Thermal classes of solid electrical insulation — highest temperature limit Which insulation thermal class has the highest permissible temperature limit?

Introduction / Context:
Electrical insulation systems are categorized into thermal classes based on their maximum permissible operating temperatures. Knowing these classes is important for selecting materials for transformers, motors, and other equipment.

Given Data / Assumptions:

• Standard thermal classes include E (120°C), B (130°C), F (155°C), H (180°C), and C (> 180°C, typically unspecified upper bound).
• We seek the class with the highest temperature capability among the given options.

Concept / Approach:
Class C historically denotes materials capable of operation at temperatures above 180°C (often including mica, ceramics, glass with suitable binders). Classes E, F, and H have lower specified limits. Therefore, Class C provides the highest temperature limit.

Step-by-Step Solution:
List the typical limits: E ≈ 120°C, F ≈ 155°C, H ≈ 180°C.Class C: greater than 180°C.Therefore, select Class C as the highest temperature class among the choices.Confirm that no option exceeds C in the set provided.

Verification / Alternative check:
Standards (IEC/NEMA) summarize these temperature indices similarly; modern materials may be assigned specific thermal indices, but C remains the highest generic class.

Why Other Options Are Wrong:
E, F, and H have lower maximum temperatures than Class C by definition.

Common Pitfalls:
Confusing thermal class letters with insulation “types”; assuming Class H is the highest because it is commonly used in motor ratings.

Final Answer:
C