Monitoring device with a first conductor section with a first connecting point and a second connecting point spaced from the first connecting point in direction of conducting and with a control unit and a first current detecting unit, comprising - a first current source, wherein the first current source is connected by a first connecting line with the first connecting point and by a second connecting line with the second connecting point and outputs a first current, - a first switch with a control input, wherein the first switch is connected into the second connecting line and connects the first current source with or separates the first current source from the second connecting point and - a first differential amplifier with a first input, a second input and an output, wherein the first input is connected by a third connecting line with the first connecting point and the second input is connected by a fourth connecting line with the second connecting point, wherein the control unit is connected between the output of the first differential amplifier and the control input of the first switch, the first conductor section is acted on by an actual current and in a first state the first switch is closed and the first current is imposed in the first conductor section additionally to the first actual current and a voltage attributable to the magnitude of the actual current and the magnitude of the first current is present between the first input and the second input of the first differential amplifier and in a second state the first switch is open and a second voltage attributable exclusively to the actual current is present between the first input and the second input of the first differential amplifier (D1F1), characterised in that a second conductor section with a second current detecting unit with a second differential amplifier is provided and a second current is supplied by means of a second current source to the second conductor section, the first current and the second current are substantially equal but opposite, the control unit comprises a third differential amplifier and a first input of the third differential amplifier is connected with the output of the first differential amplifier and a second input of the third differential amplifier is connected with the output of the second differential amplifier, the first differential amplifier together with the second differential amplifier and the third differential amplifier forming a multi-stage amplifier unit (INST1) and a sum of the voltage differences measured at the first conductor section and the second conductor section being present at the output of the multi-stage amplifier unit, and the control unit is arranged to determine the magnitude of the actual current from the voltages of the first state and the second state. - a first current source, wherein the first current source is connected by a first connecting line with the first connecting point and by a second connecting line with the second connecting point and outputs a first current, - a first switch with a control input, wherein the first switch is connected into the second connecting line and connects the first current source with or separates the first current source from the second connecting point and - a first differential amplifier with a first input, a second input and an output, wherein the first input is connected by a third connecting line with the first connecting point and the second input is connected by a fourth connecting line with the second connecting point,