A monitoring device (100, 101, 102) for monitoring an insulation resistance (R f) for an ungrounded electric system which comprises a liquid cooling which is operated to ground using a refrigerant and has a supply line and a return line, the supply line and the return line each being realized as a tube section which is connected to a ground potential and is electrically conductive and to which an electrically insulated tube section is connected which has a tube length (l i) and is connected to the electric system, characterized by one or two low-impedance measuring signal sources each configured for generating a measuring signal having a measuring voltage (U m), each comprising a ground-potential connection connected to the electrically conductive tube section, and each comprising a coupling connection, a coupling-current measuring sensor for measuring a coupling current (I AK) which flows in the refrigerant flowing through the electrically insulated tube section, a fault-current measuring sensor which is switched downstream of the coupling-current measuring sensor in the direction of the electric system and is configured for measuring a fault current (I Ri) which flows in the refrigerant flowing through the electrically insulated tube section, a conductive coupling tube element which contacts the refrigerant and is electrically connected to the coupling connection of the measuring signal source for coupling the measuring signal, the coupling tube element being disposed in such a manner on the electrically insulated tube section between the coupling-current measuring sensor and the fault-current measuring sensor that the tube length (l i) of the electrically insulated tube section is divided into a coupling length (I AK) extending between the electrically conductive tube section and the coupling tube element and a resistance length (l Ri) extending from the coupling tube element to the electric system, a computing unit which is configured for computing the insulation resistance (Rf) from the measuring voltage (U m), the detected coupling current (I AK), the detected fault current (I Ri), the coupling length (I AK) and the resistance length (l Ri). one or two low-impedance measuring signal sources each configured for generating a measuring signal having a measuring voltage (U m), each comprising a ground-potential connection connected to the electrically conductive tube section, and each comprising a coupling connection, a coupling-current measuring sensor for measuring a coupling current (I AK) which flows in the refrigerant flowing through the electrically insulated tube section, a fault-current measuring sensor which is switched downstream of the coupling-current measuring sensor in the direction of the electric system and is configured for measuring a fault current (I Ri) which flows in the refrigerant flowing through the electrically insulated tube section, a conductive coupling tube element which contacts the refrigerant and is electrically connected to the coupling connection of the measuring signal source for coupling the measuring signal, the coupling tube element being disposed in such a manner on the electrically insulated tube section between the coupling-current measuring sensor and the fault-current measuring sensor that the tube length (l i) of the electrically insulated tube section is divided into a coupling length (I AK) extending between the electrically conductive tube section and the coupling tube element and a resistance length (l Ri) extending from the coupling tube element to the electric system, a computing unit which is configured for computing the insulation resistance (Rf) from the measuring voltage (U m), the detected coupling current (I AK), the detected fault current (I Ri), the coupling length (I AK) and the resistance length (l Ri).