Which of the following are known causes of flashovers on insulators?

• A. Lightning
• B. Moisture
• C. Dirt
• D. All of the above

Answer: (D)

Flashovers happen when the path along or across an insulator becomes conductive enough to carry current in an arc, instead of staying blocked by the insulator’s insulating strength. A lightning surge brings a very high transient overvoltage that can overwhelm the insulator and start an arc across its surface or to ground. Moisture on the surface, from rain, dew, or fog, lowers the surface resistance and creates a leakage path, making arcing much more likely. Dirt and pollution on the surface form conductive films and can hold moisture, further reducing surface resistance and promoting leakage currents that lead to a flashover. Since lightning, moisture, and dirt each can independently cause a flashover, the option that includes all these causes is the best choice.