The Dielectric Voltage Withstand Test Executive Summary

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  • 1. execuTive summary ON: The Dielectric Voltage Withstand TestThe dielectric voltage withstand test is an integral part of the setting is indicative of dielectric breakdown. While this is true in manyproduct safety evaluation of electrical and electronic devices cases, a current above the trip-out threshold may actually reflect theand provides manufacturers with important information natural dielectric current of the tested device. This uncertainty mayregarding the quality and appropriateness of the chosen lead to test results deemed unacceptable, when the results actuallyinsulation system. However, while the test is widely used, the mask a favorable outcome.real objective of the test is often misunderstood, which maylead to incomplete testing or misleading test results. In such cases, further testing or evaluation should be conducted, including DC dielectric testing, use of dielectric testers with arc detection, specialized diagnostic testers and reapplication of theThe dielectric voltage withstand test involves placing an extra-high dielectric withstand test voltage to locate the site of the breakdown.voltage across the insulation barrier of the device for one minute.If the insulation holds the voltage, the device is deemed to have The objective of dielectric testing can be met with DC as well as ACpassed the test. However, if the applied voltage leads to the sudden voltages, and many product safety standards include DC voltagesbreakdown of the insulation material and allows current to flow, the in their dielectric test methods. However, once DC testing has beeninsulation is determined to be insufficient since it might pose a shock completed, the inherent capacitance of the device under test hashazard to users. been fully charged and poses a potential shock hazard. Therefore, certain products may need to be discharged via a short with aThe dielectric breakdown of an insulating material represents a sudden bleeder resistor.change in the resistance of the insulation due to an applied voltage.The applied charge accelerates the movement of electrons through The dielectric voltage withstand test can provide important insightthe material. If the electric field remains present, it can accelerate into the capabilities and limits of a device’s insulation system. Whenthese free electrons to such high energies that they, in turn, free other the tripping device is properly set to allow the expected current toelectrons in an avalanche-like process, dramatically increasing the flow through an acceptable dielectric current path, the dielectric testconductivity of the material. This transformation is called dielectric equipment will give trustworthy results. However, in cases where thebreakdown, and the electric field necessary to start the breakdown is expected current is not known or the trip-out threshold is selectedcalled the dielectric strength, or breakdown strength. generically, the trip-out may produce a false failure result. An indication of trip-out should always be followed by an investigationThe objective of the dielectric voltage withstand test is to establish into the root cause of the trip-out.the minimum level of electrical insulation necessary to preventhuman contact with a potentially harmful voltage and resulting To download a copy of the technical white paper or learn morecurrent. The test may also reveal faults in mechanically damaged about related UL standards, testing and certification services, visitinsulation or the presence of a foreign material, which may bridge www.uluniversity.com/thoughtleadership.the insulation. However, the test’s use of a trip current setting todetermine dielectric breakdown assumes that current above the trip 01 For more information e:: uLuniversity@us.ul.com / T:: 888.503.5536 Copyright © 2010 Underwriters Laboratories Inc. All rights reserved. No part of this document may be copied or distributed without the prior written consent of Underwriters Laboratories Inc.12/10 BDI 100807.7