UL 746B-2020 pdf free download.Polymeric Materials – Long Term Property Evaluations.
6.3 The relative thermal index of a material is to be based upon an evaluation of long-term thermal-aging data obtained under the program described In Relative Thermal Index — Based Upon Long-Term Thermal- Aging Programs, Section 8. Thermal indices on a generic basis have been established through knowledge of extensive field-service records, as outlined in Relative Thermal Index — Based Upon Historical Record, Section 7. Relative thermal indices may also be established based upon a study and evaluation of the interrelationship of all of the data mentioned in Supplementary Test Procedures. Section 3, which can also be coupled with knowledge concerning the materials performance in insulating systems gained through experience or long-term aging tests.
6.4 A comparison of the thermal-aging characteristics of one material of proven field service at a particular temperature level with the thermal-aging characteristics of another material with no field service history provides a means for estimating the relative thermal index level at which the second material might also provide acceptable field service.
6.5 Another explanation of a relative thermal index is the maximum temperature below which a material maintains its characteristics over a reasonable period. This relative thermal index serves the very great need to evaluate materials that are exposed to heat sources in electrical products in which they are not used as part of an insulating system and in which they are not subjected to other major degradation influences. It is to be assumed that neither excessively long nor excessively short duty cycles are invo4ved.
6.6 To be valid for use in a specific application, a relative thermal index of a material must be established by a study of the degradation rates of all properties that are relied upon in that application, As a corollary to this principle, more than one relative thermal index can be assigned to a material depending on the relative degradation rates of the properties of the material and depending on which of these properties are considered in establishing the indices.
7 Relative Thermal Index — Based Upon Historical Record
7.1 Table 7.1 presents a list of materials, each of which is assigned a relative thermal index based on acceptable service experience, the chemical structure of the material, and a knowledge of the performance of the material in tests of insulating systems and electrical equipment. The assigned relative thermal index is applicable to each member of the generic material class.
7.2 Unless otherwise indicated In Table 7.1, the generic thermal index of a material is to be considered
50°C (122°F).
7.3 Unless otherwise indicated in Table 7.1, the generic thermal index of a material is independent of thickness and pigmentation.UL 746B pdf download.