IEC 60172: Winding Wire Temperature Index — Extrapolating Life With Arrhenius

How Enamelled Wire Temperature Class Is Really Determined — The IEC 60172 Test

IEC 60172:2015 specifies temperature index determination for enamelled winding wires. The “Class F (155 °C)” on every motor nameplate is verified through this standard test procedure.

Test principle: Wire specimens are aged at three different temperatures (e.g., 180 °C, 200 °C, 220 °C), minimum 10 specimens per temperature. Time-to-failure is recorded (endpoint: dielectric breakdown), and Arrhenius extrapolation determines the temperature corresponding to 20,000-hour life — the Temperature Index (TI).

Why three temperatures? The Arrhenius equation ln(t) = A + B/T has two unknowns (A, B); theoretically, two temperatures suffice. But a third temperature validates linearity — if the third data point deviates significantly from the line, the ageing mechanism has changed at that temperature (e.g., polyester-imide oxidation above 240 °C differs from oxidation at 180 °C), and simple extrapolation is invalid.

TN Lab — The temperature index is not “measured”; it is “extrapolated” using the Arrhenius equation. Three temperature points are the linearity verification key.