Puncture arc of composite insulators

 The electrical breakdown of a composite insulator due to excessive voltage can occur in one of two ways:

A puncture arc is a breakdown and conduction of the material of the insulator, causing an electric arc through the interior of the insulator. The heat resulting from the arc usually damages the insulator irreparably. Puncture voltage is the voltage across the insulator (when installed in its normal manner) that causes a puncture arc.
A flashover arc is a breakdown and conduction of the air around or along the surface of the insulator, causing an arc along the outside of the insulator. They are usually designed to withstand this without damage. Flashover voltage is the voltage that causes a flash-over arc.
Most high voltage insulators are designed with a lower flashover voltage than puncture voltage, so they flash over before they puncture, to avoid damage.
Dirt, pollution, salt, and particularly water on the surface of a high voltage composite insulator can create a conductive path across it, causing leakage currents and flashovers. The flashover voltage can be reduced by more than 50% when the composite insulator is wet. High voltage composite insulators for outdoor use are shaped to maximise the length of the leakage path along the surface of composite insulators from one end to the other, called the creepage length, to minimise these leakage currents.
puncture arc of composite insulator
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