This work presents a set of rules to determine the voltage sag source location in electric power systems. The rule set is extracted using subgroup discovery (SD). The SD objective is to discover characteristics of subgroups with respect to a specific property of interest. Our interest is to obtain the origin of sag events, upstream or downstream from the measurement point. Voltage sag features registered in electric substations are used as input data to SD algorithm. The SD algorithm used is CN2-SD to learn descriptive rules. Results show the rules extracted can be easily interpreted by a domain expert, allowing the formulation of heuristic classification rules with high accuracy.