Underwriters Laboratories Inc. (UL) has been developing and enforcing safety standards for over one hundred years. UL 1446, Standard of Safety for Systems of Insulating Materials – General, outlines the test procedures required for the thermal evaluation of electrical insulation systems (EIS). The procedures also cover the evaluation of insulation components for the addition or substitution into an established EIS. UL 1446 also specifies the test procedures required to qualify some of the individual components such as varnishes, magnet wires, and magnet wire coatings.
An established EIS is one that has known service life at operating temperatures and conditions or has been previously evaluated by a long-term thermal aging procedure recognized under UL 1446 requirements. Thermal aging is an accelerated aging program that allows the results to be extrapolated to yield a relative thermal index (RTI) for a proposed EIS that is then compared to the TI of a known EIS. This then allows the proposed EIS to be assigned a thermal class, e.g., Class 155(F) or Class 180(H).
UL 1446 deals mostly with EIS intended for use in electrical equipment connected to a low-voltage distribution network, defined as not more than 600 V nominal in the US, not more than 750 V in Canada, and 1000 V ac or less in overvoltage category IV installations according to International Electrotechnical Commission (IEC) 60364. These EIS may be used in equipment with higher output voltage, such as control transformers or HID ballasts, but secondary voltage limits should be addressed in end-product standards and test procedures. Usually, this requires that the ground and interwinding thicknesses be increased proportionately so that the V/mm stress does not exceed that of the material as originally tested.
UL 1446 is not intended to cover EIS exposed to radiation or operating in liquids such as oils or refrigerants, or other media that may degrade insulating materials. Nor does it intend to evaluate the effect of the manufacturing process or design factors on the life of the EIS.
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