Patent ID: 8854846

Claim:
A method of determining a fault current portion I F in a differential current i diff (t) of an inverter, comprising: measuring the differential current i diff (t) at the inverter; measuring an AC voltage u AC (t) present at an AC current output of the inverter: generating a periodic reference function y(t) of alternating sign as a function of the measured AC voltage u AC (t), wherein the reference function y(t), at least for one operating state of the inverter, has at least a predefined phase offset with respect to the measured AC voltage u AC (t) or has a frequency which is an integral multiple of the frequency of the measured AC voltage u AC (t); multiplying the differential current i diff (t) by the reference function y(t) to form a product; and averaging the product of the differential current i diff (t) and the reference function y(t) over an integral number of periods (T) of the reference function y(t) to determine an AC fault current portion I F AC in the differential current i diff (t).