A method for testing the function of a standard insulation monitoring device, installed in an ungrounded power supply system, during operation said method comprising a fully automated, perpetually running testing cycle, comprising the method steps: - (a) measuring a first insulation resistance (Riso1) of the ungrounded power supply system via the insulation monitoring device, - (b) transmitting the first insulation resistance value (Riso1) to a testing apparatus and saving the first insulation resistance value (Riso1) in the testing apparatus, - (c) calculating an uncritical test fault resistance value (Rf) via the testing apparatus, - (d) adding the test fault resistance (Rf) between at least one of the active conductors of the ungrounded power supply system and ground via the testing apparatus, - (e) measuring a second insulation resistance (Riso2) of the ungrounded power supply system via the insulation monitoring device, said second insulation resistance (Riso2) resulting from the first insulation resistance (Riso1) and the test fault resistance (Rf) added parallel thereto, - (f) transmitting the second insulation fault resistance (Riso2) to the testing apparatus and saving the second insulation fault resistance in the testing apparatus, - (g) calculating a comparative insulation fault value (Rref), via the testing apparatus, from the first insulation resistance value (Riso1) and the test fault resistance value (Rf) added parallel thereto, - (h) comparing the second insulation resistance (Riso2) measured by the insulation monitoring device to the comparative insulation resistance value (Rref) calculated by the testing apparatus in the testing apparatus and generating a result log, - (i) activating an alarm message via the testing apparatus, should the comparison result in the second insulation resistance (Riso2) measured by the insulation monitoring device deviating from the calculated comparative insulation resistance value (Rref), said deviation indicating a faulty function of the insulation monitoring device, - (j) removing the test fault resistance (Rf) via the testing apparatus. - (a) measuring a first insulation resistance (Riso1) of the ungrounded power supply system via the insulation monitoring device, - (b) transmitting the first insulation resistance value (Riso1) to a testing apparatus and saving the first insulation resistance value (Riso1) in the testing apparatus, - (c) calculating an uncritical test fault resistance value (Rf) via the testing apparatus, - (d) adding the test fault resistance (Rf) between at least one of the active conductors of the ungrounded power supply system and ground via the testing apparatus, - (e) measuring a second insulation resistance (Riso2) of the ungrounded power supply system via the insulation monitoring device, said second insulation resistance (Riso2) resulting from the first insulation resistance (Riso1) and the test fault resistance (Rf) added parallel thereto, - (f) transmitting the second insulation fault resistance (Riso2) to the testing apparatus and saving the second insulation fault resistance in the testing apparatus, - (g) calculating a comparative insulation fault value (Rref), via the testing apparatus, from the first insulation resistance value (Riso1) and the test fault resistance value (Rf) added parallel thereto, - (h) comparing the second insulation resistance (Riso2) measured by the insulation monitoring device to the comparative insulation resistance value (Rref) calculated by the testing apparatus in the testing apparatus and generating a result log, - (i) activating an alarm message via the testing apparatus, should the comparison result in the second insulation resistance (Riso2) measured by the insulation monitoring device deviating from the calculated comparative insulation resistance value (Rref), said deviation indicating a faulty function of the insulation monitoring device, - (j) removing the test fault resistance (Rf) via the testing apparatus.