What are the compliance criteria in 188.8.131.52.3 for solid insulation used between one earthed part and another having an hazardous voltage?
More specifically you asked: RTI - What is the exact meaning of 184.108.40.206.3, Compliance criteria, concerning solid insulation, where it is called the “relative temperature index”? In particular many application are made by a single insulating sheet as a basic safeguard. The clause specifies that “for a single insulating material, the declared relative temperature index information from the material manufacturer can be used if it is suitable for the applicable class of insulation”. In the simplest case where the insulating sheet is applied between two metal parts, for example one earthed part and another having an hazardous voltage, there is no risk of chemical interaction between different polymer parts. In this case the application is not an EIS so the question is it sufficient to provide the manufacturer documentation providing the maximum temperature at which the material can be used? The concern is that any RTI index based on the UL 746B is not formally referred in this IEC standard and the term “relative temperature index” is not defined before this clause in the standard. Flammability certification based on IEC 60695-11-10 and UL 94, dielectric strength based testing at its maximum temperature (while the unit is well heated) and manufacturer information concerning the maximum temperature look as a sufficient base for the acceptability.
In response, as the complete answer to this topic is complex, and since there appears to be a specific, detailed construction / application that needs review / analysis, you are encouraged to contact UL for an in-depth consultation, either via your local account executive or via https://62368-ul-solutions.com/contact-ul.html.
However, in general, per 220.127.116.11.3, a single material used alone is not required to comply with IEC 60085, although this 60085 standard also covers thermal (endurance) indexes for individual materials which may be considered. Alternatively, an RTI based on UL 746B, Polymeric Materials – Long Term Property Evaluations, may be considered for USA/Canada. (See Clauses 6-9, Determination of Relative Thermal Indices of Polymeric Materials, including the generic data in Table 7.1, Relative thermal indices based upon past field-test performance and chemical structure.) Other NCBs also may accept this published data like they do for other UL data associated with thermoplastics.
IEC 62368-1 allows to use declared temperature index; however an accepting NCB may have additional requirements for acceptance and evaluation of materials. Therefore, based on the specific construction, there may be differing approaches for compliance with IEC 62368-1 versus a certification program like UL Mark, GS Mark etc.
Again, due to the complexity of this topic, you are encouraged to contact UL for an in-depth consultation.