A circuit arrangement (20) having an active measuring voltage (UG) for determining an insulation resistance (RF) or a complex-valued insulation impedance (ZF) of an ungrounded power supply system (12) against ground potential (PE), the circuit arrangement (20) having a measuring path (24) which runs between an active conductor (L1, L2) of the power supply system (12) and the ground potential (PE) and includes a measuring-voltage generator (VG) for generating the measuring voltage (UG), a measuring resistance (RM) for capturing a measured voltage (UM) and a coupling resistance (RA), the circuit arrangement (20) having a signal evaluation circuit (26) which includes a signal input for evaluating the measured voltage (UM) and a ground connection (GND). The ground connection (GND) is connected to a ground potential (PE).