A method for detecting a fault in an exciter circuit, suitable for use in a brushless generator, the exciter circuit including an exciter armature winding, having a plurality of armatures (106-1, 106-2, 106-3), arranged to generate a multiphase AC signal, each armature (106-1, 106-2, 106-3) of the plurality of armatures (106-1, 106-2, 106-3) providing a respective signal component of the multiphase AC signal, and a rectifier circuit for receiving the signal components from the respective armatures (106-1, 106-2, 106-3), the rectifier circuit comprising a plurality of diodes arranged to rectify the signal component to provide a rectified output signal, wherein the method includes the steps of: acquiring, for each of at least two signal components, a respective characteristic value for a first characteristic of the signal components wherein each characteristic value is indicative of the current flowing in the respective armature (106-1, 106-2, 106-3) or of the voltage across the respective armature (106-1, 106-2, 106-3) or of the voltage drop across a resistor connected in series with the respective armature (106-1, 106-2, 106-3); characterised in that the method further includes the steps of: acquiring, for each of at least two signal components, a respective further characteristic value for a second characteristic of the respective signal component wherein each further characteristic value is indicative of the current flowing in the respective armature (106-1, 106-2, 106-3) or of the voltage across the respective armature (106-1, 106-2, 106-3) or of the voltage drop across a resistor connected in series with the respective armature (106-1, 106-2, 106-3); calculating the magnitude of the positive sequence third harmonic component for each acquired characteristic value; calculating the magnitude of the positive sequence third harmonic component for each acquired further characteristic value; on the basis of the calculated magnitudes, calculating the positive sequence third harmonic admittance and/or impedance of the rectifier circuit; and determining whether a fault has occurred in the exciter circuit on the basis of the positive sequence third harmonic admittance and/or impedance of the rectifier circuit. acquiring, for each of at least two signal components, a respective characteristic value for a first characteristic of the signal components wherein each characteristic value is indicative of the current flowing in the respective armature (106-1, 106-2, 106-3) or of the voltage across the respective armature (106-1, 106-2, 106-3) or of the voltage drop across a resistor connected in series with the respective armature (106-1, 106-2, 106-3); characterised in that the method further includes the steps of: acquiring, for each of at least two signal components, a respective further characteristic value for a second characteristic of the respective signal component wherein each further characteristic value is indicative of the current flowing in the respective armature (106-1, 106-2, 106-3) or of the voltage across the respective armature (106-1, 106-2, 106-3) or of the voltage drop across a resistor connected in series with the respective armature (106-1, 106-2, 106-3); calculating the magnitude of the positive sequence third harmonic component for each acquired characteristic value; calculating the magnitude of the positive sequence third harmonic component for each acquired further characteristic value; on the basis of the calculated magnitudes, calculating the positive sequence third harmonic admittance and/or impedance of the rectifier circuit; and determining whether a fault has occurred in the exciter circuit on the basis of the positive sequence third harmonic admittance and/or impedance of the rectifier circuit. acquiring, for each of at least two signal components, a respective further characteristic value for a second characteristic of the respective signal component wherein each further characteristic value is indicative of the current flowing in the respective armature (106-1, 106-2, 106-3) or of the voltage across the respective armature (106-1, 106-2, 106-3) or of the voltage drop across a resistor connected in series with the respective armature (106-1, 106-2, 106-3); calculating the magnitude of the positive sequence third harmonic component for each acquired characteristic value; calculating the magnitude of the positive sequence third harmonic component for each acquired further characteristic value; on the basis of the calculated magnitudes, calculating the positive sequence third harmonic admittance and/or impedance of the rectifier circuit; and determining whether a fault has occurred in the exciter circuit on the basis of the positive sequence third harmonic admittance and/or impedance of the rectifier circuit.