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The Itabera substation is part of the Furnas 765 kV AC transmission system, an important portion of the Brazilian national interconnected grid. On February 2007, a heavy rain, with a high number of lightning strokes, was observed in the Itabera region. This thunderstorm brought about several short-circuits and, consequently, the tripping of transmission lines and one of the 765 kV substation busbar. This paper presents part of the investigation performed for the analysis of the disturbance involving this 765 kV AC transmission system, typically related with fault initiation and fault clearing overvoltages. Detailed transient simulations were performed with the alternative transient program (ATP) in order to reproduce the multiple fault events. The ZnO arresters performance was investigated and it is discussed in the paper. Simulation results, alike to the field measurements, have shown that short-circuits occurred for relatively low values of overvoltages. A possible relation between heavy rain and the reduction of insulator 60 Hz flashover strength is also discussed, based on historical precipitation rate data in the Itabera region and on literature.