A major factor affecting the long term performance of insulating materials is thermal degradation. Other factors, such as moisture and vibration, may cause failures after the material has been weakened by thermal degradation.

An electrical insulating varnish is effective in protecting electrical equipment only as long as it retains its physical and electrical integrity.

The thermal degradation of the varnish results in weight loss, porosity, crazing, and generally a reduction in flexibility. Degradation of the varnish can be detected by a decrease in dielectric strength, which is therefore used as the failure criterion for this test method.

Electrical insulating varnishes undergo flexing in service due to vibration and thermal expansion. For this reason, this functional test includes flexing and elongation of the insulation. The electrodes used in this test method are designed to elongate the outer surface of the specimen 2 % with respect to the neutral axis of the base fiber while being tested for dielectric breakdown.

1.1 This test method covers the determination of the relative thermal endurance of flexible electrical insulating varnishes by determining the time necessary at elevated temperatures to decrease the dielectric breakdown of the varnish to an arbitrarily selected value when applied to a standard glass fiber fabric.

1.2 This test method does not apply to varnishes that lose a high percentage of their dielectric breakdown voltage when flexed before elevated temperature exposure as prescribed in the screening test (Section 9). Examples of such varnishes are those used for high speed armatures and laminated structures. Also, this test method is not applicable to varnishes which distort sufficiently during thermal elevated temperature exposure so that they cannot be tested using the curved electrode assembly.

1.3 Thermal endurance is expressed in terms of a temperature index.

1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific hazard statements, see Section 7.

1.5 The values stated in SI units are the standard.

There is no equivalent IEC or ISO standard.