A method for measuring and processing data pertaining to an activity of partial electrical discharges taking place in a three-phase electric machine (1) powered with square wave voltage, said data comprising a plurality of discharge signals and a plurality of sets of three values (eu, ev, ew) of power supply voltage of the machine measured in concurrence with a corresponding discharge signal, comprises the following steps: —transforming each set (eu, ev, ew) of three voltage values measured into a corresponding pair (ed, eq) of voltage values; —deriving a phase angle value from each of said pairs (ed, eq) of voltage values; —attributing to each discharge signal the corresponding phase value, to assess the measured discharge signals as a function of corresponding phase values. Prior art techniques for processing data pertaining to activity of partial electrical discharges taking place in three phase electric machines powered with square wave voltage provide for assessing the measured discharged signals as a time sequence of events, with no possibility to place them effectively in relation to the power supply voltages, this entailing that any subsequent statistical processing of the measured data will have little significance.