A good quality insulator has a high dielectric strength value. ASTM D149 test method determines the Dielectric Strength of solid electrical insulating materials at commercial power frequencies. Any of the three prescribed methods can be used to determine the dielectric strength of an insulator.
Dielectric Strength Test as per ASTM D149 evaluates solid electrical insulating materials for their use in commercial power applications. The dielectric strength of a material is defined as the maximum voltage required to produce a dielectric breakdown through the material. Dielectric strength is expressed as Volts per unit thickness, and it determines the electrical strength of a material as an insulator. A good quality insulator is characterized by a high dielectric strength value.
ASTM D149 tests insulating materials at commercial power frequencies, and at dielectric strength between 25 and 800 Hz, generally at 60 Hz. Three basic short-time methods — slow rate-of-rise method, short-time method, and step-by-step method —are used to determine the dielectric strength of an insulator.All three methods have the same basic setup where the test specimen is placed between two electrodes in the air or in oil.
(a) Out of these three methods, the short-time method is the most common one. In this method, an initial zero voltage is applied across the two electrodes, which is then raised at a uniform rate till dielectric breakdown is achieved. The breakdown is marked by electrical burn-through punctures or decomposition in the specimen. Time taken by the sample to reach dielectric breakdown determines the rate of voltage rise.
(b) The point of 50% of the breakdown voltage determined by the short-time method is considered the starting point of the slow-rate-of rise method. After starting point, voltage is increased uniformly.
(c) The step-by-step method also starts at 50% of the short-time method, and after that, voltage is increased at equal increments for a specified period till breakdown is met. To prevent any arcing from the electrode to the ground, it is better to perform the test in oil as a medium.
The recommended specimen type for ASTM D149 test is a plaque of 4 inches or more and any suitable thickness value. However, thickness is kept between 0.8 to 3.2 mm (0.032 to 0.125 inch) mostly. To avoid any chance of flashover before breakdown the specimens over 2 mm thick are typically tested in oil.
The recorded value of the breakdown voltage is divided by the thickness of the specimen to calculate the Dielectric Strength by ASTM D149 value which is expressed in volts per meter (volts/m). Failure location is also recorded.