The dielectric breakdown voltage and dielectric strength of an insulating gas in a uniform field depends primarily on the molecular structure of the gas. As different gases are mixed either by plan or by contamination any change in dielectric breakdown voltage and dielectric strength will depend on both the nature and proportion of the individual gases. This test method uses plane and spherical electrodes which provide a nearly uniform field (see Appendix) in the area of electrical discharge. It is suitable for determining the dielectric breakdown voltage and dielectric strength of different gases and mixtures thereof for research and application evaluations and also as a field test. A more complete discussion of the significance of the dielectric strength test is given in the Appendix.

1.1 This test method covers the determination of the dielectric breakdown voltage and dielectric strength of insulating gases used in transformers circuit breakers cables and similar apparatus as an insulating medium. The test method is applicable only to gases with boiling points below room temperature at atmospheric pressure.

1.2 This standard may involve hazardous materials operations and equipment. 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.

1.3 Mercury has been designated by EPA and many state agencies as a hazardous material that can cause central nervous system kidney and liver damage. Mercury or its vapor may be hazardous to health and corrosive to materials. Caution should be taken when handling mercury and mercury containing products. See the applicable product Material Safety Data Sheet (MSDS) for details and EPA''s website 8212; http://www.epa.gov/mercury/faq.htm for additional information. Users should be aware that selling mercury and/or mercury containing products into your state may be prohibited by state law.

Insulating materials used in high-voltage equipment may be subjected to transient voltage stresses resulting from such causes as nearby lightning strokes. This is particularly true of apparatus such as transformers and switchgear used in electrical-power transmission and distribution systems. The ability of insulating materials to withstand these transient voltages is important in establishing the reliability of apparatus insulated with these materials.

Transient voltages caused by lightning may be of either positive or negative polarity. In a symmetrical field between identical electrodes the polarity has no effect on the breakdown strength. However with dissimilar electrodes there may be a pronounced polarity effect. It is common practice when using dissimilar electrodes to make negative that electrode at which the higher gradient will appear. When asymmetrical electrodes are used for testing materials with which the tester has no previous experience or knowledge it is recommended that he make comparative tests with positive polarity and negative polarity applied to the higher gradient or smaller electrode to determine which polarity produces the lower breakdown voltage.

The standard wave shape is a 1.2 by 50-x03BC;s wave reaching peak voltage in approximately 1.2 x03BC;s and decaying to 50 % of peak voltage in approximately 50 x03BC;s after the beginning of the wave. This wave is intended to simulate a lightning stroke that may strike a system without causing failure on the system.

For most materials the impulse dielectric strength will be higher than either its power frequency alternating voltage or its direct voltage dielectric strengths. Because of the short time involved dielectric heating and other thermal effects are largely eliminated during impulse testing. Thus the impulse test gives values closer to the intrinsic breakdown strength than do longer time tests. From comparisons of the impulse dielectric strength with the values obtained from longer time tests inferences may be drawn as to the modes of failures under the various tests for a given material. Appendix X1 of Test Method D 149 should be referred to for further information on this subject.

1.1 This test method covers the determination of dielectric strength of solid electrical insulating materials under simulated-lightning impulse conditions.

1.2 Procedures are given for tests using standard 1.2 by 50 s full-wave impulses.

1.3 This test method is intended for use in determining the impulse dielectric strength of insulating materials either using simple electrodes or functional models. It is not intended for use in impulse testing of apparatus.

1.4 This test method is similar to IEC Publication 243-3. All procedures in this test method are included in IEC 243-3. Differences between this test method and IEC 243-3 are largely editorial.

1.5 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. Specific precaution statements are given in Section 9.