Dielectric or electrical insulating material is a substance which possesses two electrical properties – Ability to persist in electrostatic fields for a long time (or low Conduction) and ability to be polarised under the action of an external electric field. Aging of Electrical Insulation can be caused by higher conduction and/or higher activities of Polarisation. Both phenomena are caused by the motion in space of the electrically charged particles of a dielectric, so there is a certain affinity between them. But it is the difference and the ability to distinguish between these two phenomena which allow Trouble in electrical insulation to be solved correctly, thus the life of equipment can be prolonged.

PDC Analysis is the latest on-site off-line technique which can easily identify the two phenomena in non-destructive way. Its ability to measure current in the range of picoamperes (pA) has made the dielectric test with low voltage become possible. The electrical insulation of an extra-high voltage transformer can now be tested using the voltage as low as 100V and the high voltage cable can be tested only with 30 V

The principle of test arrangement for PDC measuring technique is shown in the figure below.