A method and an electric circuit arrangement for determining a branch insulation resistance and a branch leakage capacitance of a line branch to be monitored in a branched, ungrounded power supply system having active conductors and a measuring voltage fed centrally by a controllable measuring voltage source and a residual current caused by the measuring voltage being registered using a current sensor in the line branch to be monitored. The centrally supplied measuring voltage is formed over a generator period having a characteristic signal form defined via the frequency composition. By this (frequency) modulation of the measuring voltage, information is transmitted from the central feed location of the measuring voltage to the current sensor disposed in the line branch to be monitored. Based on this information, the current sensor can establish with which resistance value the coupling circuit feeds the measuring voltage in the corresponding generator period, without another communication channel being required.