The dielectric breakdown voltage is a measure of the ability of an insulating liquid to withstand electrical stress. The power-frequency breakdown voltage of a liquid is reduced by the presence of contaminants such as cellulosic fibers, conducting particles, dirt, and water. A low result in this test method indicates the presence of significant concentrations of one or more of these contaminants in the liquid tested. See Appendix X1.

A high breakdown voltage measured in this test method does not necessarily indicate that the amount of the contaminants present in a liquid from which the sample was taken are sufficiently low for the sampled liquid to be acceptable in all electrical equipment. Test Method D 877 is not sensitive to low levels of these contaminants. Breakdown in this test method is dominated by events occurring at the electrode edges. The voltage stress distribution between the parallel disk electrodes used in this test method are quasi-uniform and there is substantial stress concentration at the sharp edges of the flat disk faces.

This test method may be used for evaluation of insulating liquids in equipment that is designed to be filled with unprocessed liquids as delivered by a vendor.

This test method is not recommended for evaluation of the breakdown voltage of liquids used in equipment that requires the application of vacuum and filtering of the oil before being placed into service. Test Method D 1816 should be used to determine the breakdown voltage of filtered and degassed liquids.

This test method is used in laboratory or field tests. For field breakdown results to be comparable to laboratory results, all criteria including room temperature (20 to 30°C) must be met.

1.1 This test method describes two procedures, A and B, for determining the electrical breakdown voltage of insulating liquid specimens. The breakdown test uses ac voltage in the power-frequency range from 45 to 65 Hz.

1.2 This test method is used to judge if the disk electrode breakdown voltage requirements are met for insulating liquids, as delivered from the manufacturer, that have never been filtered or dried. See Specification D 3487, Specification D 4652, and Guide D 5222 for the minimum specified electrical breakdown. This test method should be used as recommended by professional organization standards such as IEEE C57.106.

1.3 Limitations of the Procedures

1.3.1 The sensitivity of this test method to the general population of contaminants present in a liquid sample decreases as applied test voltages used in this test method become greater than approximately 25 kV rms.

1.3.2 If the concentration of water in the sample at room temperature is less than 60 % of saturation, the sensitivity of this test method to the presence of water is decreased. For further information refer to RR: D27-1006.

1.3.3 The suitability for this test method has not been determined for a liquid's viscosity higher than 900 cSt at 40C.

1.4 Procedure Applications

1.4.1 Procedure A

Procedure A is used to determine the breakdown voltage of liquids in which any insoluble breakdown products easily settle during the interval between the required repeated breakdown tests. These liquids include petroleum oils, hydrocarbons, and askarels (PCB) used as insulating and cooling liquids in transformers, cables, and similar apparatus.

Procedure A may be used to obtain the dielectric breakdown of silicone fluid as specified in Test Methods D 2225, provided the discharge energy into the sample is less than 20 mJ (milli joule) per breakdown for five consecutive breakdowns.

1.4.2 Procedure BThis procedure is used to determine the breakdown voltage of liquids in which any insoluble breakdown products do not completely settle from the space between the disks during the 1-min interval required in Procedure A. Procedure B, modified in accordance with Section 17 of Test Methods D 2225, is acceptable for testing silicone dielectric liquids if the requirements of can not be achieved.

Procedure B should also be applied for the determination of the breakdown voltage of liquid samples containing insoluble materials that settle from the specimen during testing. These may include samples taken from circuit breakers, load tap changers, and other liquids heavily contaminated with insoluble particulate material. These examples represent samples that may have large differences between replicate tests. The use of Procedure B will result in a more accurate value of breakdown voltage when testing such liquids.

Use Procedure B to establish the breakdown voltage of an insulating liquid where an ASTM specification does not exist or when developing a value for an ASTM guide or standard. Procedure A may be used once the single operator precision of has been demonstrated.

1.5 Both the SI and inch-pound units are equally acceptable.

This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.