ASTM D149 Dielectric Strength

ASTM D149 Dielectric Strength, IEC 60243 Scope:

Dielectric strength test as per ASTM D 149 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.

The tests carried out in accordance with this ASTM D 149 document are suitable for use in identifying changes or deviations from normal characteristics brought on by processing variables, aging conditions, or other manufacturing or environmental circumstances. They can also be used to provide part of the information required to determine whether a material is suitable for a particular application. For process control, acceptability testing, or research testing, this test approach is helpful.

Test Procedure:

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  in the ASTM D 149 test method 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. Electrical burn-through punctures or decomposition in the specimen are indicators of the breakdown. The 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 the 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 until breakdown is met. To prevent any arcing from the electrode to the ground, it is better to perform the test with oil as a medium.

The dielectric strength of materials that are fluid during testing is not intended to be measured using this test method.

Rarely can the dielectric behavior of a material in a real application be directly predicted from the results of this test procedure. To determine the importance of these results for a certain material, it is typically necessary to compare them to results from experiments on different materials, other functional tests, or both.

Video 01: ASTM D149 Material & Oil Breakdown Testing

Specimen size:

The recommended specimen type for the ASTM D149 test is a plaque of 4 inches or more and any suitable thickness value. However, thickness is kept mostly between 0.8 and 3.2 mm (0.032 and 0.125 inch). To avoid any chance of flashover before breakdown, specimens over 2 mm thick are typically tested in oil.

Data:

The recorded value of the breakdown voltage is divided by the thickness of the specimen to calculate the dielectric strength using the ASTM D149 value, which is expressed in volts per meter (volts/m). The failure location is also recorded.

Conclusion:

Hence, ASTM D 149 is the standard test method for dielectric breakdown voltage and dielectric strength of solid electrical insulating materials at commercial power frequencies.