— Correlation time was changed to 60,000 hours —
UL746B has been revised (5th ed., 2018/8/15)
The maximum limit of correlation time in evaluating RTI (Relative Thermal Index) was reduced from 100,000 to 60,000 hours by the standard revision.- Instead of correlation time obtained by a control material, it is now possible to use extrapolation evaluation of 60,000 hours (which was formerly evaluated by 100,000 hours).
- Even if the control material was used, evaluate the RTI by 60,000 hours if the correlation time of the control material is longer than 60,000 hours.
There is an opinion that the impact to RTI value by the reduction from 100,000 to 60,000 hours is small. However, as you can see on the plot example below (the data plot stated in the section 20.2 in UL746B), the calculated RTI is about 8℃ higher.
The Example of the Difference of Calculated RTI based on Correlation Time
RTI is assigned according to the rules below.
- Up to calculated RTI of 130℃, the interval is at 5℃
- From calculated RTI of 130 to 180℃, the interval is at 10℃ (155℃ as an exception)
- If calculated RTI exceeded 180℃, the interval is at 20℃
On examples below, calculated RTI 129℃ is assigned to 125℃, while calculated RTI 121℃ is assigned to 120℃. Although RTI is 120℃ based on the conventional method, RTI increases to 125℃ when it is calculated by the correlation time of 60,000 hours. If the RTI is lower than 130℃, the impact of the revision is more evident. RTI may change even if it is more than 130℃.
(60,000 hours: Blue Line, 129℃, 100,000 hours:
Green Line, 121℃)
At this point, it is not clear whether RTI will be reviewed based on the long term thermal test data as in the past. The subject was not discussed in the UL94/746 STP (standard formulation meeting) held on June, 2018. It is expected that the subject would be discussed with UL on an individual basis.
Please feel free to contact us regarding both new and past cases, as well as the standard revision.
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