A method for determining a branch insulation resistance (Rz) and a branch leakage capacitance (Cz) of a line branch to be monitored in a branched, ungrounded power supply system having active conductors (L1, L2), the method comprising the following steps: generating a measuring voltage (Um) by means of a controllable measuring voltage source; feeding the measuring voltage (Um) in each active conductor (L1, L2) at a central location of the power supply system by means of a coupling circuit; registering, by means of a current sensor, a residual current (Iz) caused by the measuring voltage (Um) in the line branch to be monitored, characterized in that the measuring voltage (Um) is formed alternatingly via a generator period from a first frequency composition and a second frequency composition, respectively, the first frequency composition is fed via a first coupling resistance by means of a coupling circuit switchable using a switching unit and the second frequency composition is fed via a second coupling resistance; the value of the first coupling resistance and the value of the second coupling resistance and the spectral composition of the first frequency composition and the spectral composition of the second frequency composition are stored in a data storage of the current sensor; the current sensor executes a frequency analysis of the residual current (Iz) in a computing unit over two subsequent generator periods in order to identify which frequency composition (F1, F2) is fed presently; the coupling resistance (Ra1, Ra2) presently switched is determined from the assignment of the identified frequency composition (F1, F2) to the corresponding coupling resistance (Ra1, Ra2) in the computing unit; the branch insulation resistance (Rz) and the branch leakage capacitance (Cz) are computed in the computing unit from the measured residual current (Iz), the measuring voltage (Um), the frequencies (ω1, ω2) contained in the frequency composition (F1, F2), and the coupling resistance (Ra1, Ra2). generating a measuring voltage (Um) by means of a controllable measuring voltage source; feeding the measuring voltage (Um) in each active conductor (L1, L2) at a central location of the power supply system by means of a coupling circuit; registering, by means of a current sensor, a residual current (Iz) caused by the measuring voltage (Um) in the line branch to be monitored, characterized in that the measuring voltage (Um) is formed alternatingly via a generator period from a first frequency composition and a second frequency composition, respectively, the first frequency composition is fed via a first coupling resistance by means of a coupling circuit switchable using a switching unit and the second frequency composition is fed via a second coupling resistance; the value of the first coupling resistance and the value of the second coupling resistance and the spectral composition of the first frequency composition and the spectral composition of the second frequency composition are stored in a data storage of the current sensor; the current sensor executes a frequency analysis of the residual current (Iz) in a computing unit over two subsequent generator periods in order to identify which frequency composition (F1, F2) is fed presently; the coupling resistance (Ra1, Ra2) presently switched is determined from the assignment of the identified frequency composition (F1, F2) to the corresponding coupling resistance (Ra1, Ra2) in the computing unit; the branch insulation resistance (Rz) and the branch leakage capacitance (Cz) are computed in the computing unit from the measured residual current (Iz), the measuring voltage (Um), the frequencies (ω1, ω2) contained in the frequency composition (F1, F2), and the coupling resistance (Ra1, Ra2).