W ITH THE TREND toward reducing the size of electric equipment for a given rating, the electrical insulation is being operated closer to its maximum thermal and electrical capabilities. Therefore, the need for functional evaluation of electrical insulation prior to its use in the field to assure good performance of the equipment has become more important. Prior to func tional testing of a complete insulation system within the equipment, it is necessary to evaluate the various component materials. The insulating varnish which is a part of the insulation system is being evaluated in industry by two different methods. One method is known as the "Proof Test Procedure" and the other "Breakdown Test Procedure." These tests can be of value only if they provide a truly adequate basis for temperature classification of varnishes. The tests should be accurate and reproducible, and there should be a reasonable correlation between the temperature limi tation of the varnish as determined by these tests and the thermal life of the varnish in an insulation system as measured by functional tests such as described in AIEE No. 510.These two test procedures for measuring thermal life have been suggested by ASTM (American Society for Testing Materials) and one of these, a proof test, has also been proposed by AIEE. Thé procedures differ in the method of applying the breakdown voltage to the specimen and the amount of thermal shock the speci mens are subjected to during test. Both procedures measure the time required for the electric strength of the varnished glass fabric to drop to a fixed endpoint and employ a curved electrode test fixture which elon gates the outer surface of the specimen about 2%.With both of these procedures, there can be a wide dispersion of test results due to inadequate control in the preparation of the test specimen, in the aging condi tions, and in the method of testing. For these reasons, the test results are interpreted in a statistical manner.In comparing the two test methods, there were sev eral disadvantages encountered in using the proof test:(1) only a single failure point was measured, thus there was no knowledge of the rate of decrease in the electric strength of the varnish; (2) the results obtained using this procedure were subject to large errors due to the limited number of test values and the method used in determining the endpoint; (3) a time cycle was es tabare with the Westinghouse Electric Corporation, East Pittsburgh, Pa. lished prior to the start of the aging period and speci mens were removed at fixed intervals of time, which sometimes led to the depletion of the test samples be fore the failure point was reached resulting in a waste of time since the test will have to be repeated; and (4) if in the future the end-point voltage is raised to a higher value, the data that has been taken using the proof test will be worthless.The breakdown procedure overcame most of the disadvantages associated with the proof test for the following reasons.1. The procedure gave the history of the decline in...
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