The invention relates to a method and to devices for extended insulation-fault search in an IT power supply system using a multifunctional test current, wherein, selectively and depending on the application, the test current functions as a voltage compensation current so as to compensate a voltage increase in an active conductor of the IT power supply system, as a tripping current so as to trip a residual current protection device arranged in a subsystem of the IT power supply system and/or as a leakage-capacitance compensation current so as to compensate a capacitive leakage current. The test current can fulfil more than one of the cited functions simultaneously.