Insulation Class (A/E/B/F/H)
Standardized temperature ratings for transformer insulation materials defining maximum operating temperatures.
Insulation classes define maximum continuous temperature: Class A (105C, oil-immersed); E (120C); B (130C); F (155C, most dry-type); H (180C); C (220C).
Determines permissible temperature rise. Class F in 40C ambient = 115K max winding rise. Operating above rated temperature halves life every 6-10C.
Essential for high-temperature industrial or high-altitude selection.
QDTB offers all classes: Class F standard for dry-type, optimized Class A for oil-immersed.
Related Terms
Temperature Rise
The temperature increase of transformer components above ambient at rated load, a key design parameter.
Hot-Spot Temperature
The highest temperature within transformer winding insulation, determining aging rate and loading capability.
Dry-Type Transformer
A transformer using air or solid insulation instead of liquid, ideal for indoor applications with fire safety priority.
Dielectric Strength
The maximum electric field an insulating material can withstand before breakdown, measured in kV/mm.
More Terms Starting with “I”
Impedance (%Z)
Total opposition to current flow in a transformer as a percentage, determining fault current and voltage regulation.
Inrush Current
A high transient current when a transformer is first energized, caused by core saturation, reaching 10-30x rated.
IEC 60076
The IEC standard series defining requirements for power transformers, the global reference standard.
IEEE C57
The IEEE standard series for transformers, the primary reference for the North American market.
More in “Insulation & Testing”
BIL (Basic Impulse Level)
The ability of transformer insulation to withstand high-voltage surges from lightning or switching, measured in kV peak.
Dielectric Strength
The maximum electric field an insulating material can withstand before breakdown, measured in kV/mm.
Lightning Impulse Test
A high-voltage test simulating lightning surges to verify transformer insulation withstands transient overvoltages.
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