Transformer faults have now become a major cause of power system failures, with transformers being more susceptible to malfunctions in adverse weather conditions. This article takes the example of a fault in the on-load tap changer switch of a transformer caused by a localized overvoltage short circuit during severe thunderstorm weather. The author elaborates on three types of lightning-induced overvoltage, combining fault analysis with extensive on-site experimental data and comparisons with standard values to convincingly identify the fault causes. The article briefly discusses five types of overvoltage in neutral point ungrounded systems and, in conjunction with one of the transformer’s crucial components, the on-load tap changer switch, provides a concise analysis of four main fault types associated with OLTC. Finally, the author traces the root cause of the fault, summarizes the findings, and presents effective recommendations to prevent similar incidents in the future.