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How Lightning Causes an Electrical Fault

Printable Version

You're driving along at night during a thunderstorm when a lightning bolt suddenly slices through the sky in front of you. Immediately afterwards, you see a wierd blue glow coming from the ground that lasts for a couple of seconds. You then spend the rest of the night trying to figure out the source of the mysterious blue light.

Most likely, what you saw was a common event that occurs when lightning strikes an energized power line- the flashover or electrical arc. Storm chasers and weather enthusiasts often call the bright blue arcs power flashes. Power flashes are the result of a short circuit (called a fault) caused by the accidential contact of two energized power lines or an energized power line and a grounded object. Many events can cause such short circuits, such as fallen trees, high winds, or an unfortunate squirrel.

Lightning causes faults in electrical power circuits, but in an unusual way:

As stated in the last paragraph, a short is caused by two live power lines (of differing phases) becoming connected in some way OR one live line becoming connected to ground. This connection can be made by the two lines touching each other, or if something conductive bridges the gap such as a squirrel or a metal bar. A lightning channel is no more than conductive air (ionized air). When lightning strikes a live power line and then jumps to ground (such as a metal transmission tower), the section of lightning channel between the power line and ground essentially connects the two, creating a path for power line current to flow to ground (a short circuit). This current continues to flow even after the lightning flash is over. This arc (or fault) will continue until the circuit breaker protecting the line opens. A fault caused by lightning strike is often called a flashover.

Most power lines have grounded shield wires running directly above the live wires to intercept a lightning strike and prevent flashovers.

The image below shows how a flashover occurs when lightning strikes a live high-voltage transmission line. In most cases, lightning will hit the protective shield wire instead of the live wires, but it is not uncommon for lightning to completely miss the shield wire and strike the live wire (as in the image below).

Flashovers can occur even if lightning hits the shield wire: If the shield wire and towers are not sufficiently grounded, the huge voltage surge will cause the lightning to 'overflow' and jump from the grounded tower to the live wires, initiating a flashover. In many instances flashovers will occur on more than one of the live wires (phases).

• The PTI Lightning Photo Gallery has spectacular close-up pictures of lightning striking power lines and causing flashovers. The photos are captured using special lightning-triggered cameras mounted on telephone poles and transmission towers.
• The PTI web site also contains more in-depth information about lightning and electrical power distribution/transmission systems.

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