When determining a fault current portion IF in a differential current idiff(t) measured by an inverter, an AC voltage uAC(t) applied to an AC output of the inverter is measured and a periodic reference function y(t) of alternating sign is generated as a function of the measured AC voltage uAC(t) in order to determine an AC fault current portion IFAC in the differential current idiff(t). The differential current idiff(t) is multiplied by the periodic reference function y(t), and the product of the differential current idiff(t) and the reference function (y(t)) is averaged over an integral number of periods of the reference function y(t). The reference function y(t), at least for one operating state of the inverter, is generated with a predefined phase offset with respect to the measured AC voltage uAC(t) and/or with a frequency which is an integer multiple of the frequency of the measured AC voltage uAC(t).