5.1 This test method is intended for use as a control and acceptance test for direct-voltage applications. It can also be used in the partial evaluation of material for specific end uses and as a means for detecting changes in material due to specific deteriorating causes.

5.2 Experience indicates that the breakdown value obtained with direct voltage usually will be approximately 2 to 4 times the rms value of the 60-Hz alternating-voltage breakdown.

5.3 For a nonhomogeneous test specimen, the distribution of voltage stress within the specimen is determined by impedance (largely capacitive) with alternating voltage. With an increasing direct voltage, the voltage distribution will still be largely capacitive, although this depends partly on the rate of voltage increase. After steady application of direct voltage the voltage division across the test specimen is determined by resistance. The choice of direct or alternating voltage depends upon the purpose for which the breakdown test is to be used, and to some extent, on the intended application of the material.

5.4 A more complete discussion of the significance of dielectric breakdown tests is given in Appendix X1 of this method and in Appendix X1 of Test Method D149. Those appendix sections of Test Method D149 that refer to alternating voltage are not applicable to the direct-voltage method.