assigned to the grid-side terminal, - wherein the mechanical break contact unit (MK) can be switched by opening contacts for avoiding a current flow or closing the contacts for a current flow in the low-voltage circuit, - wherein the electronic interrupt unit (EU) can be switched by semiconductor-based switching elements into a high-resistance state of the switching elements for avoiding a current flow or a low-resistance state of the switching elements for current flow in the low-voltage circuit, - a current sensor unit (SI) for determining the level of the current of the low-voltage circuit, - a control unit (SE) which is connected to the current sensor unit (SI), the mechanical break contact unit (MK) and the electronic interrupt unit (EU), wherein avoidance of a current flow of the low-voltage circuit is initiated if current or/and current/time limit values are exceeded, characterized - in that a measuring impedance is provided between two wires of the low-voltage circuit, wherein on one side, the measuring impedance is connected to the connection between mechanical break contact unit (MK) and electronic interrupt unit (EU), - in that the circuit breaker is designed in such a manner that, to function test the circuit breaker when contacts of the mechanical break contact unit (MK) are open and the electronic interrupt unit (EU) is switched to high-resistance, the electronic interrupt unit (EU) is switched to a low-resistance state for a first time period. - a housing (GEH) having at least one grid-side terminal and one load-side terminal, - a mechanical break contact unit (MK) which is connected in series with an electronic interrupt unit (EU), wherein the mechanical break contact unit is assigned to the load-side terminal and the electronic interrupt unit (EU) is assigned to the grid-side terminal, - wherein the mechanical break contact unit (MK) can be switched by opening contacts for avoiding a current flow or closing the contacts for a current flow in the low-voltage circuit, - wherein the electronic interrupt unit (EU) can be switched by semiconductor-based switching elements into a high-resistance state of the switching elements for avoiding a current flow or a low-resistance state of the switching elements for current flow in the low-voltage circuit, - a current sensor unit (SI) for determining the level of the current of the low-voltage circuit, - a control unit (SE) which is connected to the current sensor unit (SI), the mechanical break contact unit (MK) and the electronic interrupt unit (EU), wherein avoidance of a current flow of the low-voltage circuit is initiated if current or/and current/time limit values are exceeded, characterized - in that a measuring impedance is provided between two wires of the low-voltage circuit, wherein on one side, the measuring impedance is connected to the connection between mechanical break contact unit (MK) and electronic interrupt unit (EU), - in that the circuit breaker is designed in such a manner that, to function test the circuit breaker when contacts of the mechanical break contact unit (MK) are open and the electronic interrupt unit (EU) is switched to high-resistance, the electronic interrupt unit (EU) is switched to a low-resistance state for a first time period. <EOS>