Lightning strokes represent a considerable cause of short interruptions in overhead line transmission line networks. The over voltages caused by lightning cannot be avoided but their influence can be limited by appropriate overvoltage protection. Lightning is one of the main causes of problems on transmission lines; this is usually happening when a stroke hits: the phase conductor, the ground wire or directly the structure. If the first case occurs, the line may have a shielding failure. Meanwhile in the second or third cases, there may be a back flashover, which generally depends on a high...