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The question of readily obtaining suitable test values that more fully and correctly define the uniformity and quality of electrical insulating materials has been uppermost in our minds in connection with the evaluation, application and inspection of these materials. We have found that the use of certain ASTM test methods and procedures in evaluating and inspecting sheet insulating materials did not give us adequate information that we could translate into intelligent facts that the design engineer and/or manufacturing people could use in building their equipment. These ASTM tests dealing with electric strength and thickness measurements do serve a useful purpose in determining the spread of the particular property tested. Knowing the spread of a particular property in a material is important in its end use but by itself is insufficient to act as a quality control guide, since it does not readily give the lower limits of the property tested easily and quickly. This is further aggravated by the very few specimens, five in most cases, called for in most ASTM test methods, from which one is expected to know the quality level of a particular shipment of material. We in the materials laboratory of the Buffalo divisions were checking many incoming materials to ASTM test values of electric strength and yet often when they reached the shop they would not stand up anywhere near the limits suggested by test data. It was virtually impossible to estimate whether the materials would stand up to 1/4, 1/3, or 1/2 of the values obtained in tests on a small numbers of specimens. All this even after we carefully screened damages caused by general shop handling. A better approach and a better tool had to be developed for preventing such a wide disparity between existing test procedures and ultimate values of the material when it reached manufacturing. It was a must. It was at this point that our concept of large-area-testing was born. We do not claim any priority of this concept nor do we suggest that the two large-area-test methods to be presented below are the only ones that can be used or that they cannot be improved. All we are trying to do is alert ASTM and particularly Committee D-9 on Electrical Insulating Materials, as well as industry in general to the concept and benefits of large-area-testing so that perhaps we may start now in introducing into our test methods ways and means of determining the minimum quality levels of insulating materials.
Photiadis, C. J.
Westinghouse Electric, Buffalo, N. Y.