What Is ASTM D149 - 09(2013) Standard Test Method ?
Scope:
Dielectric Strength is a measure of the electrical strength of a material as an insulator. Dielectric strength is defined as the maximum voltage required to produce a dielectric breakdown through the material and is expressed as Volts per unit thickness. A higher dielectric strength represents a better quality of insulator.
Test Procedure:
There are three basic procedures that can be used to determine the dielectric strength of an insulator. These procedures are the short-time method, the slow rate-of-rise method and the step-by-step method. Each of these three methods has the same basic set-up, which consists of the test specimen placed between two electrodes in air or oil.
For the most common test, the short-time method, voltage is applied across the two electrodes and raised from zero to dielectric breakdown at a uniform rate. Breakdown is when an electrical burn-through punctures the sample, or decomposition occurs in the specimen. The rate of voltage rise is determined by the time it takes the sample to reach dielectric breakdown.
The slow-rate-of rise method starts at 50% of the breakdown voltage as determined by the short-time-method and is increased at a uniform rate. The step-by-step method starts at 50% of the short-time-test then voltage is increased at equal increments for a specified time period until breakdown. The test is sometimes performed in oil to prevent arcing from the electrode to the ground.
Specimen size:
The recommended specimen type for this test is a 4 inch plaque or larger. Any specimen thickness can be used; however the most common thickness is between 0.8 to 3.2 mm (0.032 to 0.125 inch). Specimens over 2 mm thick are typically tested in oil to decrease the chance of flashover before breakdown.
Data:
Dielectric strength is calculated by dividing the breakdown voltage by the thickness of the sample. The data is expressed in Volts/mil. The location of the failure is also recorded. A higher dielectric strength represents a better quality of insulator.
Note:
For details on ASTM D149 - 09 (2013) Standard PDF, please refer to Free Download ASTM D149 - 09 (2013) Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at Commercial Power Frequencies.
For ASTM D149 - 09 (2013) test equipment, please go to KP8009 Electric Strength Voltage Breakdown Tester