ASTM D3251-19

Standard Test Method for Thermal Endurance Characteristics of Electrical Insulating Varnishes Applied Over Film-Insulated Magnet Wire

SIGNIFICANCE AND USE
5.1 Different combinations of varnishes and film-insulated magnet wire behave differently when exposed to elevated temperatures. This includes different varnishes tested with the same film-insulated magnet wire and a single varnish tested with different film-insulated magnet wire.  
5.2 This test method is used to determine the effect on the electrical properties of a varnish applied to film-insulated magnet wire when the combination is exposed to prescribed elevated temperatures.
SCOPE
1.1 This test method covers the determination of the thermal endurance characteristics of electrical insulating varnishes and film-insulated magnet wire in combination.  
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
Note 1: This test method is equivalent to IEC 60172.  
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-Dec-2018
• ICS: 29.035.60 - Varnished fabrics
• Technical Committee: D09 - Electrical and Electronic Insulating Materials
• Drafting Committee: D09.01 - Electrical Insulating Products

Relations

• Replaces: ASTM D3251-04(2013) - Standard Test Method for Thermal Endurance Characteristics of Electrical Insulating Varnishes Applied Over Film-Insulated Magnet Wire
• Effective Date: 01-Jan-2019
• Refers: ASTM D115-17(2024) - Standard Test Methods for Testing Solvent Containing Varnishes Used for Electrical Insulation
• Effective Date: 01-Mar-2024
• Refers: ASTM D1711-24 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-Mar-2024
• Refers: ASTM D115-17 - Standard Test Methods for Testing Solvent Containing Varnishes Used for Electrical Insulation
• Effective Date: 01-Nov-2017
• Refers: ASTM D1711-15 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-Nov-2015
• Refers: ASTM D115-14 - Standard Test Methods for Testing Solvent Containing Varnishes Used for Electrical Insulation
• Effective Date: 01-Nov-2014
• Refers: ASTM D1711-14a - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-Nov-2014
• Refers: ASTM D1711-14 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-May-2014
• Refers: ASTM D1711-13 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-Nov-2013
• Refers: ASTM D115-07(2012) - Standard Test Methods for Testing Solvent Containing Varnishes Used for Electrical Insulation
• Effective Date: 01-Apr-2012
• Refers: ASTM D1711-11a - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-Aug-2011
• Refers: ASTM D1711-08 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-May-2008
• Refers: ASTM D2307-07a - Standard Test Method for Thermal Endurance of Film-Insulated Round Magnet Wire
• Effective Date: 01-Oct-2007
• Refers: ASTM D115-07 - Standard Test Methods for Testing Solvent Containing Varnishes Used for Electrical Insulation
• Effective Date: 01-May-2007
• Refers: ASTM D2307-07 - Standard Test Method for Thermal Endurance of Film-Insulated Round Magnet Wire
• Effective Date: 01-Jan-2007

Overview

ASTM D3251-19, Standard Test Method for Thermal Endurance Characteristics of Electrical Insulating Varnishes Applied Over Film-Insulated Magnet Wire, is an internationally recognized method developed by ASTM International. This standard provides a uniform procedure to determine the thermal endurance properties of electrical insulating varnishes when used in combination with film-insulated magnet wire. The aim is to assess how different varnish and wire pairings behave when subjected to prolonged elevated temperatures. Reliable assessment of thermal endurance is critical for manufacturers and users of electrical components to ensure durability, safety, and consistent performance of electrical insulation systems.

Key Topics

• Thermal Endurance Determination: Specifies procedures for exposing varnished and unvarnished film-insulated magnet wire specimens to prescribed elevated temperatures.
• Performance Comparison: Includes side-by-side testing of coated versus uncoated magnet wire, highlighting the impact varnish has on electrical properties under heat stress.
• Specimen Preparation: Outlines detailed methods for sample varnishing, including solvent and solventless varnishes, as well as the construction of twisted wire pairs for testing.
• Testing Procedures: References specific conditioning, proof testing, and temperature exposure cycles aligned with ASTM D2307.
• Reporting and Analysis: Follows explicit guidance for reporting data and calculating thermal endurance indexes.
• Safety and Handling: Emphasizes safe handling of varnishes, high temperatures, and hazardous voltages, requiring proper grounding and operator training.

Applications

ASTM D3251-19 is essential for multiple industries that rely on high-quality electrical insulation, especially where magnet wire is used:

• Electrical Equipment Manufacturing: Ensures that electrical machines such as transformers, motors, generators, and coils are produced with varnish-wire combinations that offer proven thermal stability.
• Product Development & QC: Supports R&D and quality control labs in evaluating new varnish formulations or film-insulated magnet wires for improved longevity and performance.
• Approval and Certification: Provides a benchmark for certifying magnet wire and varnish systems before market release or regulatory submission.
• Failure Analysis: Assists engineers in identifying causes of insulation breakdown and suitability for specific service environments.

Organizations that implement ASTM D3251-19 can minimize risk of insulation failure, enhance safety, and optimize the lifespan of their electrical systems, making this standard a valuable tool for compliance and product reliability.

Related Standards

For effective thermal endurance testing and a comprehensive insulation assessment, ASTM D3251-19 is used alongside several other key standards:

• ASTM D115 - Test Methods for Testing Solvent Containing Varnishes Used for Electrical Insulation
• ASTM D1711 - Terminology Relating to Electrical Insulation
• ASTM D2307 - Test Method for Thermal Endurance of Film-Insulated Round Magnet Wire
• IEC 60172 - International Equivalent: Test Procedure for Determination of the Temperature Index of Enameled Winding Wires

These standards collectively help standardize vocabulary, sampling, preparation, and evaluation methods for electrical insulating materials, ensuring consistent and internationally accepted results.

Keywords: ASTM D3251-19, thermal endurance, electrical insulating varnishes, film-insulated magnet wire, electrical insulation, magnet wire testing, varnish thermal aging, twisted pair method, insulation reliability, IEC 60172 equivalent.

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: D3251 − 19
Standard Test Method for
Thermal Endurance Characteristics of Electrical Insulating
Varnishes Applied Over Film-Insulated Magnet Wire
This standard is issued under the fixed designation D3251; 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* 3. Terminology
1.1 Thistestmethodcoversthedeterminationofthethermal 3.1 Definitions:
endurance characteristics of electrical insulating varnishes and 3.1.1 For definitions of terms used in this test method refer
film-insulated magnet wire in combination. to Terminology D1711.
1.2 The values stated in SI units are to be regarded as
4. Summary of Test Method
standard. No other units of measurement are included in this
4.1 A varnish is applied to twisted pairs of film-insulated
standard.
magnet wire and exposed to elevated temperatures. Life test
1.3 This standard does not purport to address all of the
results are compared to those of unvarnished twisted pairs.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
5. Significance and Use
priate safety, health, and environmental practices and deter-
5.1 Different combinations of varnishes and film-insulated
mine the applicability of regulatory limitations prior to use.
magnet wire behave differently when exposed to elevated
NOTE 1—This test method is equivalent to IEC 60172.
temperatures. This includes different varnishes tested with the
same film-insulated magnet wire and a single varnish tested
1.4 This international standard was developed in accor-
with different film-insulated magnet wire.
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
5.2 This test method is used to determine the effect on the
Development of International Standards, Guides and Recom-
electrical properties of a varnish applied to film-insulated
mendations issued by the World Trade Organization Technical
magnet wire when the combination is exposed to prescribed
Barriers to Trade (TBT) Committee.
elevated temperatures.
2. Referenced Documents
6. Hazards
2.1 ASTM Standards:
6.1 It is unsafe to use varnish at temperatures above the
D115 Test Methods for Testing Solvent Containing Var-
flash point without adequate ventilation, especially if the
nishes Used for Electrical Insulation
possibility exists that flames or sparks are present. Store
D1711 Terminology Relating to Electrical Insulation
varnish in sealed containers. This also applies to specified
D2307 Test Method for Thermal Endurance of Film-
reagents and solvents. (Warning—Lethal voltages are a po-
Insulated Round Magnet Wire
tential hazard during the performance of this test. It is essential
2.2 IEC Standard:
thatthetestapparatus,andallassociatedequipmentelectrically
IEC 60172 Test Procedure for the determination of the
connected to it, be properly designed and installed for safe
temperature index of enameled winding wires
operation. Solidly ground all electrically conductive parts
which it is possible for a person to contact during the test.
Provide means for use at the completion of any test to ground
This test method is under the jurisdiction of ASTM Committee D09 on
any parts which were at high voltage during the test or have the
Electrical and Electronic Insulating Materials and is the direct responsibility of
Subcommittee D09.01 on Electrical Insulating Products.
potential for acquiring an induced charge during the test or
Current edition approved Jan. 1, 2019. Published January 2019. Originally
retaining a charge even after disconnection of the voltage
approved in 1973. Last previous edition approved in 2013 as D3251 – 04 (2013).
source. Thoroughly instruct all operators as to the correct
DOI: 10.1520/D3251-19.
procedures for performing tests safety. When making high
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
voltage tests, particularly in compressed gas or in oil, it is
Standards volume information, refer to the standard’s Document Summary page on
possiblefortheenergyreleasedatbreakdowntobesufficientto
theASTM website.
result in fire, explosion, or rupture of the test chamber. Design
Available from American National Standards Association, 25 W. 43rd St., 4th
Floor, New York, NY 10036. test equipment, test chambers, and test specimens so as to
*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
D3251 − 19
minimize the possibility of such occurrences and to eliminate 9.2.2.1 Test solventless varnishes in the “as made” condi-
the possibility of personal injury. If the potential for fire exists, tion.
have fire suppression equipment available.) 9.2.2.2 The number of dips used for a solventless varnish is
to be agreed upon between the interested parties. Repo
...

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: D3251 − 04 (Reapproved 2013) D3251 − 19
Standard Test Method for
Thermal Endurance Characteristics of Electrical Insulating
Varnishes Applied Over Film-Insulated Magnet Wire
This standard is issued under the fixed designation D3251; 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 Scope*
1.1 This test method covers the determination of the thermal endurance characteristics of electrical insulating varnishes and
film-insulated magnet wire in combination.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
NOTE 1—This test method is equivalent to IEC 60172.
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:
D115 Test Methods for Testing Solvent Containing Varnishes Used for Electrical Insulation
D1711 Terminology Relating to Electrical Insulation
D2307 Test Method for Thermal Endurance of Film-Insulated Round Magnet Wire
2.2 IEC Standard:
IEC 60172 Test Procedure for the determination of the temperature index of enameled winding wires
3. Terminology
3.1 Definitions:
3.1.1 For definitions of terms used in this test method refer to Terminology D1711.
4. Summary of Test Method
4.1 A varnish is applied to twisted pairs of film-insulated magnet wire and exposed to elevated temperatures. Life test results
are compared to those of unvarnished twisted pairs.
5. Significance and Use
5.1 Different combinations of varnishes and film-insulated magnet wire behave differently when exposed to elevated
temperatures. This includes different varnishes tested with the same film-insulated magnet wire and a single varnish tested with
different film-insulated magnet wire.
5.2 This test method is used to determine the effect on the electrical properties of a varnish applied to film-insulated magnet
wire when the combination is exposed to prescribed elevated temperatures.
This test method 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, 2013Jan. 1, 2019. Published April 2013January 2019. Originally approved in 1973. Last previous edition approved in 20092013 as
D3251 – 04 (2009).(2013). DOI: 10.1520/D3251-04R13.10.1520/D3251-19.
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.
Available from American National Standards Association, 25 W. 43rd St., 4th Floor, New York, NY 10036.
*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
D3251 − 19
6. Hazards
6.1 It is unsafe to use varnish at temperatures above the flash point without adequate ventilation, especially if the possibility
exists that flames or sparks are present. Store varnish in sealed containers. This also applies to specified reagents and solvents.
(Warning—Lethal voltages are a potential hazard during the performance of this test. It is essential that the test apparatus, and
all associated equipment electrically connected to it, be properly designed and installed for safe operation. Solidly ground all
electrically conductive parts which it is possible for a person to contact during the test. Provide means for use at the completion
of any test to ground any parts which were at high voltage during the test or have the potential for acquiring an induced charge
during the test or retaining a charge even after disconnection of the voltage source. Thoroughly instruct all operators as to the
correct procedures for performing tests safety. When making high voltage tests, particularly in compressed gas or in oil, it is
possible for the energy released at breakdown to be sufficient to result in fire, explosion, or rupture of the test chamber. Design
test equipment, test chambers, and test specimens so as to minimize the possibility of such occurrences and to eliminate the
possibility of personal injury. If the potential for fire exists, have fire suppression equipment available.)
7. Sampling
7.1 Sample the varnish in accordance with Te
...