The present invention relates to a device (1) for detecting a failed rectifier in three-phase AC/DC converter having its AC side connected to a three-phase AC source (90) and its DC side connected to a DC load (50), the AC/DC converter including a rectifier circuit (30) comprising at least three legs (31, 31', 31"), each of the legs (31, 31', 31") connected to a corresponding AC phase (40) and including at least one rectifier (32, 34, 32', 34', 32", 34"), the device (1) including a measuring unit (20) for measuring and sampling AC current for each of the phases and a calculating unit (10) configured to estimate an instantaneous DC current based on the measured instantaneous AC currents during one period of the power supply frequency. The calculating unit (10) is further configured to calculate a ripple value of the estimated instantaneous DC current, determine if the calculated ripple value exceeds a pre-defined threshold value, initiate an alarm accordingly, calculate a rectifier status value based on a maximum instantaneous AC phase current and a minimum instantaneous AC phase current during the same one period, and determine, for each of the legs, if the rectifier in the leg has a non-conducting fault based on the calculated rectifier status value.