Monitoring device for monitoring an impedance of a protective conductor, wherein the monitoring device comprises: a first voltage divider (R1, R2), arranged between a first input terminal and a second input terminal, which is connectable to a voltage source, wherein the first voltage divider comprises a series connection of a first resistor and a second resistor, a second voltage divider, which comprises a series connection of a third resistor, the third resistor having a first end and a second end, and a bridge diode, the bridge diode having an anode and a cathode, wherein the anode is connected to second end of the third resistor, and wherein the second voltage divider is connected with a first end of the third resistor to the first input terminal, and is connectable with the cathode of the diode to the protective conductor, a measuring device, which is connected between a first node that is arranged between the first resistor and the second resistor, and a second node, which is arranged between the third resistor and the bridge diode, wherein the measuring device comprises an optical coupler having an anode terminal, a cathode terminal, a collector terminal, and an emitter terminal, the anode terminal of which is connected to the first node, and the cathode terminal of which is connected to the second node, wherein the second resistor has a resistance value which corresponds to a threshold value for the impedance of the protective conductor, and wherein the measuring device is operable to detect a bridge voltage between the first and second nodes (K1, K2), and wherein the bridge voltage constitutes the input signal of the optical coupler, such that an output signal is delivered on the emitter terminal of the optical coupler, if the impedance of the protective conductor is greater than the value of the second resistor. a first voltage divider (R1, R2), arranged between a first input terminal and a second input terminal, which is connectable to a voltage source, wherein the first voltage divider comprises a series connection of a first resistor and a second resistor, a second voltage divider, which comprises a series connection of a third resistor, the third resistor having a first end and a second end, and a bridge diode, the bridge diode having an anode and a cathode, wherein the anode is connected to second end of the third resistor, and wherein the second voltage divider is connected with a first end of the third resistor to the first input terminal, and is connectable with the cathode of the diode to the protective conductor, a measuring device, which is connected between a first node that is arranged between the first resistor and the second resistor, and a second node, which is arranged between the third resistor and the bridge diode, wherein the measuring device comprises an optical coupler having an anode terminal, a cathode terminal, a collector terminal, and an emitter terminal, the anode terminal of which is connected to the first node, and the cathode terminal of which is connected to the second node, wherein the second resistor has a resistance value which corresponds to a threshold value for the impedance of the protective conductor, and wherein the measuring device is operable to detect a bridge voltage between the first and second nodes (K1, K2), and wherein the bridge voltage constitutes the input signal of the optical coupler, such that an output signal is delivered on the emitter terminal of the optical coupler, if the impedance of the protective conductor is greater than the value of the second resistor.