Experimental researches have been proceeded from the viewpoint of diagonostics on insulation systems to determine the relationship between the voltage endurance life and internal partial discharge. The test-pieces were epoxy resin impregnated mica composites to be used for insulation of electric rotating machine and epoxy resin/filler composite to be used for insulation of casting equipments. Investigation was made for measuring the discharge quantities in the course of time and for residual breakdown voltage. The results of the tests revealed that the discharge quantity is an index that indicates how much and how seriously the insulation are damaged at given point of time, but it can hardly show how fast the damage would develop thereafter. The endurance life of insulation varies in accodance with the discharge quantities which depends on development of breakdown path, thickness of insulation and the applied voltage.
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