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What is the maximum temperature limit of class E insulation?
120°C
180°C
50°C
90°C
120°C
According to the IS 1271 (International Standard IEC 60085), insulation materials are classified into thermal classes based on their maximum operating temperatures. Class E insulation is assigned a maximum safe operating temperature limit of 120°C.
According to the IS 1271 (International Standard IEC 60085), insulation materials are classified into thermal classes based on their maximum operating temperatures. Class E insulation is assigned a maximum safe operating temperature limit of 120°C.
Tmax=120°C — Maximum permissible temperature for Class E
The insulation class defines the thermal endurance of the electrical insulating material used in motors, transformers, and other electrical equipment. Operating beyond this temperature threshold leads to the chemical degradation of the insulating material, significantly reducing the lifespan of the insulation and risking dielectric breakdown.
Class Y: 90°C
Class A: 105°C
Class E: 120°C
Class B: 130°C
Class F: 155°C
Class H: 180°C
Class C: >180°C
Provides moderate thermal stability for general purpose motors
Balanced cost-to-performance ratio compared to lower classes
Cannot withstand high overload temperatures compared to Class F or H
Susceptible to faster degradation in high-humidity or contaminated environments
Small to medium induction motors
Low-voltage distribution transformers
Stator winding insulation in industrial electrical machinery
Option B (180°C) corresponds to Class H insulation.
Option D (90°C) corresponds to Class Y insulation.
These classifications are critical for ensuring the longevity of windings under standard duty cycles.
A is correct — Class E insulation is rated for a maximum operating temperature of 120°C under the international insulation classification system.
Memorize the sequence 'Y-A-E-B-F-H-C' with temperature intervals (90, 105, 120, 130, 155, 180, >180) as this is a frequently asked table in competitive engineering exams.