The dielectric,usually the insulator between the plates of a
capacitor, can be overstressed by the application of too high
voltages applied to the capacitor plates. The dielectric breaks
down and a current flows between the plates until,either they are
discharged, or an equilibrium is reached,below the working voltage
of the capacitor. If the dielectric is damaged in this process he
capacitor must be replaced. Some dielectric material self heal and
can recover from an over voltage.