Thermal life of varnished glass cloth

WITH 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 functional 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 limitation 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.