Components like capacitors have two plates in parallel to them separated by Dielectric material. This dielectric acts as an insulator. Similarly, the PCB has dielectric material which acts as separator between two copper layers. Whatever may be the use of dielectric material it shall have specific strength called the dielectric strength. Dielectric Strength is measured in kV/cm. Dielectric Strength is a measure of maximum voltage that shall be applied before a breakdown occurs. As a design engineer if you have observed any spark between the capacitor terminals while testing, this could be because of the high voltage unintentionally applied across the terminals.
Dielectric tests are used to measure the strength of the PCB. During these dielectric tests, high level of alternating current (AC) or direct current (DC) is applied at the input power terminals of the PCB. Dielectric testers also called hipot testers or high voltage testers are used to provide high voltage at the input power terminal of the PCB to see if there is any breakdown of the PCB or any capacitors. When this high voltage is applied, Electrical safety standards define the maximum current that should flow. Any voltage above this current meant that there is more leakage that is occurring and indicates hipot test failure.
Below is a snapshot of a FR4 datasheet which indicates the dielectric breakdown.
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