Abstract: The invention relates to a method and an apparatus for measuring the impedance of an energy supply system at a rated frequency by impressing a testing current (3) into the system and measuring (4) the resulting changes in a system current and a system voltage. According to the invention, the system current and the system voltage are measured during a first preselected time interval (T1) without impressing of the testing current and are measured during a second preselected time interval (T2) with the impressing of the testing current. The testing current is composed of at least one periodic signal with at least one testing frequency (f1, f2) that deviates from the rated frequency. Based on the measured system currents and system voltages, a Fourier analysis is used to determine a resulting impedance for the testing frequency (f1, f2). The impedance of the energy supply system at the rated frequency is derived from the impedance obtained for the testing frequency (f1, f2) (FIG. 1).