ASTM D2225 Test Methods for Silicone Fluids Used for Electrical Insulation
ELECTRICAL METHODS
16. Relative Permittivity (Dielectric Constant)
16.1 Significance - Silicone insulating fluids are used to insulate components of an electrical network from each other and from ground. For this use, it is generally desirable to have the capacitance as small as possible, consistent with acceptable chemical properties and design considerations.
16.2 Procedure - Determine the relative permittivity in accordance with Test Method D924. An alternative method of cleaning the test cells is to use multiple rinses of isopropanol followed by a thorough rinsing with methylene chloride and dry with clean, dry, warm air.
17. Dielectric Breakdown Voltage
17.1 Significance - The importance of the dielectric breakdown voltage of a silicone liquid is as a measure of its ability to withstand electrical stress without failure. It may also indicate the presence of contaminating materials, such as water, conducting solid particles, dissolved contaminants, or the decomposition products resulting from an electric arc. A high dielectric breakdown voltage, however, is not a certain indication of the absence of all contaminants.
17.2 Procedure - Determine the dielectric breakdown voltage in accordance with Method D877, with the following modification:
17.2.1 Fill the test cup by tilting it at a 45° angle. As the liquid approaches the tilted top edge of the cup, slowly rotate the cup to an upright position while continuing to pour sample into the cup. This will reduce the amount of air bubbles in the sample and prevent bubbles from being trapped under the electrodes.
17.2.2 Make one breakdown on each of the specified fillings of the test cup.
17.2.3 Clean the electrode surfaces after each breakdown by one of the following methods:
17.2.3.1 Method A - After each breakdown and before the cup is emptied, pass the electrode-spacing gage through the electrode gap twice. Then empty the cup. This will clean the electrodes of any semisolid breakdown products and they will flow out when the cup is emptied.
17.2.3.2 Method B - Empty the cup. Wipe the electrode surfaces with a lintless paper or cloth. This is best accomplished by folding the lintless paper over a clinical tongue depressor. Flush the test cup with clean silicone fluid and empty the cup.
18. Dissipation (Power) Factor
18.1 Significance - Dissipation (power) factor of a silicone fluid is an indication of the energy dissipated as heat in the fluid. It is useful as a means for quality control and as an indication of changes in the fluid resulting from deteriorating and contaminating influences.
18.2 Procedures - Determine the dissipation factor in accordance with Test Method D924. An alternative method of cleaning the test cells is to use multiple rinses of isopropanol followed by a thorough rinsing with methylene chloride and dry with clean, dry, warm air.
19. Specific Resistance (Resistivity)
19.1 Significance - The specific resistance of a silicone insulating fluid is a measure of its electrical insulating capability in d-c apparatus. High resistivity reflects low content of free ions and ion-forming particles, and normally indicates a low concentration of conductive contaminants.
19.2 Procedure - With the exception of the procedure for cleaning the test cells, determine the specific resistance in accordance with Test Method D1169. An alternative method of cleaning the test cells is to use multiple rinses of isopropanol followed by a thorough rinsing with methylene chloride and dry with clean, dry, warm air.
