What is STT?

What is STT?

This article provides an overview of UL 2597, "Outline of Investigation for Test Method for Materials for Use in Transportation Applications: Surface Tracking Test (STT) Method" (hereinafter referred to as "STT"), a new tracking-resistance test standard published by UL Standards & Engagement in May 2025.

Demand for electric vehicles (EVs) has been rising in recent years, yet there remains room for improvement in areas such as driving range and battery charging time. To increase the charging speed in particular, the industry is shifting from conventional DC charging voltages of 300–400 V toward 800 V architectures, and the electrical performance requirements for materials used in these systems are expected to become increasingly stringent.

However, the conventional tracking-resistance test standard — "Method for the Determination of the Proof and the Comparative Tracking Index of Solid Insulating Materials" (hereinafter "CTI"), based on IEC 60112 and ASTM D3638 — is limited to a maximum voltage of 600 V, leaving a gap for higher-voltage applications.

With the publication of UL 2597 by UL Standards & Engagement, tracking-resistance testing is now possible at voltages of up to 900 V.

Key Differences Between STT and CTI

The most significant difference between STT and CTI is the maximum testable voltage (STT: 900 V; CTI: 600 V). The two methods also differ in electrode spacing and electrode orientation. The table below summarizes the key differences.

　

Item	STT	CTI
	Surface Tracking Test	Comparative Tracking Index Test
Test standard	UL2596	IEC60112
Maximum voltage	〜900V	〜600V
Electrode spacing	6mm±0.1mm	4mm±0.1mm
Electrode orientation	Inverted (sharp edge facing outward)	Standard (sharp edge facing inward)
Other	The test equipment and electrolyte used are similar

 

STT Evaluation Results for LEONA™ Polyamide Resin

The STT in accordance with UL 2597 standard were conducted to assess LEONA™ “Development Grade” and LEONA™ SN series which is halogen Free (Certified to UL94 V-0). 
LEONA™ “Development Grade” achieved the highest rating available in the STT with 900 V and LEONA™ SN Series achieving the rating of 800 V. 

Feel free to contact us about our tracking-resistance materials.
Asahi Kasei offers engineering plastics tailored to a range of applications and design requirements, with compliance to tracking-resistance tests including IEC 60112 and UL 2597. Please feel free to contact us with any questions, consultation requests, or sample inquiries.

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