Abstract. In this paper, events registered in power distribution systems are analyzed to recognize sequences of events associated to faults occurred in the network. The events considered in this study are basically voltage sags generated by homopolar faults and registered by power quality monitors installed in the secondary of transformers in distribution substations. The events registered in a measuring point have associated the time of occurrence, and the list of increasingtime ordered events corresponds to a sequence. The aim of this work is to discover the collection of events associated with failures in the network that can be viewed as sequences of events related with the actuation of the protection system. Two algorithms are proposed to recognize these sequences. The methodology is tested with data gathered in different substations which have been manual grouped by the utility 1 .