An insulators breakdown voltage is the minimal reverse voltage that makes the diode conduct appreciably in reverse. Breakdown voltage is an attribute of an insulator that delineates the maximum voltage difference that can be applied over the material before the insulator conductors.
In solid insulating materials, this routinely creates a weakened path the material by creating permanent molecular or physical changes by the sudden current. The breakdown voltage of a substance is not definite. There is a statistical probability that the material will fail.
