An insulator is so designed that it should fail only by:

The design of an insulator primarily aims to prevent failures that could compromise the reliability and efficiency of a power distribution system. Among the types of failures listed, insulators are specifically designed to prevent a puncture, because a puncture failure implies that the insulation material itself has been breached, which can lead to catastrophic failure modes where the insulator does not just lose its insulating properties but also risks damage to the structure it supports.

A flash-over, in contrast, involves an electrical discharge around the surface of the insulator, but the insulator itself does not necessarily get damaged. This is generally less severe compared to a puncture, as it does not directly harm the integrity of the insulator.