This has to do with a stochastic or random discharge between two voltage-bearing electrodes which bridge over only a partial distance of the clearance between the electrodes. They appear first at the contact surface or also sometimes physically displaced within an insulation configuration. If this occurs in a solid insulation material, then it is referred to as an internal partial discharge (PD), the causes of which are to be found either in defective manufacturing technology or in the use of unsuitable materials. Numbered among the latter for actual insulation materials are hollow spaces, voids and non-homogeneities which cannot be ruled out to 100%.