A method and device for determining the phases in a multi-phase electrical system includes detecting a first waveform on a phase of the multi-phase system In a first position of the multi-phase electrical system timed data is stored, synchronized with the first waveform. A second waveform is detected in a second position of the multi-phase electrical system, on an indeterminate phase of the system. Data is obtained relative to the phase on which the second waveform was read on the basis of the phase shift between the timed data and the second waveform.