Designation: D 149 – 97a （Reapproved 2004）
Standard Test Method for
Dielectric Breakdown Voltage and Dielectric Strength of
Solid Electrical Insulating Materials at Commercial Power
1
Frequencies
This standard is issued under the fixed designation D 149; the number immediay 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 over）. With the addition of instructions modifying Section 12,
this test method may be used for proof testing.
1.1 This test method covers procedures for the determina-
1.8 ThistestmethodissimilartoIECPublication243-1.All
tion of dielectric strength of solid insulating materials at
2,3 procedures in this method are included in IEC 243-1. Differ-
commercial power frequencies, under specified conditions.
ences between this methodand IEC 243-1 are largely editorial.
1.2 Unless otherwise specified, the tests shall be made at 60
1.9 This standard does not purport to address all of the
Hz. However, this test method may be used at any frequency
safety concerns, if any, associated with its use. It is the
from 25 to 800 Hz. At frequencies above 800 Hz, dielectric
responsibility of the user of this standard to establish appro-
heating may be a problem.
priate safety and health practices and determine the applica-
1.3 This test method is intended to be used in conjunction
bility of regulatory limitations prior to use. Specific hazard
with anyASTM standard or other document that refers to this
statements are given in Section 7. Also see 6.4.1.
test method. References to this document should specify the
particular options to be used （see 5.5）.
2. Referenced Documents
1.4 It may be used at various temperatures, and in any
4
2.1 ASTM Standards:
suitable gaseous or liquid surrounding medium.
D 374 Test Methods for Thickness of Solid Electrical Insu-
1.5 This test method is not intended for measuring the
lation
dielectric strength of materials that are fluid under the condi-
D 618 Practice for Conditioning Plastics for Testing
tions of test.
D 877 Test Method for Dielectric Breakdown Voltage of
1.6 This test method is not intended for use in determining
Insulating Liquids Using Disk Electrodes
intrinsic dielectric strength, direct-voltage dielectric strength,
D 1711 Terminology Relating to Electrical Insulation
or thermal failure under electrical stress （see Test Method
D 2413 Practice for Preparation of Insulating Paper and
D3151）.
Board Impregnated with a Liquid Dielectric
1.7 This test method is most commonly used to determine
D 3151 Test Method forThermal Failure of Solid Electrical
thedielectricbreakdownvoltagethroughthethicknessofatest
Insulating Materials Under Electric Stress
specimen （puncture）. It may also be used to determine dielec-
D 3487 Specification for Mineral Insulating Oil Used in
tric breakdown voltage along the interface between a solid
Electrical Apparatus
specimen and a gaseous or liquid surrounding medium （flash-
D 5423 Specification for Forced-Convection Laboratory
Ovens for Electrical Insulation
1
This test method is under the jurisdiction of ASTM Committee D09 on 2.2 IEC Standard:
Electrical and Electronic Insulating Materials and is the direct responsibility of
Pub. 243-1 Methods of Test for Electrical Strength of Solid
Subcommittee D09.12 on Electrical Tests. 5
Insulating Materials—Part 1: Tests at Power Frequencies
Current edition approved March 1, 2004. Published March 2004. Originally
approved in 1922. Last previous edition approved in 1997 as D 149–97a.
2
Bartnikas, R., Chapter 3, “High Voltage Measurements,” Electrical Properties
4
of Solid Insulating Materials, Measurement Techniques, Vol. IIB, Engineering For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Dielectrics, R. Bartnikas, Editor, ASTM STP 926, ASTM, Philadelphia, 1987. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
3
Nelson, J. K., Chapter 5, “Dielectric Breakdown of Solids,” Electrical Standards volume information, refer to the standard’s Document Summary page on
Properties of Solid Insulating Materials: Molecular Structure and Electrical the ASTM website.
5
Behavior, Vol. IIA, Engineering Dielectrics, R. Bartnikas and R. M. Eichorn, Available from the International Electrotechnical Commission, Geneva, Swit-
Editors, ASTM STP 783, ASTM, Philadelphia, 1983. zerland.
Copyright （C） ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.

D 149 – 97a （2004）
2.3 ANSI Standard: environmentalsituations.Thistestmethodisusefulforprocess
C68.1 Techniques for Dielectric Tests, IEEE Standard No. control, acceptance or research testing.
6
4 5.3 Resultsobtainedbythistestmethodcanseldombeused
directly to determine the dielectric behavior of a material in an
3. Terminology actual application. In most cases it is necessary that these
results be evaluated by comparison with results obtained from
3.1 Definitions:
other functional tests or from tests on other materials, or both,
3.1.1 dielectric breakdown voltage （electric breakdown
in order to estimate their significance for a particular material.
voltage）, n—the potential difference at which dielectric failure
5.4 Three methods for voltage application are specified in
occurs under prescribed conditions in an electrical insulating
Section 12: Method A, Short-Time Test; Method B, Step-by-
material located between two electrodes. （See also Appendix
StepTest; and Method C, Slow Rate-of-RiseTest. MethodAis
X1.）
the most commonly-used test for quality-control tests. How-
3.1.1.1 Discussion—The term dielectric breakdown voltage
ever, the longer-time tests, Methods B and C, which usually
is sometimes shortened to “breakdown voltage.”
will give lower test results, may give more meaningful results
3.1.2 dielectric failure （under test）, n—an event that is
whendifferentmaterialsarebeingcomparedwitheachother.If
evidencedbyanincreaseinconductanceinthedielectricunder
a test set with motor-driven voltage control is available, the
test limiting the electric field that can be sustained.
slow rate-of-rise test is simpler and preferable to the step-by-
3.1.3 dielectric strength, n—the voltage gradient at which
step test. The results obtained from Methods B and C are
dielectric failure of the insulating material occurs under spe-
comparable to each other.
cific conditions of test.
5.5 Documents specifying the use of this test method shall
3.1.4 electric strength, n—see dielectric strength.
also specify:
3.1.4.1 Discussion—Internationally, “electric strength” is
5.5.1 Method of voltage application,
used almost universally.
5.5.2 Voltage rate-of-rise, if slow rate-of-rise method is
3.1.5 flashover, n—a disruptive electrical discharge at the
specified,
surface of electrical insulation or in the surrounding medium,
5.5.3 Specimen selection, preparation, and conditioning,
which may or may not cause permanent damage to the
5.5.4 Surrounding medium and temperature during test,
insulation.
5.5.5 Electrodes,
3.1.6 For definitions of other terms relating to solid insulat-
5.5.6 Wherever possible, the failure criterion of the current-
ing materials, refer to Terminology D 1711.
sensing element, and
4. Summary of Test Method 5.5.7 Any desired deviations from the recommended proce-
dures as given.
4.1 Alternating voltage at a commercial power frequency
5.6 If any of the requirements listed in 5.5 are missing from
（60 Hz, unless otherwise specified） is applied to a test
the specifying document, then the recommendations for the
specimen. The voltage is increased from zero or from a level
several variables shall be followed.
well below the breakdown voltage, in one of three prescribed
5.7 Unless the items listed in 5.5 are specified, tests made
methods of voltage application, until dielectric failure of the
with such inadequate reference to this test method are not in
test specimen occurs.
conformancewiththistestmethod.Iftheitemslistedin5.re
4.2 Mostcommonly,thetestvoltageisappliedusingsimple
not closely controlled during the test, the precisions stated in
test electrodes on opposite faces of specimens. The specimens
15.2 and 15.3 may not be realized.
may be molded or cast, or cut from flat sheet or plate. Other
5.8 Variations in the failure criteria （current setting and
electrode and specimen configurations may be used to accom-
response time） of the current sensing element significantly
modate the geometry of the sample material, or to simulate a
affect the test results.
specific application for which the material is being evaluated.
5.9 Appendix X1. contains a more complete discussion of
the significance of dielectric strength tests.
5. Significance and Use
5.1 The dielectric strength of an electrical insulating mate- 6. Apparatus
rial is a property of interest for any application where an
6.1 Voltage Source—Obtain the test voltage from a step-up
electrical field will be present. In many cases the dielectric
transformer supplied from a variable sinusoidal low-voltage
strength of a material will be the determining factor in the
source. The transformer, its voltage source, and the associated
design of the apparatus in which it is to be used.
controls shall have the following capabilities:
5.2 Tests made as specified herein may be used to provide
6.1.1 The ratio of crest to root-mean-square （rms） test
part of the information needed for determining suitability of a
voltage shall be equal to =2 6 5% （1.34 to 1.48）, with the
materialforagivenapplication;andalso,fordetectingchanges
test specimen in the circuit, at all voltages greater than 50 % of
or deviations from normal characteristics resulting from pro-
the breakdown voltage.
cessing variables, aging conditions, or other manufacturing or
6.1.2 The capacity of the source shall be sufficient to
maintainthetestvoltageuntildielectricbreakdownoccurs.For
most materials, using electrodes similar to those shown in
6 Table 1, an output current capacity of 40 mA is usually
Available fromAmerican National Standards Institute （ANSI）, 25 W. 43rd St.,
4th Floor, New York, NY 10036. satisfactory. For more complex electrode structures, or for

D 149 – 97a （2004）
A
TABLE 1 Typical Electrodes for Dielectric Strength Testing of Various Types of Insulating Materials
Electrode
B,C
Description of Electrodes Insulating Materials
Type
1 Opposing cylinders 51 mm （2 in.） in diameter, 25 mm （1 in.） thick with flat sheets of paper, films, fabrics, rubber, molded plastics, laminates,
edges rounded to 6.4 mm （0.25 in.） radius boards, glass, mica, and ceramic
2 Opposing cylinders 25 mm （1 in.） in diameter, 25 mm （1 in.） thick with same as for Type 1, particularly for glass, mica, plastic, and ceramic
edges rounded to 3.2 mm （0.125 in.） radius
3 Opposing cylindrical rods 6.4 mm （0.25 in.） in diameter with edges same as for Type 1, particularly for varnish, plastic, and other thin film and
D
rounded to 0.8 mm （0.0313 in.） radius tapes: where small specimens necessitate the use of smaller electrodes,
or where testing of a small area is desired
4 Flat plates 6.4 mm （0.25 in.） wide and 108 mm （4.25 in.） long with edges same as for Type 1, particularly for rubber tapes and other narrow widths
square and ends rounded to 3.2 mm （0.125 in.） radius of thin materials
E
5 Hemispherical electrodes 12.7 mm （0.5 in.） in diameter filling and treating compounds, gels and semisolid compounds and greases,
embedding, potting, and encapsulating materials
6 Opposing cylinders; the lower one 75 mm （3 in.） in diameter, 15 mm same as for Types 1 and 2
（0.60 in.） thick; the upper one 25 mm （1 in.） in diameter, 25 mm
F
thick; with edges of both rounded to 3 mm （0.12 in.） radius
G
7 Opposing circular flat plates, 150 mm diameter , 10 mm thick with flat sheet, plate, or board materials, for tests with the voltage gradient
H
edges rounded to 3 to 5 mm radius parallel to the surface
A
TheseelectrodesarethosemostcommonlyspecifiedorreferencedinASTMstandards.WiththeexceptionofType5electrodes,noattempthasbeenmadetosuggest
electrode systems for other than flat surface material. Other electrodes may be used as specified in ASTM standards or as agreed upon between seller and purchaser
where none of these electrodes in the table is suitable for proper evaluation of the material being tested.
B
Electrodes are normally made from either brass or stainless steel. Reference should be made to the standard governing the material to be tested to determine which,
if either, material is preferable.
C
The electrodes surfaces should be polished and free from irregularities resulting from previous testing.
D
Refer to the appropriate standard for the load force applied by the upper electrode assembly. Unless otherwise specified the upper electrodes shall be 50 6 2g.
E
Refer to the appropriate standard for the proper gap settings.
F
The Type 6 electrodes are those given in IEC Publication 243-1 for testing of flat sheet materials. They are less critical as to concentricity of the electrodes than are
the Types 1 and 2 electrodes.
G
Other diameters may be used, provided that all parts of the test specimen are at least 15 mm inside the edges of the electrodes.
H G
The Type 7 electrodes, as described in the table and in Note , are those given in IEC Publication 243-1 for making tests parallel to the surface.
testing high-loss materials, higher current capacity may be one current setting. The electrode area may have a significant
needed.Thepowerratingformosttestswillvaryfrom0.5kVA effect upon what the current setting should be.
for testing low-capacitance specimens at voltages up to 10 kV, 6.1.7 The specimen current-sensing element may be in the
to 5 kVA for voltages up to 100 kV. primary of the step-up transformer. Calibrate the current-
6.1.3 The controls on the variable low-voltage source shall sensing dial in terms of specimen current.
be capable of varying the supply voltage and the resultant test 6.1.8 Exercise care in setting the response of the current
voltage smoothly, uniformly, and without overshoots or tran- control. If the control is set too high, the circuit will not
sients, in accordance with 12.2. Do not allow the peak voltage respondwhenbreakdownoccurs;ifsettoolow,itmayrespond
to exceed 1.48 times the indicated rms test voltage under any to leakage currents, capacitive currents, or partial discharge
circumstance. Motor-driven controls are preferable for making （corona）currentsor,whenthesensingelementislocatedinthe
short-time （see 12.2.1） or slow-rate-of-rise （see 12.2.3） tests. primary, to the step-up transformer magnetizing current.
6.1.4 Equip the voltage source with a circuit-breaking 6.2 Voltage Measurement—A voltmeter must be provided
device that will operate within three cycles. The device shall for measuring the rms test voltage. A peak-reading voltmeter
disconnect the voltage-source equipment from the power may be used, in which case divide the reading by =2toget
service and protect it from overload as a result of specimen rms values. The overall error of the voltage-measuring circuit
breakdown causing an overload of the testing apparatus. If shall not exceed 5 % of the measured value. In addition, the
prolonged current follows breakdown it will result in unnec- response time of the voltmeter shall be such that its time lag
essary burning of the test specimens, pitting of the electrodes, will not be greater than 1% of full scale at any rate-of-rise
and contamination of any liquid surrounding medium. used.
6.1.5 The circuit-breaking device should have an adjustable 6.2.1 Measure the voltage using a voltmeter or potential
current-sensing element in the step-up transformer secondary, transformer connected to the specimen electrodes, or to a
to allow for adjustment consistent with the specimen charac- separate voltmeter winding, on the test transformer, that is
teristics and arranged to sense specimen current. Set the unaffected by the step-up transformer loading.
sensing element to respond to a current that is indicative of 6.2.2 It is desirable for the reading of the maximum applied
specimen breakdown as defined in 12.3. test voltage to be retained on the voltmeter after breakdown so
6.1.6 The current setting can have a significant effect on the that the breakdown voltage can be accuray read and re-
test results. Make the setting high enough that transients, such corded.
as partial discharges, will not trip the breaker but not so high 6.3 Electrodes—For a given specimen configuration, the
thatexcessiveburningofthespecimen,withresultanectrode dielectric breakdown voltage may vary considerably, depend-
damage, will occur on breakdown. The optimum current inguponthegeometryandplacementofthetesectrodes.For
setting is not the same for all specimens and depending upon this reason it is important that the electrodes to be used be
the intended use of the material and the purpose of the test, it described when specifying this test method, and that they be
may be desirable to make tests on a given sample at more than described in the report.

D 149 – 97a （2004）
6.3.1 One of the electrodes listed in Table 1 should be the test values. Testing in air may require excessively large
specified by the document referring to this test method. If no specimens or cause heavy surface discharges and burning
electrodes have been specified, select an applicable one from before breakdown. Some electrode systems for testing in air
Table 1, or use other electrodes mutually acceptable to the make use of pressure gaskets around the electrodes to prevent
parties concerned when the standard electrodes cannot be used flashover. The material of the gaskets or seals around the
due to the nature or configuration of the material being tested. electrodes may influence the breakdown values.
See references in Appendix X2 for examples of some special 6.4.1 When tests are made in insulating oil, an oil bath of
electrodes.Inanyeventtheelectrodesmustbedescribedinthe adequate size shall be provided. （Caution—The use of glass
report. containers is not recommended for tests at voltages above
6.3.2 The electrodes of Types 1 through 4 and Type 6 of about10kV,becausetheenergyreleasedatbreakdownmaybe
Table 1 should be in contact with the test specimen over the sufficient to shatter the container. Metal baths must be
entire flat area of the electrodes. grounded.）
6.3.3 The specimens tested using Type 7 electrodes should It is recommended that mineral oil meeting the requirements
be of such size that all portions of the specimen will be within of Specification D 3487, Type I or II, be used. It should have a
andnolessthan15mmfromtheedgesoftheelectrodesduring dielectric breakdown voltage as determined by Test Method
test. In most cases, tests usingType 7 electrodes are made with D 877 of at least 26 kV. Other dielectric fluids may be used as
the plane of the electrode surfaces in a vertical position. Tests surrounding mediums if specified. These include, but are not
made with horizontal electrodes should not be directly com- limited to, silicone fluids and other liquids intended for use in
pared with tests made with vertical electrodes, particularly transformers, circuit breakers, capacitors, or cables.
when the tests are made in a liquid surrounding medium.
6.4.1.1 The quality of the insulating oil may have an
6.3.4 Keep the electrode surfaces clean and smooth, and appreciable effect upon the test results. In addition to the
freefromprojectingirregularitiesresultingfromprevioustests. dielectric breakdown voltage, mentioned above, particulate
If asperities have developed, they must be removed. contaminants are especially important when very thin speci-
6.3.5 It is important that the original manufacture and mens （25 μm （1 mil） or less） are being tested. Depending upon
subsequent resurfacing of electrodes be done in such a manner the nature of the oil and the properties of the material being
that the specified shape and finish of the electrodes and their tested, other properties, including dissolved gas content, water
edges are maintained. The flatness and surface finish of the content, and dissipation factor of the oil may also have an
electrode faces must be such that the faces are in close contact effect upon the results. Frequent replacement of the oil, or the
with the test specimen over the entire area of the electrodes. use of filters and other reconditioning equipment may be
Surface finish is particularly important when testing very thin necessary to minimize the effect of variations of the quality of
materials which are subject to physical damage from improp- the oil on the test results.
erly finished electrodes. When resurfacing, do not change the 6.4.1.2 Breakdown values obtained using liquids having
transition between the electrode face and any specified edge different electrical properties may not be comparable. （See
radius. X1.4.7.）Iftestsaretobemadeatotherthanroomtemperature,
6.3.6 Whenever the electrodes are dissimilar in size or the bath must be provided with a means for heating or cooling
shape, the one at which the lowest concentration of stress the liquid, and with a means to ensure uniform temperature.
exists, usually the larger in size and with the largest radius, Small baths can in some cases be placed in an oven （see 6.4.2）
should be at ground potential. in order to provide temperature control. If forced circulation of
6.3.7 In some special cases liquid metal electrodes, foil the fluid is provided, care must be taken to prevent bubbles
electrodes, metal shot, water, or conductive coating electrodes from being whipped into the fluid. The temperature shall be
are used. It must be recognized that these may give results maintainedwithin65°Cofthespecifiedtesttemperatureatthe
differing widely from those obtained with other types of electrodes, unless otherwise specified. In many cases it is
electrodes. specified that specimens to be tested in insulating oil are to be
6.3.8 Because of the effect of the electrodes on the test previously impregnated with the oil and not removed from the
results, it is frequently possible to obtain additional informa- oilbeforetesting（seePracticeD2413）.Forsuchmaterials,the
tion as to the dielectric properties of a material （or a group of bath must be of such design that it will not be necessary to
materials） by running tests with more than one type of expose the specimens to air before testing.
electrode. This technique is of particular value for research 6.4.2 If tests in air are to be made at other than ambient
testing. temperature or humidity, an oven or controlled humidity
6.4 Surrounding Medium—The document calling for this chamber must be provided for the tests. Ovens meeting the
test method should specify the surrounding medium and the requirementsofSpecificationD 5423andprovidedwithmeans
test temperature. Since flashover must be avoided and the for introducing the test voltage will be suitable for use when
effects of partial discharges prior to breakdown mimimized, only temperature is to be controlled.
even for short time tests, it is often preferable and sometimes 6.4.3 Testsingassesotherthanairwillgenerallyrequirethe
necessary to make the tests in insulating liquid （see 6.4.1）. use of chambers that can be evacuated and filled with the test
Breakdown values obtained in insulating liquid may not be gas, usually under some controlled pressure. The design of
comparable with those obtained in air. The nature of the such chambers will be determined by the nature of the test
insulating liquid and the degree of previous use may influence program to be undertaken.

D 149 – 97a （2004）
6.5 Test Chamber—The test chamber or area in which the 8.2 Sampling procedures for quality control purposes
tests are to be made shall be of sufficient size to hold the test should provide for gathering of sufficient samples to estimate
equipment, and shall be provided with interlocks to prevent both the average quality and the variability of the lot being
accidental contact with any electrically energized parts. A examined; and for proper protection of the samples from the
number of different physical arrangements of voltage source, time they are taken until the preparation of the test specimens
measuring equipment, baths or ovens, and electrodes are in the laboratory or other test area is begun.
possible, but it is essential that （1） all gates or doors providing 8.3 For the purposes of most tests it is desirable to take
access to spaces in which there are electrically energized parts samples from areas that are not immediay adjacent to
be interlocked to shut off the voltage source when opened; （ 2） obvious defects or discontinuities in the material. The outer
clearances are sufficiently large that the field in the area of the few layers of roll material, the top sheets of a package of
electrodes and specimen are not distorted and that flashovers sheets, or material immediay next to an edge of a sheet or
and partial discharges （corona） do not occur except between roll should be avoided, unless the presence or proximity of
the test electrodes; and （3） insertion and replacement of defects or discontinuities is of interest in the investigation of
specimens between tests be as simple and convenient as the material.
possible.Visualobservationoftheelectrodesandtestspecimen 8.4 The sample should be large enough to permit making as
during the test is frequently desirable. many individual tests as may be required for the particular
material （see 12.4）.
7. Hazards
9. Test Specimens
7.1 Warning—Lethal voltages may be present during this
9.1 Preparation and Handling:
test. It is essential that the test apparatus, and all associated
9.1.1 Prepare specimens from samples collected in accor-
equipment that may be electrically connected to it, be properly
dance with Section 8.
designed and installed for safe operation. Solidly ground all
9.1.2 When flat-faced electrodes are to be used, the surfaces
electrically conductive parts that any person might come into
of the specimens which will be in contact with the electrodes
contact with during the test. Provide means for use at the
shall be smooth parallel planes, insofar as possible without
completion of any test to ground any parts which: were at high
actual surface machining.
voltage during the test; may have acquired an induced charge
9.1.3 The specimens shall be of sufficient size to prevent
duringthetest;mayretaina chargeeven after disconnection of
flashover under the conditions of test. For thin materials it may
the voltage source. Thoroughly instruct all operators in the
be convenient to use specimens large enough to permit making
proper way to conduct tests safely. When making high-voltage
more than one test on a single piece.
tests, particularly in compressed gas or in oil, the energy
9.1.4 For thicker materials （usually more than 2 mm thick）
released at breakdown may be sufficient to result in fire,
the breakdown strength may be high enough that flashover or
explosion, or rupture of the test chamber. Design test equip-
intense surface partial discharges （corona） may occur prior to
ment, test chambers, and test specimens so as to minimize the
breakdown. Techniques that may be used to prevent flashover,
possibility of such occurrences and to eliminate the possibility
or to reduce partial discharge （corona） include:
of personal injury.
9.1.4.1 Immerse the specimen in insulating oil during the
7.2 Warning—Ozone is a physiologically hazardous gas at
test. See X1.4.7 for the surrounding medium factors influenc-
elevated concentrations. The exposure limits are set by gov-
ingbreakdown.Thismaybenecessaryforspecimensthathave
ernmental agencies and are usually based upon recommenda-
not been dried and impregnated with oil, as well as for those
tions made by the American Conference of Governmental
7
whichhavebeenpreparedinaccordancewithPracticeD 2413,
Industrial Hygienists. Ozone is likely to be present whenever
for example. （See 6.4.）
voltagesexistwhicharesufficienttocausepartial,orcomplete,
9.1.4.2 Machinearecessordrillaflat-bottomholeinoneor
discharges in air or other atmospheres that contain oxygen.
both surfaces of the specimen to reduce the test thickness. If
Ozone has a distinctive odor which is initially discernible at
dissimilar electrodes are used （such as Type 6 of Table 1） and
low concentrations but sustained inhalation of ozone can cause
only one surface is to be machined, the larger of the two
temporary loss of sensitivity to the scent of ozone. Because of
electrodes should be in contact with the machined surface.
thisitisimportanttomeasuretheconcentrationofozoneinthe
Caremustbetakeninmachiningspecimensnottocontaminate
atmosphere, using commercially available monitoring devices,
or mechanically damage them.
whenever the odor of ozone is persistently present or when
9.1.4.3 Apply seals or shrouds around the electrodes, in
ozone generating conditions continue. Use appropriate means,
contact with the specimen to reduce the tendency to flashover.
such as exhaust vents, to reduce ozone concentrations to
9.1.5 Materials that are not in flat sheet form shall be tested
acceptable levels in working areas.
using specimens （and electrodes） appropriate to the material
8. Sampling and the geometry of the sample. It is essential that for these
materials both the specimen and the electrodes be defined in
8.1 The detailed sampling procedure for the material being
the specification for the material.
tested should be defined in the specification for that material.
9.1.6 Whatever the form of the material, if tests of other
than surface-to-surface puncture strength are to be made,
7 define the specimens and the electrodes in the specification for
Available from the American Conference of Governmental Industrial Hygien-
ists, Building No. D-7, 6500 Glenway Ave., Cincinnati, OH 45211. the material.

D 149 – 97a （2004）
9.2 In nearly all cases the actual thickness of the test
specimenisimportant.Unlessotherwisespecified,measurethe
thickness after the test in the immediate vicinity of the area of
breakdown. Measurements shall be made at room temperature
（25 6 5°C）, using the appropriate procedure of Test Methods
D374.
10. Calibration
10.1 In making calibration measurements, take care that the
valuesofvoltageattheelectrodescanbedeterminedwithinthe
accuracy given in 6.2, with the test specimens in the circuit. Rates
（V/s） 6 20 %
10.2 Use an independently calibrated voltmeter attached to
100
the output of the test voltage source to verify the accuracy of 200
500
the measuring device. Electrostatic voltmeters, voltage divid-
1000
ers,orpotentialtransformershavingcomparableaccuracymay
2000
be used for calibration measurement. 5000
10.3 At voltages above about 12 kV rms （16.9 kV peak） a FIG. 1 Voltage Profile of the Short-Time Test
sphere gap may be used to calibrate the readings of the
voltage-measuring device. Follow procedures as specified in
ANSI C68.1 in such calibration.
occasionalaveragetimetobreakdownfallingoutsidetherange
of 10 to 20 s. In this case, the times to failures shall be made
11. Conditioning
a part of the report.
11.1 The dielectric strength of most solid insulating mate- 12.2.1.3 In running a series of tests comparing different
rials is influenced by temperature and moisture content. Mate- material, the same rate-of-rise shall be used with preference
rials so affected should be brought to equilibrium with an given to a rate that allows the average time to be between 10
atmosphere of controlled temperature and relative humidity and 20 s. If the time to breakdown cannot be adhered to, the
before testing. For such materials, the conditioning should be time shall be made a part of the report.
included in the standard referencing this test method. 12.2.2 Method B, Step-by-Step Test—Apply voltage to the
11.2 Unless otherwise specified, follow the procedures in test electrodes at the preferred starting voltage and in steps and
Practice D618. duration as shown in Fig. 2 until breakdown occurs.
12.2.2.1 From the list in Fig. 2, select the initial voltage, V ,
11.3 For many materials the moisture content has more s
to be the one closest to 50 % of the experimentally determined
effect on dielectric strength than does temperature. Condition-
or expected breakdown voltage under the short time test.
ing times for these materials should be sufficiently long to
12.2.2.2 If an initial voltage other than one of the preferred
permit the specimens to reach moisture equilibrium as well as
values listed in Fig. 2 is selected, it is recommended that the
temperature equilibrium.
voltage steps be 10% of the preferred initial voltage immedi-
11.4 If the conditioning atmosphere is such that condensa-
ay below the selected value.
tionoccursonthesurfaceofthespecimens,itmaybedesirable
12.2.2.3 Apply the initial voltage by increasing the voltage
to wipe the surfaces of the specimens immediay before
from zero as rapidly as can be accomplished without introduc-
testing. This will usually reduce the probability of surface
ing a peak voltage exceeding that permitted in 6.1.3. Similar
flashover.
requirements shall apply to the procedure used to increase the
voltagebetweensuccessivesteps.Aftertheinitialstep,thetime
12. Procedure
required to raise the voltage to the succeeding step shall be
12.1 （Caution—see Section 7 before commencement of
counted as part of the time at the succeeding step.
any test.）
12.2.2.4 If breakdown occurs while the voltage is being
12.2 Methods of Voltage Application:
increased to the next step, the specimen is described as having
12.2.1 Method A, Short-Time Test—Apply voltage uni- sustained a dielectric withstand voltage, V , equal to the
ws
formlytothetesectrodesfromzeroatoneoftheratesshown voltage of the step just ended. If breakdown occurs prior to the
inFig.1untilbreakdownoccurs.Usetheshort-timetestunless end of the holding period at any step, the dielectric withstand
otherwise specified. voltage,V ,forthespecimenistakenasthevoltageatthelast
ws
12.2.1.1 When establishing a rate initially in order for it to completedstep.Thevoltageatbreakdown,V ,istobeusedto
bd
beincludedinanewspecification,selectaratethat,foragiven calculate dielectric breakdown strength. The dielectric with-
set of specimens, will give an average time to breakdown of stand strength is to be calculated from the thickness and the
between 10 and 20 s. It may be necessary to run one or two dielectric withstand voltage, V . （See Fig. 2.）
ws
preliminary tests in order to determine the most suitable 12.2.2.5 It is desirable that breakdown occur in four to ten
rate-of-rise. For many materials a rate of 500 V/s is used. steps, but in not less than 120 s. If failure occurs at the third
12.2.1.2 If the document referencing this test method speci- steporless,orinlessthan120s,whicheverisgreater,onmore
fied a rate-of-rise, it shall be used consistently in spite of thanonespecimeninagroup,thetestsshouldberepeatedwith
6

D 149 – 97a （2004）
Rates （V/s） 6 20 % Constraints
1 tbd > 120 s
2
5
Preferred starting voltages, V are 0.25, 0.50, 1, 2, 5, 10, 20, 50, and 100 kV.
s
10 Vbd = > 1.5 Vs
Step Voltage 12.5
when Increment 20
A
Vs（kV） is （kV） 25
50
5 or less 10 % of Vs
100
over 5 to 10 0.50
over 10 to 25 1 FIG. 3 Voltage Profile of Slow Rate-of-Rise Test
over 25 to 50 2
over 50 to 100 5
over 100 10
greater than 2.5 times the initial value （and at a time of over
A
Vs = 0.5 （ Vbd for Short-Time Test） unless constraints cannot be met.
________________________________________________________________ 120 s）, increase the initial voltage.
Constraints
12.3 Criteria of Breakdown—Dielectric failure or dielectric
（t 1 - t0）=（t2 - t1） = ... = （60 6 5）s
Alternate step times, （20 6 3）s and （300 6 10）s breakdown （as defined in Terminology D 1711） consists of an
120s # t # 720s, for 60s steps
bd increase in conductance, limiting the electric field that can be
________________________________________________________________
sustained. This phenomenon is most commonly evidenced
FIG. 2 Voltage Profile of Step-by-Step Test
duringthetestbyanabruptvisibleandaudiblerupturethrough
the thickness of the specimen, resulting in a visible puncture
a lower initial voltage. If failure does not occur before the and decomposition of the specimen in the breakdown area.
twelfth step or greater than 720 s, increase the initial voltage. This form of breakdown is generally irreversible. Repeated
12.2.2.6 Record the initial voltage, the voltage steps, the applicationsofvoltagewillsometimesresultinfailureatlower
breakdown voltage, and the length of time that the breakdown
voltages （sometimes unmeasurably low）, usually with addi-
voltage was held. If failure occurred while the voltage was
tional damage at the breakdown area. Such repeated applica-
being increased to the starting voltage the failure time shall be
tions of voltage may be used to give positive evidence of
zero.
breakdown and to make the breakdown path more visible.
12.2.2.7 Other time lengths for the voltage steps may be
12.3.1 Arapid rise in leakage current may result in tripping
specified, depending upon the purpose of the test. Commonly
of the voltage source without visible decomposition of the
used lengths are 20 s and 300 s （5 min）. For research purposes,
specimen. This type of failure, usually associated with slow-
it may be of value to conduct tests using more than one time
rise tests at elevated temperatures, may in some cases be
interval on a given material.
reversible,thatis,recoveryofthedielectricstrengthmayoccur
12.2.3 Method C, Slow Rate-of-Rise Test—Apply voltage to
the test electrodes, from the starting voltage and at the rate if the specimen is allowed to cool to its original test tempera-
shown in Fig. 3 until breakdown occurs. ture before reapplying voltage. The voltage source must trip
12.2.3.1 Selecttheinitialvoltagefromshort-timetestsmade rapidlyatrelativelylowcurrentforthistypeoffailuretooccur.
as specified in 12.2.1. The initial voltage shall be reached as 12.3.2 Tripping of the voltage source may occur due to
specified in 12.2.2.3.
flashover, to partial discharge current, to reactive current in a
12.2.3.2 Use the rate-of-voltage rise from the initial value
highcapacitancespecimen,ortomalfunctioningofthebreaker.
specified in the document calling for this test method. Ordi-
Such interruptions of the test do not constitute breakdown
narily the rate is selected to approximate the average rate for a
（except for flashover tests） and should not be considered as a
step-by-step test.
satisfactory test.
12.2.3.3 Ifmorethanonespecimenofagroupofspecimens
12.3.3 If the breaker is set for too high a current, or if the
breaks down in less than 120 s, reduce either the initial voltage
breaker malfunctions, excessive burning of the specimen will
or the rate-of-rise, or both.
occur.
12.2.3.4 Ifmorethanonespecimenofagroupofspecimens
breaks down at less than 1.5 times the initial voltage, reduce 12.4 Number of Tests—Make five breakdowns unless oth-
the initial value. If breakdown repeatedly occurs at a value erwise specified for the particular material.

D 149 – 97a （2004）
13. Calculation 15. Precision and Bias
13.1 CalculateforeachtestthedielectricstrengthinkV/mm 15.1 The results of an interlaboratory study with four
or V/mil at breakdown, and for step-by-step tests, the gradient laboratories and eight materials are summarized in Table 2.
at the highest voltage step at which breakdown did not occur. This study made use of one electrode system and one test
8
13.2 Calculate the average dielectric strength and the stan- medium.
dard deviation, or other measure of variability. 15.2 Single-Operator Precision—Depending upon the vari-
ability of the material being tested, the specimen thickness,
14. Report
method of voltage application, and the extent to which tran-
14.1 Report the following information: sient voltage surges are controlled or suppressed, the coeffi-
14.1.1 Identification of the test sample. cientofvariation（standarddeviationdividedbythemean）may
14.1.2 For Each Specimen: varyfromalow1%toashighas20 %ormore.Whenmaking
14.1.2.1 Measured thickness, duplicate tests on five specimens from the same sample, the
14.1.2.2 Maximum voltage withstood （for step-by-step coefficient of variation usually is less than 9 %.
tests）, 15.3 Multilaboratory Precision—The precision of tests
14.1.2.3 Dielectric breakdown voltage, made in different laboratories （or of tests made using different
14.1.2.4 Dielectric strength （for step-by-step tests）, equipment in the same laboratory） is variable. Using identical
14.1.2.5 Dielectric breakdown strength, and
A
TABLE 2 Dielectric Strength Data Summary From Four Laboratories
Dielectric Strength （V/mil）
Thickness Standard Coefficient of
Material
（in. nom.） Deviation Variation （%）
mean max min
Polyethylene 0.001 4606 5330 4100 332 7.2
Terephthalate
Polyethylene 0.01 1558 1888 1169 196 12.6
Terephthalate
Fluorinated 0.003 3276 3769 2167 333 10.2
Ethylene
Propylene
Fluorinated 0.005 2530 3040 2140 231 9.1
Ethylene
Propylene
PETP fiber 0.025 956 1071 783 89 9.3
reinforced
epoxy resin
PETP fiber 0.060 583 643 494 46 7.9
reinforced
epoxy resin
Epoxy-Glass 0.065 567 635 489 43 7.6
Laminate
Crosslinked 0.044 861 948 729 48 5.6
Polyethylene
Average 8.7
A
Tests performed with specimens in oil using Type 2 electrodes （see Table 1）.
14.1.2.6 Location of failure （center of electrode, edge, or types of equipment and controlling specimen preparation,
outside）. electrodes and testing procedures closely, the single-operator
14.1.3 For Each Sample: precision is approachable. When making a direct comparison
14.1.3.1 Average dielectric withstand strength for step-by- ofresultsfromtwoormorelaboratories,evaluatetheprecision
step test specimens only, between the laboratories.
14.1.3.2 Average dielectric breakdown strength,
15.4 If the material under test, the specimen thickness, the
14.1.3.3 Indication of variability, preferably the standard
electrode configuration, or the surrounding medium differs
deviation and coefficient of variation,
from those listed in Table 1, or if the failure criterion of the
14.1.3.4 Description of test specimens,
current-sensing element of the test equipment is not closely
14.1.3.5 Conditioning and specimen preparation,
controlled, the precisions cited in 15.2 and 15.3 may not be
14.1.3.6 Ambient atmosphere temperature and relative hu-
realized. Standards which refer to this method should deter-
midity,
mineforthematerialwithwhichthatstandardisconcernedthe
14.1.3.7 Surrounding medium,
applicability of this precision statement to that particular
14.1.3.8 Test temperature,
material. Refer to 5.4-5.8 and 6.1.6.
14.1.3.9 Description of electrodes,
14.1.3.10 Method of voltage application,
14.1.3.11 If specified, the failure criterion of the current-
sensing element, and 8
The complete report is available from ASTM International. Request RR:D09-
14.1.3.12 Date of test. 1026.

D 149 – 97a （2004）
15.5 Use special techniques and equipment for materials 16. Keywords
having a thickness of 0.001 in. or less.The electrodes must not
16.1 breakdown; breakdown voltage; calibration; criteria of
damage the specimen upon contact. Accuray determine the
breakdown; dielectric breakdown voltage; dielectric failure;
voltage at breakdown.
dielectric strength; electrodes; flashover; power frequency;
15.6 Bias—This test method does not determine the intrin-
process-control testing; proof testing; quality-control testing;
sic dielectric strength. The test values are dependent upon
rapid rise; research testing; sampling; slow rate-of-rise; step-
specimen geometry, electrodes, and other variable factors, in
by-step; surrounding medium; voltage withstand
addition to the properties of the sample, so that it is not
possible to make a statement of bias.
APPENDIXES
（Nonmandatory Information）
X1. SIGNIFICANCE OF THE DIELECTRIC STRENGTH TEST
X1.1 Introduction directly between the electrodes. Weak spots within the volume
under stress sometimes determine the test results.
X1.1.1 A brief review of three postulated mechanisms of
breakdown, namely: （1） the discharge or corona mechanism,
X1.4 Influence of Test and Specimen Conditions
（2）thethermalmechanism,and（3）theintrinsicmechanism,as
well as a discussion of the principal factors affecting tests on
X1.4.1 Electrodes— In general, the breakdown voltage will
practical dielectrics, are given here to aid in interpreting the
tend to decrease with increasing electrode area, this area effect
data. The breakdown mechanisms usually operate in combina-
being more pronounced with thin test specimens. Test results
tionratherthansingly.Thefollowingdiscussionappliesonlyto
are also affected by the electrode geometry. Results may be
solid and semisolid materials.
affected also by the material from which the electrodes are
constructed, since the thermal and discharge mechanism may
X1.2 Postulated Mechanisms of Dielectric Breakdown
be influenced by the thermal conductivity and the work
X1.2.1 Breakdown Caused by Electrical Discharges—In function, respectively, of the electrode material. Generally
many tests on commercial materials, breakdown is caused by speaking, the effect of the electrode material is difficult to
electrical discharges, which produce high local fields. With
establish because of the scatter of experimental data.
solid materials the discharges usually occur in the surrounding
X1.4.2 Specimen Thickness—The dielectric strength of
medium, thus increasing the test area and producing failure at
solid commercial electrical insulating materials is greatly
or beyond the electrode edge. Discharges may occur in any
dependentuponthespecimenthickness.Experiencehasshown
internal voids or bubbles that are present or may develop.
that for solid and semi-solid materials, the dielectric strength
These may cause local erosion or chemical decomposition.
varies inversely as a fractional power of the specimen thick-
These processes may continue until a complete failure path is
ness, and there is a substantial amount of evidence that for
formed between the electrodes.
relatively homogeneous solids, the dielectric strength varies
X1.2.2 Thermal Breakdown—Cumulative heating develops
approximay as the reciprocal of the square root of the
inlocalpathswithinmanymaterialswhentheyaresubjectedto
thickness. In the case of solids that can be melted and poured
high electric field intensities, causing dielectric and ionic
to solidify between fixed electrodes, the effect of electrode
conduction losses which generate heat more rapidly than can
separationislessclearlydefined.Sincetheelectrodeseparation
be dissipated. Breakdown may then occur because of thermal
can be fixed at will in such cases, it is customary to perform
instability of the material.
dielectricstrengthtestsonliquidsandusuallyonfusiblesolids,
X1.2.3 Intrinsic Breakdown—If electric discharges or ther-
with electrodes having a standardized fixed spacing. Since the
mal instability do not cause failure, breakdown will still occur
when the field intensity becomes sufficient to accelerate elec- dielectric strength is so dependent upon thickness it is mean-
trons through the material. This critical field intensity is called ingless to report dielectric strength data for a material without
the intrinsic dielectric strength. It cannot be determined by this stating the thickness of the test specimens used.
test method, although the mechanism itself may be involved. X1.4.3 Temperature—The temperature of the test specimen
and its surrounding medium influence the dielectric strength,
X1.3 Nature of Electrical Insulating Materials although for most materials small variations of ambient tem-
X1.3.1 Solid commercial electrical insulating materials are perature may have a negligible effect. In general, the dielectric
generally nonhomogeneous and may contain dielectric defects strength will decrease with increasing temperatures, but the
of various kinds. Dielectric breakdown often occurs in an area extent to which this is true depends upon the material under
of the test specimen other than that where the field intensity is test. When it is known that a material will be required to
greatest and sometimes in an area remote from the material function at other than normal room temperature, it is essential

D 149 – 97a （2004）
that the dielectric strength-temperature relationship for the properties are usually such that edge breakdown will generally
material be determined over the range of expected operating occur if the electric strength, E , approaches the value given
s
temperatures. by:
X1.4.4 Time—Test results will be influenced by the rate of
4.2 63
E kV/mm （X1.4）
voltage application. In general, the breakdown voltage will s 5 Sts 1e8sD
tend to increase with increasing rate of voltage application.
In cases of large thickness of specimen and low permittivity
This is to be expected because the thermal breakdown mecha-
of specimen, the term containing t becomes relatively insig-
s
nismistime-dependentandthedischargemechanismisusually
nificant and the product of permittivity and electric strength is
time-dependent, although in some cases the latter mechanism 10
approximay a constant. Whitehead also mentions （p. 261）
may cause rapid failure by producing critically high local field
that the use of moist semiconducting oil can affect an appre-
intensitives.
ciablereductioninedgedischarges.Unlessthebreakdownpath
X1.4.5 Wave Form—In general, the dielectric strength is
between the electrodes is solely within the solid, results in one
influenced by the wave form of the applied voltage.Within the
medium cannot be compared with those in a different medium.
limitsspecifiedinthismethodtheinfluenceofwaveformisnot
It should also be noted that if the solid is porous or capable of
significant.
being permeated by the immersion medium, the breakdown
X1.4.6 Frequency—The dielectric strength is not signifi-
strength of the solid is directly affected by the electrical
cantly influenced by frequency variations within the range of
properties of immersion medium.
commercial power frequencies provided for in this method.
X1.4.8 Relative Humidity—The relative humidity influ-
However, inferences concerning dielectric strength behavior at
ences the dielectric strength to the extent that moisture ab-
other than commercial power frequencies （50 to 60 Hz） must
sorbed by, or on the surface of, the material under test affects
not be made from results obtained by this method.
the dielectric loss and surface conductivity. Hence, its impor-
X1.4.7 Surrounding Medium—Solid insulating materials
tance will depend to a large extent upon the nature of the
havingahighbreakdownvoltageareusuallytestedbyimmers-
material being tested. However, even materials that absorb
ing the test specimens in a liquid dielectric such as transformer
little or no moisture may be affected because of greatly
oil, silicone oil, or chlorofluorocarbons, in order to minimize
increased chemical effects of discharge in the presence of
theeffectsofsurfacedischargespriortobreakdown.Ithasbeen
9 moisture. Except in cases where the effect of exposure on
shownbyS.Whitehead thatinordertoavoiddischargesinthe
dielectric strength is being investigated, it is customary to
surrounding medium prior to reaching the breakdown voltage
control or limit the relative humidity effects by standard
of the solid test specimen, in alternating voltage tests it is
conditioning procedures.
necessary that
2 2 X1.5 Evaluation
E D 1 E D 1 （X1.1）
me8m = m 1 . se8s = s 1
X1.5.1 A fundamental requirement of the insulation in
If the liquid immersion medium is a low loss material, the electrical apparatus is that it withstand the voltage imposed on
criterion simplifies to it in service. Therefore there is a great need for a test to
evaluatetheperformanceofparticularmaterialsathighvoltage
2
E E D 1 （X1.2）
me8m . se8s = s 1 stress. The dielectric breakdown voltage test represents a
and if the liquid immersion medium is a semiconducting convenient preliminary test to determine whether a material
material the criterion becomes merits further consideration, but it falls short of a complete
evaluation in two important respects. First, the condition of a
E 2 f E （X1.3）
msm . p er e0 s
material as installed in apparatus is much different from its
condition in this test, particularly with regard to the configu-
where: ration of the electric field and the area of material exposed to
E = electric strength,
it, corona, mechanical stress, ambient medium, and association
f = frequency,
with other materials. Second, in service there are deteriorating
e and e8 = permittivity,
influences, heat, mechanical stress, corona and its products,
D = dissipation factor, and
contaminants, and so forth, which may reduce the breakdown
s = conductivity （S/m）.
voltage far below its value as originally installed. Some of
Subscripts:
these effects can be incorporated in laboratory tests, and a
m refers to immersion medium,
better estimate of the material will result, but the final
r refers to relative,
consideration must always be that of the performance of the
0 refers to free space,
-12 material in actual service.
（e0 =8.854310 F/m） and
X1.5.2 The dielectric breakdown test may be used as a
s refers to solid dielectric.
material inspection or quality control test, as a means of
X1.4.7.1 Whitehead points out that it is therefore desirable
to increase E and ,or , if surface discharges are to be
m em sm
avoided. Transformer oil is usually specified and its dielectric 10
Starr, R. W., “Dielectric Materials Ionization Study” Interim Engineering,
Report No. 5, Index No ME-111273.Available from Naval Sea Systems Command
Technical Library, Code SEA 09B 312, National Center 3, Washington, DC
9
Whitehead, S., Dielectric Breakdown of Solids, Oxford University Press, 1951. 20362-5101.

D 149 – 97a （2004）
inferring other conditions such as variability, or to indicate the test it is the relative value of the breakdown voltage that is
deteriorating processes such as thermal aging. In these uses of important rather than the absolute value.
X2. STANDARDS REFERRING TO TEST METHOD D149
X2.1 Introduction X2.1.2 In some standards which specify that the dielectric
strength or the breakdown voltage is to be determined in
X2.1.1 The listing of documents in this appendix provides
reference to a broad range ofASTM standards concerned with accordance with Test Method D 149, the manner in which the
determination of dielectric strength at power frequencies or reference is made to this test method is not compley in
with elements of test equipment or elements of procedural conformance with the requirements of 5.5. Do not use another
details used to determine this property. While every effort has document, including those listed in this appendix, as a model
been made to include as many as possible of the standards forreferencetothistestmethodunlessthereisconformitywith
referring to Test Method D 149, the list may not be complete, 5.5.
and standards written or revised after publication of this
appendix are not included.

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