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# The basics of Relative Thermal Index

## The basics of Relative Thermal Index

(OP)
As an EE I have little background in materials properties, so I have a very basic question regarding UL 746b RTI values.

Is there an aggregate value for RTI that is calculated from the three specific RTI values shown on the UL yellow card, or is the intent of the three different values on the yellow card to apply the temperature rating for the intended use of the product, whereas a generic rating (from UL 746b, for example) would apply to all three characteristics?

In other words, if an application requires all three characteristics to be considered, and the material under consideration has three different temperature ratings, would one need to consider the lowest temperature as the upper limit for the application, or is there some math necessary to ensure the aggregate of three characteristics fall within the temperature spec?

### RE: The basics of Relative Thermal Index

From the UL Site.. http://industries.ul.com/plastics-and-components/p...
Relative temperature index (RTI) is the maximum service temperature for a material where a class of critical property will not be unacceptably compromised through chemical thermal degradation. This spans over the reasonable life of an electrical product relative to a reference material having a confirmed, acceptable corresponding performance-defined RTI.

Electrical RTI is associated with critical electrical insulating properties.

Mechanical impact RTI is associated with critical impact resistance, resilience and flexibility properties.

Mechanical strength RTI mechanical without impact is associated with critical mechanical strength where impact resistance, resilience and flexibility are not essential.

Its basically the max steady state temp that cannot be exceeding during performance testing conducted by the NRTL..
If a plastic piece is serving as an electrical insulator then its not allowed to be hotter than the electrical RTI..
if a plastic piece is serving as protection from mechanical impact then its not allowed to be hotter than the mechanical RTI..
etc..

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