Electrical insulation classes – impregnated paper Impregnated paper (e.g., paper with suitable insulating oil/varnish impregnation) belongs to which standard insulation class?

Introduction:
Insulation classes categorize materials by their maximum permissible temperature rise and operating temperature, guiding selection in transformers, motors, and other electrical apparatus. The class determines thermal endurance and safe design margins for long-term reliability.

Given Data / Assumptions:

• Impregnated paper as a composite of cellulose and impregnating agent.
• Conventional/classical temperature classes: A, E, B, F, H, etc.
• Standardized temperature limits per class.

Concept / Approach:

Cellulose-based materials such as cotton, silk, and paper with suitable impregnation are traditionally placed in Class A insulation. Class A typically corresponds to a maximum permissible temperature of about 105 °C (sum of ambient plus allowable rise and hot-spot). Higher classes (B ≈ 130 °C, F ≈ 155 °C, H ≈ 180 °C) are for more thermally robust materials like mica, glass, and certain synthetic films/resins.

Step-by-Step Solution:

Verification / Alternative check:

Transformer design references consistently list paper–oil systems under Class A, confirming the classification.

Why Other Options Are Wrong:

Classes B, F, H imply higher thermal endurance materials; Class C historically referred to mica/ceramics with very high temperature capability.

Common Pitfalls:

Assuming all impregnated materials jump to higher classes; ignoring that the base cellulose still limits thermal endurance.

Final Answer:

A