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Long-term thermal aging is designed to monitor degradation of the insulation system. Specific details regarding thermal aging can be found in UL 1446, Standard for Safety of Systems of Insulating Materials — General (Paragraphs 12.1-12, 3.5) and IEEE 117. A typical thermal aging program lasts 12-18 months with many international standards also requiring testing for this duration.
This test is conducted by aging the proposed EIS at three or more elevated temperatures with intermittent dielectric proof testing. After each heat cycle, samples are also subjected to a repeated series of environmental exposures such as cold shock, mechanical stress and humidity.
At the conclusion of the aging program, test results for the proposed EIS are compared to results from a field-proven control EIS, running concurrently. The rating issued to the proposed EIS is relative to the previously established rating of the control EIS.
Reduced test programs of one- or two-temperature thermal agings are used to evaluate modifications to an existing EIS. Recognition of a modified EIS is usually issued within three to six months of testing, depending on test results.