AC vs. DC over-potential testing of AC equipment

The existing equipment and testing standards for electrical power switchgear clearly define the requirement of ac over-voltage withstand testing. However, some of the field testing standards have deemed dc over-voltage as sufficient to test the dielectric strength of the insulation system and do not require an ac over-potential test. The question is whether or not dc is sufficient to test electrical distribution equipment, such as switchgear, switchboards and panel in which mounting insulators and air may be the only insulation. This paper will document a series of comparison tests in which both dc and ac voltages were used to test the dielectric strength of an air gap between conductors. It will show that a very small air gap can be significant impedance to both ac and dc. Performing both tests provides the best assurance that the insulation system will be adequate.