Circuit breaker device (SG) for protecting an electrical low-voltage circuit, comprising: - a housing (GEH) having grid-side connections and at least one load-side connection, - a mechanical isolating contact unit (MK) connected in series with an electronic interruption unit (EU), wherein the mechanical isolating contact unit is assigned to the load-side connection and the electronic interruption unit (EU) is assigned to the grid-side connections, - wherein the mechanical isolating contact unit (MK) can 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) can 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 measuring impedance (ZM) between conductors of the low-voltage circuit in such a way that, when the contacts of the mechanical isolating contact unit (MK) are open and the electronic interruption unit (EU) is switched to being low-impedance, a measuring current flows through the electronic interruption unit (EU) via the network-side connections. - a housing (GEH) having grid-side connections and at least one load-side connection, - a mechanical isolating contact unit (MK) connected in series with an electronic interruption unit (EU), wherein the mechanical isolating contact unit is assigned to the load-side connection and the electronic interruption unit (EU) is assigned to the grid-side connections, - wherein the mechanical isolating contact unit (MK) can 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) can 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 measuring impedance (ZM) between conductors of the low-voltage circuit in such a way that, when the contacts of the mechanical isolating contact unit (MK) are open and the electronic interruption unit (EU) is switched to being low-impedance, a measuring current flows through the electronic interruption unit (EU) via the network-side connections. <EOS>