Circuit breaker device (SG) for protecting an electrical low-voltage circuit, comprising: - a housing (GEH) having a first and a second network-side connection and a first and a second load-side connection, - a mechanical isolating contact unit (MK) which is connected in series with an electronic interruption unit (EU), wherein the mechanical isolating contact unit is assigned to the load-side connections and the electronic interruption unit (EU) is assigned to the network-side connections, - wherein the mechanical isolating contact unit (MK) is able to be switched by opening contacts in order to prevent a flow of current or closing the contacts to allow a flow of current in the low-voltage circuit, - wherein the electronic interruption unit (EU) is able to be switched, by means of semiconductor-based switching elements, to a high-impedance state of the switching elements in order to prevent a flow of current or to a low-impedance state of the switching elements to allow the flow of current in the low-voltage circuit, - a current sensor unit (SI) for ascertaining the level of the current of the low-voltage circuit, - a control unit (SE) which is connected to the current sensor unit (SI), to the mechanical isolating contact unit (MK) and to the electronic interruption unit (EU), wherein if current or/and current-time limit values is/are exceeded a process for preventing a flow of current in the low-voltage circuit is initiated, characterized - in that provision is made for a first measuring impedance (ZM1) between the first and the second load-side connection in such a way that when the contacts of the mechanical isolating contact unit (MK) are open a current could flow from the first load-side connection to the second load-side connection via the measuring impedance. - a housing (GEH) having a first and a second network-side connection and a first and a second load-side connection, - a mechanical isolating contact unit (MK) which is connected in series with an electronic interruption unit (EU), wherein the mechanical isolating contact unit is assigned to the load-side connections and the electronic interruption unit (EU) is assigned to the network-side connections, - wherein the mechanical isolating contact unit (MK) is able to be switched by opening contacts in order to prevent a flow of current or closing the contacts to allow a flow of current in the low-voltage circuit, - wherein the electronic interruption unit (EU) is able to be switched, by means of semiconductor-based switching elements, to a high-impedance state of the switching elements in order to prevent a flow of current or to a low-impedance state of the switching elements to allow the flow of current in the low-voltage circuit, - a current sensor unit (SI) for ascertaining the level of the current of the low-voltage circuit, - a control unit (SE) which is connected to the current sensor unit (SI), to the mechanical isolating contact unit (MK) and to the electronic interruption unit (EU), wherein if current or/and current-time limit values is/are exceeded a process for preventing a flow of current in the low-voltage circuit is initiated, characterized - in that provision is made for a first measuring impedance (ZM1) between the first and the second load-side connection in such a way that when the contacts of the mechanical isolating contact unit (MK) are open a current could flow from the first load-side connection to the second load-side connection via the measuring impedance. <EOS>