A method for identifying the existence of a failure of a relay device in an electrical power supply system of a device to be supplied with power, the power supply system comprising at least one power generator configured to generate a power supply signal and a load comprising at least one relay device; the power generator comprising a frequency variator; each relay device comprising a quadripole and at least one antenna linked to the quadripole. The identification method comprises the following steps: a) selecting a characteristic frequency, b) determining the impedance of the load, then when the supply signal has the selected characteristic frequency, c) repeating steps a) and b) for several distinct characteristic frequencies, d) identifying the existence of a failure from several impedances determined for different characteristic frequencies.