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The main difference between Class F and Class H is the thermal class: Class F corresponds to 155°C, while Class H corresponds to 180°C. However, when selecting a class h insulation varnish, you should not look at the temperature figure alone. You also need to check your insulation system design, curing process, electrical properties and validation results on the actual workpiece.

IEC 60085 provides a reference for the thermal evaluation and designation of electrical insulation. The thermal class stated in a material TDS does not automatically demonstrate that the complete insulation system of a motor, transformer or other equipment has received the same thermal classification.

This guide compares Changxian E57A, E57-81H and E57-89H to help you narrow the selection for Class B–F, Class H and Class H–C requirements.

1. What Do Class F and Class H Mean?

The common thermal classes can be understood using the following table based on the IEC 60085 thermal classification:

Authoritative reference: IEC 60085 official page.

These temperature classes should not be interpreted simply as the permissible operating temperature of every part of the equipment. Short-term temperature resistance, thermal aging results and equipment temperature-rise limits should also not be treated as interchangeable. Base your selection on the insulation system design and applicable standards.

2. Is a Class H Product Always More Suitable Than a Class F Product?

Not necessarily. A class h insulation varnish provides a candidate route for a higher thermal class, but its suitability for your workpiece also depends on the following conditions:

1. Whether the insulation system actually requires Class H.
2. Whether your existing impregnation and curing equipment matches the product process.
3. Whether the conductors and other insulation materials are compatible with the candidate varnish.
4. Whether the electrical properties meet your design requirements.
5. Whether the actual workpiece has completed process and reliability validation.

If your design requirement is Class F, switching directly to a Class H product does not automatically resolve incomplete impregnation, insufficient curing or other process problems. Conversely, if the design requires Class H, you should not overlook the temperature-class difference merely because a Class F product has higher values in other specifications.

3. How Do the Thermal Classes and Processes of the 3 Options Differ?

E57A: A Class B–F Baking Option

Resin system: Alkyd-amino.
Thermal class: Class B–F, 130–155°C.
Primary process: Baking.
Key specifications: Dielectric strength ≥60 kV/mm, solid content 45–55%, rotational viscosity approximately 150–235 mPa·s, and SG 0.98.
TDS applications: Small and medium-sized transformers, small and medium-sized electric motors, and other electrical products.
Selection guidance: When your design requirement is Class B–F and baking facilities are available, E57A can be included as a candidate. The dilution ratio and baking time should be determined through testing on the actual product.

E57-81H: A Class H Unsaturated Polyester Option

Resin system: Unsaturated polyester.
Thermal class: Class H.
Key specification: Dielectric strength ≥22 MV/m.
Confirmed performance: Excellent performance in the 96-hour refrigerant resistance test.
Selection guidance: When your design requires Class H and the application requires an evaluation of refrigerant resistance, E57-81H can be included in your assessment. The specific process and other unlisted parameters should be confirmed using the complete TDS and actual testing.

E57-89H: A Class H–C Solventless Impregnating Option

Resin system: Solventless epoxy-polyester.
Thermal class: Class H–C, ≥180°C.
Active ingredient content: 100%.
Primary process: Curing after vacuum impregnation or trickle impregnation.
Key specifications: Dielectric strength ≥24 MV/m; bonding strength ≥140 N at room temperature and ≥50 N at 180°C; thermal conductivity ≥0.4 W/m·K; rotational viscosity approximately 165–355 mPa·s; and SG 1.075.
TDS applications: These include small and medium-sized round-/flat-wire coils and reactor windings.
Selection guidance: When you require Class H–C, solventless impregnation, or data for bonding strength at elevated temperatures or thermal conductivity, E57-89H can be prioritized for validation.

4. Class H Insulation Varnish Specification Comparison

This class h insulation varnish comparison includes only items confirmed in the respective TDS documents. Items not listed for a particular grade are marked “—” rather than estimated from the resin chemistry.

Unit note: 1 kV/mm is numerically equivalent to 1 MV/m. However, the test conditions in different TDS documents should still be reviewed separately. Dielectric strength values should not determine selection independently of the application and process.

5. How Should You Select a Class H Insulation Varnish by Design Requirement?

Your class h insulation varnish selection should start with the required thermal class and then move to application and process compatibility.

When the Design Requires Class F

First confirm whether E57A covers your application and baking conditions. Its Class B–F range provides a candidate route for Class F requirements, but impregnation, curing and electrical performance still need to be validated on your actual workpiece.

When the Design Requires Class H

You can further screen E57-81H and E57-89H:

• When refrigerant resistance requires evaluation, focus on E57-81H.
• When you need solventless impregnation or data for bonding strength at elevated temperatures or thermal conductivity, focus on E57-89H.
• If your current process uses vacuum impregnation or trickle impregnation, the E57-89H TDS provides a more direct process route.

When the Design Requirement Exceeds Class H

Do not rely only on the words “Class H.” Continue to check the specific TDS range and insulation system requirements. E57-89H is classified as Class H–C, but its actual suitability still needs to be determined through evaluation of the complete system and testing on the workpiece.

6. Why Should Curing Conditions Not Be Checked Last?

When selecting a class h insulation varnish, the thermal class and curing process should be evaluated together.

E57A is a baking option. Its TDS requires the dilution ratio and baking time to be determined through testing according to the product’s shape, dimensions and requirements. E57-89H provides procedures for vacuum impregnation and trickle impregnation, but its impregnation and curing times also need to be determined according to the actual workpiece.

If your equipment cannot complete the process required by a candidate product, the product cannot be treated as production-ready even if its temperature class meets your requirement. For air-drying or other curing methods, surface dryness should not replace confirmation of complete curing.

7. What Should You Record During Sample Validation?

Use the following checklist to retain validation records for the actual workpiece:

1. Workpiece type, dimensions and winding structure.
2. Target thermal class and applicable insulation system requirements.
3. Product grade and sample batch.
4. Application viscosity.
5. Impregnation, draining and curing conditions.
6. Appearance and coverage.
7. Electrical performance tests.
8. Additional bonding, thermal conductivity, refrigerant resistance or other tests required by your design.
9. Process confirmation on production equipment.

The material TDS helps you establish a candidate range. Validation on the actual workpiece confirms whether that candidate can be converted into stable production conditions.

8. Related Selection Guides

Motor Winding Insulating Varnish Selection Guide

Transformer Insulating Varnish Selection Guide

Coil Winding Varnish Guide

Copper Wire Insulation Varnish Guide

Electrical Insulating Varnish Manufacturers Guide

Insulating Varnish Price Guide

9. Frequently Asked Questions

Q1: What temperatures correspond to Class F and Class H?

Class F corresponds to 155°C, while Class H corresponds to 180°C. These classes should be understood in conjunction with IEC 60085 and your actual insulation system design. They should not be treated directly as the permissible operating temperature of every part of the equipment.

Q2: Can a Class H insulating varnish be used directly in a Class F insulation system?

Technical and process evaluation is required. A class h insulation varnish provides a higher thermal class, but you still need to check its application scope, material compatibility, impregnation process, curing conditions and performance on the actual workpiece.

Q3: Can E57A be used in a Class H design?

The E57A TDS specifies Class B–F. It should therefore not be presented as a Class H option on that basis. A Class H design should evaluate candidates with the corresponding Class H designation.

Q4: How should you distinguish E57-81H from E57-89H?

E57-81H is a Class H unsaturated polyester option with confirmed excellent performance in the 96-hour refrigerant resistance test. E57-89H is a Class H–C solventless epoxy-polyester option that provides bonding strength data at elevated temperatures and thermal conductivity data, together with explicit vacuum and trickle impregnation processes.

Q5: Does a higher thermal class automatically mean higher dielectric strength?

No. Thermal class and dielectric strength are different evaluation items. Review them separately in the TDS rather than inferring one from the other.

10. Conclusion

When selecting a class h insulation varnish, first confirm whether your design requires Class F, Class H or a higher thermal class. Then compare the resin system, curing process and TDS specifications. E57A provides a Class B–F baking candidate, E57-81H offers a Class H unsaturated polyester route, and E57-89H covers Class H–C solventless impregnating requirements.

If you are unsure whether to select a Class F or Class H option, send our engineers your application, target thermal class, current impregnation process and curing conditions. We can provide the corresponding TDS documents, grade comparisons and free samples for testing.

About Changxian

Zhuhai Changxian New Material Technology Co., Ltd. has specialized in electronic chemicals for more than 30 years. Changxian is listed on China’s National Equities Exchange and Quotations (stock code: 831323) and recognized as a national-level Specialized, Sophisticated, Distinctive and Innovative “Little Giant” enterprise. Our approximately 40,000 m² production base in Zhuhai’s Gaolan Port area includes in-house resin synthesis capabilities. We have achieved ISO 9001, ISO 14001 and ISO 45001 management system certification, together with IATF 16949 certification. Relevant insulating varnish and conformal coating products are covered by UL files; specific grade coverage is subject to the applicable certification.

Changxian’s position is straightforward: specifications benchmarked item by item against international brands, with factory-direct pricing — Equal Quality, Factory Price. If you need TDS documents for Class F or Class H insulating varnishes, grade comparisons or free samples for testing, contact our engineers. We will provide targeted support based on your insulation system and production conditions.

Source basis for this article: The official IEC 60085 page, product TDS documents for E57A, E57-81H and E57-89H, and confirmed technical records. Test items not listed in the applicable TDS are marked “—” and have not been estimated.