Circuit breaker device for protecting an electrical low-voltage circuit, comprising: - a housing having a first and a second network-side connection and a first and a second load-side connection, - a mechanical isolating contact unit which is connected in series with an electronic interruption unit, wherein the mechanical isolating contact unit is assigned to the load-side connections and the electronic interruption unit is assigned to the network-side connections, - wherein the mechanical isolating contact unit 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 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 for ascertaining the level of the current of the low-voltage circuit, - a control unit which is connected to the current sensor unit, to the mechanical isolating contact unit and to the electronic interruption unit, 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 between the first and the second load-side connection in such a way that when the contacts of the mechanical isolating contact unit are open a current could flow from the first load-side connection to the second load-side connection via the measuring impedance. - a housing having a first and a second network-side connection and a first and a second load-side connection, - a mechanical isolating contact unit which is connected in series with an electronic interruption unit, wherein the mechanical isolating contact unit is assigned to the load-side connections and the electronic interruption unit is assigned to the network-side connections, - wherein the mechanical isolating contact unit 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 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 for ascertaining the level of the current of the low-voltage circuit, - a control unit which is connected to the current sensor unit, to the mechanical isolating contact unit and to the electronic interruption unit, 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 between the first and the second load-side connection in such a way that when the contacts of the mechanical isolating contact unit are open a current could flow from the first load-side connection to the second load-side connection via the measuring impedance.