An Introduction to Dielectric Withstand Testing with the HT-PR

The continuity test/dielectric withstand test is a production line test which is recognized by safety agencies worldwide as a valid criterion of safe assembly of end-use equipment. The test ensures that the primary circuit power and ground conductors were properly wired and connected for safe operation. It also applies a high-voltage potential between power and ground conductors to make sure that no unintentional leakage or arcing paths exist between power and ground. The test consists of a ground continuity check, a leakage current check and a high voltage check. It is non-destructive to the equipment under test, and can be accomplished in a short time.

The purpose of dielectric testing: Dielectric testing is a simple, non-destructive method of verifying the adequacy of electrical insulation to withstand the sort of transients that can occur during transient (surge) events. In addition, the dielectric test can verify that the insulation in question has an adequate amount of performance "headroom". This is necessary to ensure that the insulation does not fail because of degradation of the insulation due to aging, moisture, wear due to vibration, etc.

The method of dielectric testing: A high voltage (typically 1000 Volts or higher) is applied between two conductors that are "supposed" to be electrically insulated from each other. If the two conductors (an insulated "live" wire, and a metal enclosure, for example) are completely isolated from each other, then the application of a large voltage difference between the two conductors will not allow current to flow between the conductors. The insulation will "withstand" the application of a large voltage potential between the two conductors - hence the term "dielectric withstand test". In general, there are two results of the test that are considered a failure of the insulation: (1) excessive current flow during the test due to low insulation resistance of the insulating material which separates the two conductors, and (2) an abrupt dielectric breakdown due to electrical arcing or discharge, either through the insulation material, over the surface of the insulation material, or a discharge through air.

The determination of a suitable test voltage: If the test voltage is too low, the insulation material in question will not be adequately stressed during the test. This could cause inadequate insulation to pass the test, and be considered acceptable. On the other hand, if the test voltage is too high, then the test could cause permanent damage to an insulation material that is otherwise adequate for the application. A general "rule of thumb" that is used for the testing of mains wiring which operates at voltages of 120-240Vac is 1000V plus two times the operating voltage. Using this rule, 120V wiring would be tested using a voltage of 1240Vac.

Duration of the test: Generally, the test voltage is applied for one minute, in order to adequately stress the insulation. Many standards allow the test duration to be reduced to 1 second for production-line testing in order to accommodate large-volume production testing. In this case, standards quite often require that the test voltage be increased by 20% in order to ensure that the shorter test duration of one second will adequately test the insulation in question.