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In resonant grounded systems, when single phase-to-earth fault occurring, transient components of zero-sequence current in fault lines are composed of that in healthy lines. So there is a great difference between health lines and fault lines of transient information. This paper gives a new diagnostic method based on cluster. A cluster is an assemblage constituted by a set of objects with the character of similarity each other. But the objects in different clusters usually are not similar. In the process of cluster, the more similar the variables are, the more precedent they merged. Not more similar, merged later. The degree of similarity of different variables merged last is the lowest. When single phase-to-earth fault occurs, the transient information of zero-sequence current is seen as variables of cluster. A detecting criterion is built up by calculating and analyzing the statistics of cluster. The process of cluster can be shown though scatter plot and binary tree diagram. Simulation results show that the proposed method is convenient, symbolic and reliable.