ASTM D374-99
(Test Method)Standard Test Methods for Thickness of Solid Electrical Insulation
Standard Test Methods for Thickness of Solid Electrical Insulation
SCOPE
1.1 These test methods cover the determination of the thickness of several types of solid electrical insulating materials employing recommended techniques. Use these methods except as otherwise required by a material specification.
1.2 The values stated in inch-pound units are the 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 and health practices and determine the applicability of regulatory limitations prior to use.
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An American National Standard
Designation: D 374 – 99
Standard Test Methods for
Thickness of Solid Electrical Insulation
This standard is issued under the fixed designation D 374; 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 (e) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope H) employ different micrometers that exert various pressures
for varying times upon specimens of different geometries.
1.1 These test methods cover the determination of the
Table 1 and Table 2 display basic differences of each test
thickness of several types of solid electrical insulating materi-
method and identify methods applicable for use on various
als employing recommended techniques. Use these methods
categories of materials.
except as otherwise required by a material specification.
1.2 The values stated in inch-pound units are the standard.
5. Significance and Use
1.3 This standard does not purport to address all of the
5.1 Some electrical properties, such as dielectric strength,
safety concerns, if any, associated with its use. It is the
vary with the thickness of the material. Determination of
responsibility of the user of this standard to establish appro-
certain properties, such as relative permittivity (dielectric
priate safety and health practices and determine the applica-
constant) and volume resistivity, usually require a knowledge
bility of regulatory limitations prior to use.
of the thickness. Design and construction of electrical machin-
2. Referenced Documents ery require that the thickness of insulation be known.
2.1 ASTM Standards:
6. Apparatus
D 1711 Terminology Relating to Electrical Insulation
6.1 Apparatus A— Machinist’s Micrometer Caliper with
D 6054 Practice for Conditioning Electrical Insulating Ma-
Calibrated Ratchet or Friction Thimble:
terials for Testing
6.1.1 ApparatusAis a micrometer caliper without a locking
E 252 Test Method for Thickness of Thin Foil and Film by
device but is equipped with either a calibrated ratchet or a
Weighing
friction thimble. By use of a proper manipulative procedure
3. Terminology and a calibrated spring (seeAnnexA1), the pressure exerted on
the specimen is controllable.
3.1 Refer toTerminology D 1711 for definitions pertinent to
6.1.2 Use an instrument constructed with a vernier capable
this standard.
of measurement to the nearest 0.1 mil.
3.2 Definitions of Terms Specific to This Standard:
6.1.3 Use an instrument with the diameter of the anvil and
3.2.1 1 mil, n—a dimension equivalent to 0.0010 in.
spindle surfaces (which contact the specimen) of 250 6 1 mil.
3.2.2 absolute uncertainty (of a measurement), n—the
6.1.4 Use an instrument conforming to the requirements of
smallest division that may be read directly on the instrument
7.1, 7.2, 7.5, 7.6.1, and 7.6.2.
used for measurement.
6.1.5 Periodically, test the micrometer for conformance to
3.2.3 micrometer, n—an instrument for measuring any di-
the requirements of 6.1.4.
mension with absolute uncertainty of 1 mil or smaller.
6.2 Apparatus B—Machinist’s Micrometer Without a
4. Summary of Test Methods
Ratchet:
6.2.1 Apparatus B is a micrometer caliper without a locking
4.1 This standard provides eight different test methods for
device.
the measurement of thickness of solid electrical insulation
6.2.2 Use an instrument constructed with a vernier capable
materials. The test methods (identified as Methods A through
of measurement to the nearest 0.1 mil.
6.2.3 Use an instrument with the diameter of the anvil and
spindle surfaces (which contact the specimen) 250 6 1 mil.
These test methods are under the jurisdiction of ASTM Committee D-9 on
Electrical and Electronic Insulating Materials and are the direct responsibility of 6.2.4 Use an instrument conforming to the requirements of
Subcommittee D09.12 on Electrical Tests.
7.1, 7.2, 7.5.1, 7.5.2, 7.5.3, 7.6.1, and 7.6.3.
Current edition approved March 10, 1999. Published June 1999. Originally
published as D 374 – 33T. Last previous edition D 374 – 94.
Annual Book of ASTM Standards, Vol 10.01
Annual Book of ASTM Standards, Vol 10.02.
4 5
Annual Book of ASTM Standards, Vol 02.02. Hereinafter referred to as a machinist’s micrometer.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D374–99
TABLE 1 Methods Suitable for Specific Materials
6.3.2 The preferred design and construction of manually
Material Method operated dead-weight dial-type micrometers calls for a limit on
the force applied to the presser foot. The limit is related to the
Plastic sheet and film A B C or D
Paper (all thicknesses) E
compressive characteristics of the material being measured.
Paper (over 2 mils thickness) F or G
6.3.2.1 The force applied to the presser foot spindle and the
Rubber and other elastomers H
weight necessary to move the pointer upward from the zero
position shall be less than the force that will cause permanent
deformation of the specimen. The force applied to the presser
TABLE 2 Method Parameter Differences
footspindleandtheweightnecessarytojustpreventmovement
Diameter of Pressure on
Presser Foot Specimen, of the pointer from a higher to a lower reading shall be more
Method Apparatus
or Spindle, approximate,
than the minimum permissible force specified for a specimen.
mils PSI
6.4 Apparatus D—Motor-Operated Dead-Weight Dial
A Machinist micrometer with 250 not specified
Gage:
calibrated ratchet or
6.4.1 Except as additionally defined in this section, use an
thimble
B Machinist micrometer 250 unknown
instrument that conforms to the requirements of 6.3. An
without ratchet/thimble
electronic instrument having a digital readout in place of the
C Dead-weight dial type 125 to 500 0.5 to 130
dial indicator is permitted if that instrument meets the other
bench micrometer—Manual
D Dead-weight dial type 125 to 500 0.5 to 130
requirements of 6.3 and 6.4.
bench micrometer—Motor
6.4.2 Use a motor operated instrument having a presser foot
operated
spindle that is lifted and lowered by a constant speed motor
E Dead-weight dial type 250 25
bench micrometer—Motor
through a mechanical linkage such that the rate of descent (for
operated
a specified range of distances between the presser foot surface
F Dead-weight dial type 250 25
bench micrometer—Manual and the anvil) and the dwell time on the specimen are within
G Machinist micrometer with 250 25
thelimitsspecifiedforthematerialbeingmeasured.Designthe
calibrated ratchet or
mechanical linkage so that the only downward force upon the
thimble
H Dead-weight dial type 250 4 presserfootspindleisthatofgravityupontheweightedspindle
bench micrometer—Manual
assembly without any additional force exerted by the lifting/
lowering mechanism.
6.4.2.1 The preferred design and construction of motor
operated dead-weight dial-type micrometers calls for a limit on
6.2.5 Periodically, examine and test the micrometer for
the force applied to the presser foot. The limit is related to the
conformance to the requirements of 6.2.4.
compressive characteristics of the material being measured.
6.3 Apparatus C— Manually-Operated, Dead-Weight, Dial
6 6.4.2.2 The force applied to the presser foot spindle and the
Type Thickness Gage:
weight necessary to move the pointer upward from the zero
6.3.1 Use a dead-weight dial-type gage in accordance with
position shall be less than the force that will cause permanent
the requirements of 7.1, 7.3, 7.4, 7.6.1, 7.6.4, that has:
deformation of the specimen. The force applied to the presser
6.3.1.1 Apresser foot that moves in an axis perpendicular to
footspindleandtheweightnecessarytojustpreventmovement
the anvil face,
of the pointer from a higher to a lower reading must be more
6.3.1.2 The surfaces of the presser foot and the anvil (which
than the minimum permissible force specified for a specimen.
contact the specimen) parallel to within 0.1 mil (see 7.3),
6.3.1.3 A vertical dial spindle,
7. Calibration (General Considerations for Care and Use
6.3.1.4 Adial indicator essentially friction-free and capable
of Each of the Various Pieces of Apparatus for
of repeatable readings within 60.05 mil at zero setting, or on
Thickness Measurements)
a steel gage block,
7.1 Good testing practices require clean anvil and presser
6.3.1.5 A frame, housing the indicator, of such rigidity that
foot surfaces for any micrometer instrument. Prior to calibra-
a load of 3 lbf applied to the dial housing, out of contact with
tion or thickness measurements, clean such surfaces by insert-
the presser foot spindle (or any weight attached thereto) will
ing a piece of smooth, clean bond paper between the anvil and
produce a deflection of the frame not greater than the smallest
thepresserfootandslowlymovingthebondpaperbetweenthe
scale division on the indicator dial, and,
surfaces. During measurements, check the zero setting fre-
6.3.1.6 Adial diameter at least 2 in. and graduated continu-
quently. Failure to repeat the zero setting may be evidence of
ously to read directly to the nearest 0.1 mil. If necessary, equip
dirt on the surfaces.
the dial with a revolution counter that displays the number of
complete revolutions of the large hand.
NOTE 1—Avoid pulling any edge of the bond paper between the
6.3.1.7 An electronic instrument having a digital readout in
surfaces to reduce the probability of depositing any lint particles on the
place of the dial indicator is permitted if that instrument meets
surfaces.
the other requirements of 6.3.
7.2 The parallelism requirements for machinist’s microme-
ters demand that observed differences of readings on a pair of
screw-thread-pitch wires or a pair of standard 250-mil nominal
Herein referred to as a dial gage. diameter plug gages be not greater than 0.1 mil. Spring-wire
D374–99
stock or music-wire of known diameter are suitable substitutes. 7.4.2.3 A symmetrical concave or convex surface forms
The wire (or the plug gage) has a diameter dimension that is concentric circular fringes. Their number is a measure of
known to be within 60.05 mil. Diameter dimensions may vary deviation from flatness.
by an amount approximately equal to the axial movement of
7.4.2.4 An unsymmetrical concave or convex surface forms
the spindle when the wire (or the plug gage) is rotated through
a series of curved fringes that cut the periphery of the
180°.
micrometer surface. The number of fringes cut by a straight
line connecting the terminals of any fringes is a measure of the
7.2.1 Lacking a detailed procedure supplied by the instru-
deviation from flatness.
ment manufacturer, confirm the parallelism requirements of
machinist’s micrometers using the following procedure: 7.5 Machinist’s Micrometer Requirements:
7.2.1.1 Closethemicrometeronthescrew-thread-pitchwire 7.5.1 The requirements for zero reading of machinist’s
or the plug gage in accordance with the calibration procedure
micrometers are met when ten closings of the spindle onto the
of 7.6.2 or 7.6.3 as appropriate. anvil, in accordance with 7.6.2.3 or 7.6.3.3 as appropriate,
result in ten zero readings. The condition of zero reading is
7.2.1.2 Observe and record the thickness indicated.
satisfied when examinations with a low-power magnifying
7.2.1.3 Move the screw-thread-pitch wire or the plug gage
glass show that at least 66 % of the width of the zero
to a different position between the presser foot and the anvil
graduation mark on the barrel coincides with at least 66 % of
and repeat 7.2.1.1 and 7.2.1.2.
the width of the reference mark.
7.2.1.4 If the difference between any pair of readings is
7.5.2 Proper maintenance of a machinist’s micrometer may
greater than 0.1 mil, the surfaces are NOT parallel.
require adjusting the instrument for wear of the micrometer
7.3 Lacking a detailed procedure supplied by the instrument
screw so that the spindle has no perceptible lateral or longitu-
manufacturer, confirm the requirements for parallelism of
dinal looseness yet rotates with a torque load of less than 0.25
dial-type micrometers given in 6.3.1.2 by placing a hardened
ozf–in.Ifthisisnotachievableafterdisassembly,cleaning,and
steel ball (such as is used in a ball bearing) of suitable diameter
lubrication, replace the instrument.
between the presser foot and the anvil. Mount the ball in a
7.5.3 After the zero reading has been checked, use the
fork-shaped holder to allow the ball to be conveniently moved
calibration procedure of 7.6.2 or 7.6.3 (as appropriate for the
from one location to another between the presser foot and the
machinist’s micrometer under examination) to check for maxi-
anvil. The balls used commercially in ball bearings are almost
mum acceptable error in the machinist’s micrometer screw.
perfect spheres having diameters constant within a few micro-
7.5.3.1 Use selected feeler-gage blades with known thick-
inches.
nesses to within 60.02 mil to check micrometers calibrated in
NOTE 2—Exercise care with this procedure. Calculations using the
English units at approximately 2, 5, and 10-mil points. Use
equations in X1.3.2 show that the use of a 24-oz weight on a ball between
standard gage blocks at points greater than 10 mil.
the hardened surfaces of presser foot and anvil can result in dimples in the
7.5.3.2 At each point checked, take ten readings. Calculate
anvil or presser foot surfaces caused by exceeding the yield stress of the
the arithmetic mean of these ten readings.
surfaces.
7.5.3.3 The machinist’s micrometer screw error is within
7.3.1 Observe and record the diameter as measured by the
requirements if the difference between the mean value of
micrometer at one location.
7.5.3.2 and the gage block (or feeler-gage blade) thickness is
7.3.2 Move the ball to another location and repeat the
not more than 0.1 mil.
measurement.
7.5.4 Calibration of Spindle Pressure in Machinist’s Mi-
7.3.3 If the difference between any pair of readings is
crometer with Ratchet or Friction Thimble:
greater than 0.1 mil, the surfaces are NOT parallel.
7.5.4.1 See Annex A1, which details the apparatus and
7.4 Lacking a detailed procedure supplied by the instrument
procedure required for this calibration.
manufacturer, confirm the flatness of the anvil and the spindle
7.6 Calibration of Micrometers:
surface of a micrometer or dial gage by use of an optical flat
7.6.1 Calibrate all micrometers in a standard laboratory
whichhascleansurfaces.Surfacesshallbeflatwithin0.05mil.
atmosphere maintained at 50 % relative humidity and 23°C or
7.4.1 Aftercleaningthemicrometersurfaces(see7.1),place
some other sta
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