Abstract: The invention relates to a method for forming output signals of an interference current monitor or of a state monitor of an electrical rail vehicle with current being drawn from a power grid, and optionally a current also being delivered back into said power grid, via a current collector. In the case of a state monitor, not only interference currents but also further fault states are identified and signaled. The method steps include detection by measurement and digitization of the vehicle current flowing via the current collector or of other measurement quantities which make it possible to infer vehicle states which leads to interference currents, and digital processing of these measured values with determination of the spectral density of interference currents or of interference values of the other measurement quantities, a multistage system of response thresholds being used in order to evaluate the spectral density.