Thermal Classes of Solid Insulating Materials Which of the following lists correctly gives the standard thermal classes of solid dielectrics (insulation classes) in ascending order of temperature rating?

Introduction / Context:
Electrical insulation systems are categorized into thermal classes that indicate their maximum permissible operating temperatures for reliable life. Recognizing the correct order of these classes is important in motor, transformer, and power apparatus design because overstepping a class leads to accelerated ageing and failure.

Given Data / Assumptions:

• Thermal classes considered: Y, A, E, B, F, H, C.
• Ascending order corresponds to increasing allowable temperature.
• Representative ratings (typical): Y ≈ 90 °C, A ≈ 105 °C, E ≈ 120 °C, B ≈ 130 °C, F ≈ 155 °C, H ≈ 180 °C, C > 180 °C.

Concept / Approach:

Standards group insulating materials into classes to simplify temperature rise calculations and life estimates (e.g., Arrhenius-type ageing). The conventional ordering from lowest to highest thermal endurance is Y, A, E, B, F, H, C.

Step-by-Step Solution:

Verification / Alternative check:

Manufacturers’ datasheets and machine design handbooks consistently cite the above ordering with approximate temperature ratings corresponding to insulation endurance curves.

Why Other Options Are Wrong:

• Other permutations either omit a class (e.g., D, which is not a standard thermal class) or present classes out of order.

Common Pitfalls:

Confusing thermal classes with dielectric strength classes; assuming “C” stands for a specific fixed temperature rather than “above 180 °C”.

Final Answer:

Y, A, E, B, F, H, C