Flashovers are a type of fault, or short circuit, caused by a lightning strike to a live electrical system. Faults on power lines involve tremendous levels of current and cause bright arcing, showers of sparks, and loud bangs and buzzing sounds. Arcs from a large fault can illuminate the entire sky nearby, and are often visible for miles (see video clip below).
Video clip of lightning-triggered flashover
Windows Media, 157KB
Lightning-triggered flashovers aren't the only causes of faults. Such a short circuit will occur anytime power lines are allowed to touch each other or a well-grounded object. Faults occur when high winds blow wires into one another, trees fall across lines, snow and ice snap wires in two, or an unfortunate squirrel or bird bridges the gap across two live connections. They are often visible in video footage of tornadoes, as the high winds in the vortex cause energized power lines to touch each other and/or the ground. Storm chasers and weather enthusiasts often use the term 'power flash' to refer to the intense arcs from these faults.
Since power line faults are extremely bright, they are often mistaken for lightning as they light up the sky with their characteristic bluish-green glow. In fact, many reports of 'ball lightning' have been found to be nothing more than a lightning-caused flashover.
Flashovers have been around as long as man-made electricity. They can occur anywhere along power lines- at transformers, poles, towers, and substations. Arcing from flashovers can cause extensive damage to electrical equipment, even more so than the lightning than caused them. As a result, power companies have performed years of research to help prevent the effects of this phenomenon.
Grounded shield wires are often strung along the tops of high-tension towers with the intention of intercepting a lightning strike and protecting the live conductor wires. However, a lightning strike to a shield wire may still cause flashovers across insulators. Insufficient grounding of the towers may allow voltage from a strike to 'overflow' from the towers to the live wires via a spark - initiating a flashover as described above.