5.1 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.

5.2 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.

5.3 The standard wave shape is a 1.2 by 50-μs wave, reaching peak voltage in approximately 1.2 μs and decaying to 50 % of peak voltage in approximately 50 μs 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.

5.4 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 D149 should be referred to for further information on this subject.