KIKUSUI TOS9301PD AC/DC high voltage tester, test insulation resistance and partial discharge.

Basic principles and difficulties in withstand voltage, partial discharge tests

Partial discharge is a partial breakdown of the dielectric at a small part of the insulation.
It is caused by gaps, cracks or impurities inside the dielectric material.
These gaps have very small capacitance, when a high voltage is applied between the electrodes of this gap and exceeds the breakdown voltage of the air it will discharge. This phenomenon is called partial discharge

During the withstand voltage test, a high voltage (usually AC) is applied to the equipment under test (DUT) and the leakage current through it is measured and evaluated. According to this principle, dielectric breakdown is determined by an increase in current. Therefore, if there is no current surge, the product is said to be in specification.

However, there are cases when partial discharge occurs and the DUT still passes the withstand voltage test. The KIKUSUI TOS9301PD allows these partial discharges to be detected and defective products ruled out (though the product still passes the high voltage test).