Sensitivity analysis of a power system against overvoltage was performed. The analysis took into account the origin of the overvoltage, the grounding resistance magnitude and overvoltage injection point. The IEEE 13 bus test feeder system was implemented in EMTP-ATP and lightning and switching overvoltages were applied. Three cases were evaluated: the first one, applying lightning overvoltage and varying the grounding resistance magnitude; the second, applying lightning overvoltage in different points of the system with a fixed grounding resistance magnitude; and finally, applying switching overvoltage and varying the grounding resistance magnitude.