ASTM D374/D374M-23

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: D374/D374M − 23
Standard Test Methods for
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Thickness of Solid Electrical Insulation
This standard is issued under the fixed designation D374/D374M; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope 3.2 Definitions of Terms Specific to This Standard:
3.2.1 1 micron, μm, n—a dimension equivalent to
1.1 These test methods cover the determination of the
0.03937 mils.
thickness of several types of solid electrical insulating materi-
3.2.2 1 mil, n—a dimension equivalent to 0.0010 in.
als employing recommended techniques. Use these test meth-
ods except as otherwise required by a material specification.
3.2.3 absolute uncertainty (of a measurement), n—the
smallest division able to be read directly on the instrument used
1.2 The values stated in either SI units or inch-pound units
for measurement.
are to be regarded separately as standard. The values stated in
each system may not be exact equivalents; therefore, each
3.2.4 micrometer, n—an instrument for measuring any di-
system shall be used independently of the other. Combining
mension with absolute uncertainty of 1 mil [25 μm] or smaller.
values from the two systems may result in non-conformance
with the standard.
4. Summary of Test Methods
1.3 This standard does not purport to address all of the
4.1 This standard provides eight different test methods for
safety concerns, if any, associated with its use. It is the
the measurement of thickness of solid electrical insulation
responsibility of the user of this standard to establish appro-
materials. The test methods (identified as Test Methods A
priate safety, health, and environmental practices and deter-
through H) employ different micrometers that exert various
mine the applicability of regulatory limitations prior to use.
pressures for varying times upon specimens of different geom-
1.4 This international standard was developed in accor-
etries. Tables 1 and 2 display basic differences of each test
dance with internationally recognized principles on standard-
method and identify test methods applicable for use on various
ization established in the Decision on Principles for the
categories of materials.
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
5. Significance and Use
Barriers to Trade (TBT) Committee.
5.1 Some electrical properties, such as dielectric strength,
vary with the thickness of the material. Determination of
2. Referenced Documents
certain properties, such as relative permittivity (dielectric
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2.1 ASTM Standards:
constant) and volume resistivity, usually require a knowledge
D618 Practice for Conditioning Plastics for Testing
of the thickness. Design and construction of electrical machin-
D1711 Terminology Relating to Electrical Insulation
ery require that the thickness of insulation be known.
E252 Test Method for Thickness of Foil, Thin Sheet, and
Film by Mass Measurement
6. Apparatus
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3. Terminology
6.1 Apparatus A—Machinist’s Micrometer Caliper with
3.1 Refer to Terminology D1711 for definitions pertinent to Calibrated Ratchet or Friction Thimble:
6.1.1 Apparatus A is a micrometer caliper without a locking
this standard.
device but is equipped with either a calibrated ratchet or a
1 friction thimble. By use of a proper manipulative procedure
These test methods are under the jurisdiction of ASTM Committee D09 on
Electrical and Electronic Insulating Materials and are the direct responsibility of
and a calibrated spring (see Annex A1), the pressure exerted on
Subcommittee D09.12 on Electrical Tests.
the specimen is controllable.
Current edition approved Oct. 1, 2023. Published October 2023. Originally
6.1.2 Use an instrument constructed with a vernier capable
approved in 1933. Last previous edition approved in 2016 as D374/D374M – 16.
of measurement to the nearest 0.1 mil [2 μm].
DOI: 10.1520/D0374_D0374M-23.
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For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
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the ASTM website. Hereinafter referred to as a machinist’s micrometer.
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