Assessment of tests for the evaluation of thermal endurance of varnished fabrics

The existing thermal classification of insulating materials is based on long-term experience. Reliable tests employing accelerated thermal aging are needed to classify new materials and check established ones. A number of individual properties of 14 different types of varnished fabric insulation have been measured, before and after heating. The results have been analysed to find the reliability and the suitability of the test methods as guides to the thermal endurance of this type of insulation. Methods measuring tensile strength, extension-at-break and tearing strength are found most suitable, but it is accepted that this type of test is only a first stage in the evaluation of any material. The statistical approach cannot take account of some aspects of the tests, and reasons are advanced for rejecting tests which, statistically, are satisfactory. In particular, electric-strength tests on unsupported materials are not considered suitable tests for assessing thermal endurance.