This paper proposes a short-circuit diagnosis tool for electrical low-voltage (LV) distribution networks. The tool relies on analyzing Park's vector representation of the LV phase voltage in the sag produced by a short circuit. Park's vector representation is considered to be an identification mark for the type of short circuit that the installation is exposed to and the phases that it affects. In addition, it shows whether the fault is located in the medium voltage or LV network. The characteristics of the vector representation corresponding to the different types of short circuits are analyzed in this paper. Finally, the relationship between Park's vector representation and the location of the defect is also studied. The results can be used as part of an expert system for the automatic classification of disturbances.