Methods and devices are provided for testing the function of a standard insulation monitoring device, installed in an ungrounded power supply system, during operation. The basic concept of the invention rests upon adding a testing apparatus to the ungrounded power supply system, which is being monitored using an insulation monitoring device according to regulations, between the active conductors of an ungrounded power supply system and ground, said testing apparatus systematically changing an insulation resistance of the ungrounded power supply system and observing the reaction of the insulation monitoring device in a fully automated manner in perpetually repetitive testing cycles. The power supply system is monitored perpetually during its operation. In the method, the current operating state (insulation level, load state) of the power supply system is detected via network parameters, such as insulation resistance and system leakage capacity, and consulted for assessing the functionality of the insulation monitoring device.