A method is disclosed for determining a phase failure in an electrical device for driving or monitoring a three-phase AC motor, wherein the electrical device comprises a first, a second and a third line. In order to detect a phase failure in a line in the three-phase electrical device, in particular in a soft starter, the phase failure in the third line is determined by analysing the first and second lines, wherein the analysis includes: determining a value which characterizes a phase shift between a current in the first and second lines, and outputting a signal if the value which has been determined is in a range of values which characterizes a phase failure in the third line.