ASTM D3300 Test Method for Dielectric Breakdown Voltage of Insulating Oils
ASTM D3300 Standard Test Method for Dielectric Breakdown Voltage of Insulating Oils of Petroleum Origin Under Impulse Conditions
4. Apparatus
4.1 Impulse Generator, capable of producing a standard 1.2 by 50-µs full wave adjustable to positive or negative polarity. The generator shall have a nominal voltage rating of at least 300 kV adjustable in 10-kV steps. Generators having a capability of 1000 W•s (1000 J) at 300 kV have been found satisfactory.
4.2 Voltage-Control Equipment - The controls shall include a suitable measuring device for predetermining the crest voltage to within more or less 5 %. A voltage stabilizer is desirable at the input to the d-c power supply used for charging the impulse-generator capacitors.
4.3 Electrodes:
4.3.1 The electrodes shall consist of a polished steel or brass sphere of 0.5 in. (12.7 mm) diameter and a steel point. The point may be an ordinary steel phonograph needle with a 0.06 mm more or less 20 % radius of curvature of point or a No. 18 Filter Point needle. Needles with drawn tips are not recommended.
4.3.2 The effect of variation in the radius of curvature of point is subject to further investigation. Both electrodes shall be easily replaceable.
4.4 Test Cell:
4.4.1 The test cell shall be made of a material of high dielectric strength and of such dimensions that the electrical breakdown is restricted to the electrode gap. Test cell materials shall resist attack by, and be insoluble in, any of the cleaning or test liquids used. Test cells such as those shown in Fig. 1 and Fig. 2 have been found satisfactory.
4.4.2 The sphere electrode shall be rigidly fixed and the point electrode mounted such that the gap may be adjusted from zero to the required value.
5. Sampling
5.1 Obtain a sample of the liquid to be tested using appropriate ASTM sampling apparatus in accordance with Practices D923.
6. Adjustments and Care of Electrodes and Test Cell
6.1 Electrode Spacing:
6.1.1 For the cell shown in Fig. 1, reduce the electrode gap to zero spacing. Proceed very carefully to avoid damaging the point. The point of contact shall be established electrically with an ohmmeter. Open the gap to the specified spacing using a dial micrometer or other suitable method.
6.1.2 For the cell shown in Fig. 2, the gap may be set with a go-no-go gage.
6.1.3 The gap spacings shall be 1.0 in. (25.4 mm) for point-to-sphere and 0.15 in. (3.8 mm) for sphere-to-sphere electrode configuration.
6.2 Cleaning - Degrease the cell and electrodes by rinsing them with reagent grade petroleum ether, washing with detergent and hot water, rinsing thoroughly in hot tap water, and then rinsing them with distilled water. Dry the cell and hardware in an oven for 2 h at approximately 105 to 110°C, remove, and store in a desiccator until needed.
6.3 Daily Use - Use new or polished sphere electrodes at the beginning of each day's testing. Discard the point electrode and replace it after each breakdown; replace the sphere electrodes after every five breakdowns when testing point-to-sphere. More frequent replacement may be necessary when testing sphere-to-sphere. Sphere electrodes may be cleaned and polished for reuse in point-to-sphere testing. However, the use of polished spheres is not recommended for sphere-to-sphere testing. When not in use, clean and store the cell in accordance with 6.2.
7. Test Temperature
7.1 Conduct the tests with the specimen at room temperature as defined in Terminology D2864. Testing liquids at temperatures lower than that of the room may give variable and unsatisfactory results. Record the test temperature.
