The invention relates to a method and a device for determining a maximum leakage current in unearthed power supply networks with a plurality of conductors. The method comprises the steps of: feeding of a measurement signal UP with a measurement frequency fPuls into a conductor arrangement, setting the measurement frequency fPuls, measuring a line-to-line voltage UL-E for each conductor, determining a maximum occurring line-to-line voltage UL-Emax from the measured line-to-line voltages UL-E, determining a voltage UZe across a measurement resistance RM for calculating a leakage impedance ZE, calculating the leakage impedance ZE, and calculating a maximum leakage current THC. The device comprises a signal generator for feeding in the UP with the fPuls, a network coupling circuit between the signal generator and the conductor arrangement, a voltage measuring circuit for measuring the UL-E, and an impedance measuring circuit for determining the Ze by means of the determining of the UZe.