Thermal class limits for electrical insulation systems According to standard insulation classes used in electrical machines and equipment, what is the maximum permissible operating temperature for Class B insulation?

Introduction / Context:
Insulation classes (A, B, F, H, etc.) define the thermal endurance and maximum operating temperatures for insulating materials used in transformers, motors, and generators. Selecting the appropriate class is essential to ensure reliability and life expectancy under thermal stresses.

Given Data / Assumptions:

• Standard thermal classes are considered (e.g., A, B, F, H).
• We seek the maximum continuous operating temperature (hot-spot limit) for Class B.
• Ambient and temperature rise components combine to stay within this limit in design calculations.

Concept / Approach:

Typical standardized limits: Class A = 105°C, Class B = 130°C, Class F = 155°C, Class H = 180°C. These values guide machine sizing, cooling requirements, and permissible temperature rises for the windings and core.

Step-by-Step Solution:

Verification / Alternative check:

Manufacturers’ datasheets and standards consistently list 130°C for Class B systems. Derivations in thermal life models (Arrhenius-type aging) also reference these nominal limits.

Why Other Options Are Wrong:

90°C and 105°C correspond to lower classes or specific components; 120°C is not a standard class limit; 155°C corresponds to Class F, not Class B.

Common Pitfalls:

Confusing insulation class with permissible temperature rise; the class is the absolute hot-spot temperature limit, not merely the rise above ambient.

Final Answer:

130°C