5.1 Dissipation Factor and Relative Permittivity-Knowledge of these properties is important in the design of electrical equipment such as cables, transformers, insulators, and so forth. The numerical product of these two properties of a dielectric system is proportional to the energy loss converted to heat, and is called its loss index (see Terminology D1711). The energy loss reduces the efficiency of electrical equipment. The heat produced tends to chemically degrade the dielectric material and may even lead to thermal runaway. Test results of impregnated specimens can disclose significant differences between combinations of papers and oils that appear similar when the papers and the oils are tested separately. Dissipation factor, particularly at elevated temperatures, is often changed significantly by the presence of a small quantity of impurities in either the liquid or the paper. This practice is useful in the comparison of materials and in evaluating the effects of different papers on a given liquid. Judicious analysis of results with respect to time, temperature, and field strength are useful in predicting the performance and capabilities of systems using the paper and the liquid. For additional information on the significance of dissipation factor and relative permittivity, see Test Methods D150.

5.2 Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at Commercial Power Frequencies:

5.2.1 A comprehensive discussion of the significance of the dielectric strength test as applied to solid, semi-solid, and liquid materials is given in Appendix X1 of Test Method D149. Other factors peculiar to high-quality composite insulations, such as oil-impregnated papers, are considered in the following:

5.2.2 In tests involving high electrical stresses, immersion of critical parts of a test circuit in oil is a widely used technique for inhibiting corona. However, it has limitations that must be recognized when using the submerged electrode option of this practice (Note 1). Attack on the paper by corona generated in the surrounding fluid at electrode edges can occur whether the fluid is air or oil. Corona occurs at considerably higher voltages in oil than in air. Thick and dense papers are more likely to cause discharge-initiated breakdowns. For interpretation of breakdown measurements consider the number of edge breakdowns, implying discharge-initiated breakdowns.

Note 1: Two techniques are in use in the industry for testing specimens for dielectric breakdown voltage. In one, the test is made with the electrodes and test specimen submerged in the impregnating liquid while in the other the electrodes are not submerged, that is, the specimen is tested in air. Much data has been accumulated using the latter technique. These techniques yield different values of breakdown voltage. Test Method D149 states preference for testing materials in the medium in which they are used. The use of submerged electrodes follows this preference. When testing thick insulating boards, the use of submerged electrodes is greatly preferred.

5.2.3 The results of power frequency tests on oil impregnated papers are useful for screening, research, and quality control, provided that considerable judgment is exercised in interpreting the results. The application of the test results to equipment design and service requires particular caution and skill (see Appendix X1 of Test Method D149).

5.3 Dielectric Breakdown Voltage and Dielectric Strength Under Impulse Conditions-Testing impregnated paper or board under impulse conditions can yield useful data for the designer of electrical equipment. The test results are useful in the comparison of materials and for research studies. For a more general treatise on the significance of impulse testing see Test Method D3426.