ASTM D3955-20

Standard Specification for Electrical Insulating Varnishes

ABSTRACT
This specification covers the tests and values for electrical insulating varnishes, as supplied, which are suitable for the impregnation and treatment of electrical coils and windings applied by dip process. The varnishes are classified according to grades: Grade DA; Grade DO; Grade DM; Grade DS; Grade DW; and Grade DT. This specification covers both flexible and rigid, solvent and solventless insulating varnishes. The insulating varnishes are classified according to thermal classes: Class 130; Class 155; Class 180; Class 200; and Class 220. Quality conformance tests shall be performed to conform to the specified requirements.
SCOPE
1.1 This specification covers the tests and values for electrical insulating varnishes, as supplied, which are suitable for the impregnation and treatment of electrical coils and windings applied by dip process.  
1.2 Varnishes, flexible or rigid, included in this specification are:
Grade DA—Air-dry
Grade DO—Organic solvent containing, baking,
Grade DM—Reactive diluent containing,  
Grade DS—Silicone,
Grade DW—Water containing, and
Grade DT—Thixotropic.  
1.3 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.
Note 1: This specification resembles IEC 60455 in title only. The content is significantly different.  
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Jul-2020
• ICS: 29.035.60 - Varnished fabrics
• Technical Committee: D09 - Electrical and Electronic Insulating Materials
• Drafting Committee: D09.01 - Electrical Insulating Products

Relations

• Replaces: ASTM D3955-13 - Standard Specification for Electrical Insulating Varnishes
• Effective Date: 01-Aug-2020
• Refers: ASTM D1711-24 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-Mar-2024
• Refers: ASTM D115-17(2024) - Standard Test Methods for Testing Solvent Containing Varnishes Used for Electrical Insulation
• Effective Date: 01-Mar-2024
• Refers: ASTM D3056-23 - Standard Test Method for Gel Time of Solventless Varnishes
• Effective Date: 01-Oct-2023
• Refers: ASTM D93-20 - Standard Test Methods for Flash Point by Pensky-Martens Closed Cup Tester
• Effective Date: 01-Aug-2020
• Refers: ASTM D3145-20 - Standard Test Method for Thermal Endurance of Electrical Insulating Varnishes by Helical Coil Method
• Effective Date: 01-Aug-2020
• Refers: ASTM D2519-19a - Standard Test Method for Bond Strength of Electrical Insulating Varnishes by the Helical Coil Test
• Effective Date: 01-Nov-2019
• Refers: ASTM D2519-19 - Standard Test Method for Bond Strength of Electrical Insulating Varnishes by the Helical Coil Test
• Effective Date: 01-Mar-2019
• Refers: ASTM D3251-19 - Standard Test Method for Thermal Endurance Characteristics of Electrical Insulating Varnishes Applied Over Film-Insulated Magnet Wire
• Effective Date: 01-Jan-2019
• Refers: ASTM D3056-14(2018) - Standard Test Method for Gel Time of Solventless Varnishes
• Effective Date: 01-Nov-2018
• Refers: ASTM D3145-18 - Standard Test Method for Thermal Endurance of Electrical Insulating Varnishes by the Helical Coil Method
• Effective Date: 01-Nov-2018
• Refers: ASTM D4880-18 - Standard Test Method for Salt Water Proofness of Insulating Varnishes Over Enamelled Magnet Wire
• Effective Date: 01-Nov-2018
• Refers: ASTM D5638-18 - Standard Test Method for Chemical Resistance of Electrical Insulating Varnishes
• Effective Date: 01-Nov-2018
• Refers: ASTM D2519-18 - Standard Test Method for Bond Strength of Electrical Insulating Varnishes by the Helical Coil Test
• Effective Date: 01-Nov-2018
• Refers: ASTM D5637-05(2017) - Standard Test Method for Moisture Resistance of Electrical Insulating Varnishes
• Effective Date: 01-Nov-2017

Overview

ASTM D3955-20 is the internationally recognized standard specification for electrical insulating varnishes. Published by ASTM International, this standard outlines the critical performance requirements and classification criteria for varnishes used in the impregnation and treatment of electrical coils and windings via the dip process. The specification covers a wide range of varnish types, including both flexible and rigid, as well as solvent-based, solventless, and water-containing formulations. With classifications by thermal class and product grade, ASTM D3955 helps ensure reliable electrical insulation performance in various electrical and electronic applications.

Key Topics

• Varnish Types and Grades
  The standard defines several varnish grades, each suitable for specific application requirements:

  • Grade DA - Air-dry
  • Grade DO - Organic solvent containing, baking
  • Grade DM - Reactive diluent containing
  • Grade DS - Silicone-based
  • Grade DW - Water-containing
  • Grade DT - Thixotropic

• Thermal Classification
  Varnishes are classified by their thermal endurance using standardized testing methods. The following thermal classes are specified:

  • Class 130, 155, 180, 200, and 220
  • Suitability is established based on key characteristics such as thermal index and dielectric performance.

• Quality Conformance and Performance Testing
  Varnishes must pass rigorous quality tests, including:

  • Dielectric strength
  • Bond strength
  • Saltwater resistance
  • Moisture and chemical resistance
  • Viscosity, nonvolatile content, and storage life

• Sampling and Safety
  Specific instructions are provided for obtaining representative samples, as well as handling, storage, and safety precautions, especially regarding flash points and safe use.

Applications

ASTM D3955-20 is widely used across the electrical and electronics industry for defining and procuring insulating varnishes that meet high performance and safety standards. Typical uses include:

• Impregnation of Electrical Coils and Windings
  Trusted for use in transformers, electric motors, generators, and other apparatus requiring enhanced insulation and protection.

• Enhancement of Electrical, Mechanical, and Environmental Resistance
  Varnishes specified under ASTM D3955 provide critical protection against electrical breakdown, moisture ingress, chemical attack, and mechanical stresses.

• Manufacturing and Maintenance
  The standard’s rigorous criteria support the consistent production of high-quality varnishes and reliable, long-term equipment performance in manufacturing, repair, and maintenance operations.

• Custom Formulation and Procurement
  The defined grades and performance criteria facilitate clear communication between suppliers and users for selection and qualification of insulating varnishes.

Related Standards

Users of ASTM D3955-20 may also reference the following standards for complementary information and testing procedures:

• ASTM D93 - Test Methods for Flash Point by Pensky-Martens Closed Cup Tester
• ASTM D115 - Test Methods for Testing Solvent Containing Varnishes Used for Electrical Insulation
• ASTM D2519 - Test Method for Bond Strength of Electrical Insulating Varnishes
• ASTM D3056 - Test Method for Gel Time of Solventless Varnishes
• ASTM D3145, D3251 - Test Methods for Thermal Endurance
• ASTM D4733 - Test Methods for Solventless Electrical Insulating Varnishes
• ASTM D4880 - Test Method for Salt Water Proofness
• ASTM D5637, D5638 - Moisture and Chemical Resistance Testing
• IEC 60455 - Resin Based Compounds for Electrical Insulation (note: differs in content from ASTM D3955)
• NEMA MW1000 - Magnet Wire Standard

Practical Value

By specifying rigorous test criteria and clear classification schemes, ASTM D3955-20 enables manufacturers, suppliers, and users of electrical equipment to ensure that their insulating varnishes deliver the required thermal, electrical, and chemical performance. Compliance with this standard helps improve safety, equipment longevity, and product consistency across industries relying on high-quality electrical insulation.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation:D3955 −20
Standard Specification for
Electrical Insulating Varnishes
This standard is issued under the fixed designation D3955; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope* D2519 Test Method for Bond Strength of Electrical Insulat-
ing Varnishes by the Helical Coil Test
1.1 This specification covers the tests and values for elec-
D3056 Test Method for Gel Time of Solventless Varnishes
trical insulating varnishes, as supplied, which are suitable for
D3145 Test Method for Thermal Endurance of Electrical
the impregnation and treatment of electrical coils and windings
Insulating Varnishes by the Helical Coil Method
applied by dip process.
D3251 Test Method for Thermal Endurance Characteristics
1.2 Varnishes, flexible or rigid, included in this specification
of Electrical Insulating Varnishes Applied Over Film-
are:
Insulated Magnet Wire
Grade DA—Air-dry
D3278 Test Methods for Flash Point of Liquids by Small
Grade DO—Organic solvent containing, baking,
Scale Closed-Cup Apparatus
Grade DM—Reactive diluent containing,
D4733 Test Methods for Solventless Electrical Insulating
Grade DS—Silicone,
Varnishes
Grade DW—Water containing, and
D4880 Test Method for Salt Water Proofness of Insulating
Grade DT—Thixotropic.
Varnishes Over Enamelled Magnet Wire
1.3 The values stated in inch-pound units are to be regarded D5637 Test Method for Moisture Resistance of Electrical
Insulating Varnishes
as standard. The values given in parentheses are mathematical
conversions to SI units that are provided for information only D5638 Test Method for Chemical Resistance of Electrical
Insulating Varnishes
and are not considered standard.
2.2 Military Specifications:
NOTE 1—This specification resembles IEC 60455 in title only. The
MIL-PRF-17672 Hydraulic Fluid, Petroleum, Inhibited
content is significantly different.
MIL-PRF-17331 Lubricating Oil, Synthetic Base P-D-680,
1.4 This international standard was developed in accor-
Dry Cleaning Solvent
dance with internationally recognized principles on standard-
MIL-D-16791 Detergent, General Purpose, (Liquid, Non-
ization established in the Decision on Principles for the
Ionic)
Development of International Standards, Guides and Recom-
2.3 Other Standards:
mendations issued by the World Trade Organization Technical
NEMA MW1000 Magnet Wire
Barriers to Trade (TBT) Committee.
IEC 60455 Resin Based Reactive Compounds Used for
2. Referenced Documents
Electrical Insulation
2.1 ASTM Standards:
3. Terminology
D93 Test Methods for Flash Point by Pensky-Martens
Closed Cup Tester 3.1 Definitions:
D115 Test Methods for Testing Solvent Containing Var- 3.1.1 For definitions of terms used in this specification refer
nishes Used for Electrical Insulation to Terminology D1711.
D1711 Terminology Relating to Electrical Insulation 3.1.2 varnish, electrical insulating, n—a liquid resin system
that is applied to and cured on electrical components providing
electrical, mechanical, and environmental protection.
This specification is under the jurisdiction of ASTM Committee D09 on
3.1.2.1 Discussion—There are two types of electrical insu-
Electrical and Electronic Insulating Materials and is the direct responsibility of
lating varnish: solvent-containing and solventless.The solvent-
Subcommittee D09.01 on Electrical Insulating Products.
Current edition approved Aug. 1, 2020. Published September 2020. Originally containing varnish is a solution, dispersion, or emulsion of a
approved in 1980. Last previous edition approved in 2013 as D3955 – 13. DOI:
polymer or mixture of polymers in a volatile, nonreactable
10.1520/D3955-20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on Available from Global Engineering Documents, 15 Inverness Way, East
the ASTM website. Englewood, CO 80112-5704, http://www.global.ihs.com.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D3955−20
liquid. The solventless type is a liquid resin system free of be performed in accordance with the appropriate methods, and
volatile, nonreactable solvents. limits as shown in Table 1.
6.8 Preconditioning: Grade DA Varnishes Only—
4. Flexible or Rigid Classification
Precondition all Grade DA varnish specimens, both flexible
4.1 This specification covers both flexible and rigid, solvent
and rigid, in air at room temperature for seven days prior to
(including water) and solventless insulating varnishes.
carrying out dielectric strength and bond strength tests shown
in Table 1.
5. Thermal Classification
5.1 The thermal classification of insulating varnishes cov-
7. Hazards
ered by this specification is determined by using Test Methods
7.1 It is unsafe to use varnish at temperatures above the
D3145 and D3251 in conjunction with 18 AWG magnet wire
flash point without adequate ventilation, especially if the
conforming to MW 35-C and MW 16-C in accordance with
possibility exists that flames or sparks are present. Store
NEMA MW1000. Determine the temperature index at
varnish in sealed containers.
20 000 h.
5.2 The thermal class is determined from the temperature
8. Sampling
index range as follows:
8.1 Obtain varnish samples and appropriate thinner either
Thermal Class Temperature Index Range
130 130.0 to 154.9
from the manufacturer or directly from shipping containers
155 155.0 to 179.9
supplied from the manufacturer. It is also acceptable to obtain
180 180.0 to 199.9
suitable samples from material in process. It is important that
200 200.0 to 219.9
220 220.0 and above
new, or thoroughly cleaned sampling containers be used. It is
acceptable to obtain suitable samples by any of a number of
6. General Specifications and Qualification Requirements
procedures commonly used in the industry. Normally,
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D3955 − 13 D3955 − 20
Standard Specification for
Electrical Insulating Varnishes
This standard is issued under the fixed designation D3955; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope*
1.1 This specification covers the tests and values for electrical insulating varnishes, as supplied, which are suitable for the
impregnation and treatment of electrical coils and windings applied by dip process.
1.2 Varnishes, flexible or rigid, included in this specification are:
Grade DA—Air-dry
Grade DO—Organic solvent containing, baking,
Grade DM—Reactive diluent containing,
Grade DS—Silicone,
Grade DW—Water containing, and
Grade DT—Thixotropic.
1.3 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical
conversions to SI units that are provided for information only and are not considered standard.
NOTE 1—This specification resembles IEC 60455 in title only. The content is significantly different.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D93 Test Methods for Flash Point by Pensky-Martens Closed Cup Tester
D115 Test Methods for Testing Solvent Containing Varnishes Used for Electrical Insulation
D1711 Terminology Relating to Electrical Insulation
D2519 Test Method for Bond Strength of Electrical Insulating Varnishes by the Helical Coil Test
D3056 Test Method for Gel Time of Solventless Varnishes
D3145 Test Method for Thermal Endurance of Electrical Insulating Varnishes by Helical Coil Method
D3251 Test Method for Thermal Endurance Characteristics of Electrical Insulating Varnishes Applied Over Film-Insulated
Magnet Wire
This specification is under the jurisdiction of ASTM Committee D09 on Electrical and Electronic Insulating Materials and is the direct responsibility of Subcommittee
D09.01 on Electrical Insulating Varnishes, Powders and Encapsulating CompoundsProducts.
Current edition approved April 1, 2013Aug. 1, 2020. Published April 2013September 2020. Originally approved in 1980. Last previous edition approved in 20092013 as
D3955–04 (2009). – 13. DOI: 10.1520/D3955-13.10.1520/D3955-20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D3955 − 20
D3278 Test Methods for Flash Point of Liquids by Small Scale Closed-Cup Apparatus
D4733 Test Methods for Solventless Electrical Insulating Varnishes
D4880 Test Method for Salt Water Proofness of Insulating Varnishes Over Enamelled Magnet Wire
D5637 Test Method for Moisture Resistance of Electrical Insulating Varnishes
D5638 Test Method for Chemical Resistance of Electrical Insulating Varnishes
2.2 Military Specifications:
MIL-PRF-17672 Hydraulic Fluid, Petroleum, Inhibited
MIL-PRF-17331 Lubricating Oil, Synthetic Base P-D-680, Dry Cleaning Solvent
MIL-D-16791 Detergent, General Purpose, (Liquid, Non-Ionic)
2.3 Other Standards:
NEMA MW1000 Magnet Wire
IEC 60455 Resin Based Reactive Compounds Used for Electrical Insulation
3. Terminology
3.1 Definitions—For definitions of terms used in this specification refer to Terminology D1711.
3.1 Definitions:
3.1.1 For definitions of terms used in this specification refer to Terminology D1711.
3.1.2 varnish, electrical insulating, n—a liquid resin system that is applied to and cured on electrical components providing
electrical, mechanical, and environmental protection.
3.1.2.1 Discussion—
There are two types of electrical insulating varnish: solvent-containing and solventless. The solvent-containing varnish is a
solution, dispersion, or emulsion of a polymer or mixture of polymers in a volatile, nonreactable liquid. The solventless type is
a liquid resin system free of volatile, nonreactable solvents.
4. Flexible or Rigid Classification
4.1 This specification covers both flexible and rigid, solvent (including water) and solventless insulating varnishes.
5. Thermal Classification
5.1 The thermal classification of insulating varnishes covered by this specification is determined by using Test Methods D3145
and D3251 in conjunction with 18 AWG magnet wire conforming to MW 35-C and MW 16-C in accordance with NEMA
MW1000. Determine the temperature index at 20 000 h.20 000 h.
5.2 The thermal class is determined from the temperature index range as follows:
Thermal Class Temperature Index Range
130 130.0 to 154.9
155 155.0 to 179.9
180 180.0 to 199.9
200 200.0 to 219.9
220 220.0 and above
6. General Specifications and Qualification Requirements
6.1 All varnishes supplied under this specification must conform to the limits outlined in Table 1, and must meet, or exceed, all
specification requirements.
6.2 A varnish supplied under this specification is to be manufactured from one specific formula and one specific process at one
or more plants of the same company.
6.3 Changes involved solely with percent nonvolatile content or compatible solvent system, do not require requalification, if
agreed upon between supplier and user.
Available from Global Engineering Documents, 15 Inverness Way, East Englewood, CO 80112-5704, http://www.global.ihs.com.
D3955 − 20
TABLE 1 Specification Requirements
Grade Minimum Thermal Class
ASTM
Type of Magnet Wire
Test Method
Class Over DA DO DM DS DW DT
which Varnish is Applied
130 MW 35-C and MW 16-C D3251 and D3145 Class 130 for both wire types and both test methods
155 MW 35-C and MW 16-C D3251 and D3145 Class 155 for both wire types and both test methods
180 MW 35-C and
...