Abstract: A process and an apparatus for the determination of the total leakage impedance and the maximum conductor contact current of an ungrounded AC power supply system resulting from the total leakage impedance. An AC measurement current having a frequency that differs slightly from power frequency by a ratio other than a whole number is fed into the AC power supply system, which measurement current flows through the system leakage impedances to produce between supply system and ground, an interference pattern having the AC measurement voltage superimposed on the system voltage component at an interference pattern frequency (difference or beat frequency) of the pattern envelopes which is small with respect to the system frequency. By extracting and processing of the components of the interference pattern, it is possible to determine the AC measurement voltage component contained in the interference pattern.