ASTM D3426-19

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Designation: D3426 − 97 (Reapproved 2012) D3426 − 19
Standard Test Method for
Dielectric Breakdown Voltage and Dielectric Strength of
Solid Electrical Insulating Materials Using Impulse Waves
This standard is issued under the fixed designation D3426; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers the determination of dielectric strength of solid electrical insulating materials under simulated-
lightning impulse conditions.
1.2 Procedures are given for tests using standard 1.2 by 50 μs 50 μs full-wave impulses.
1.3 This test method is intended for use in determining the impulse dielectric strength of insulating materials, either using simple
electrodes or functional models. It is not intended for use in impulse testing of apparatus.
1.4 This test method is similar to IEC Publication 243-3. All procedures in this test method are included in IEC 243-3.
Differences between this test method and IEC 243-3 are largely editorial.
1.5 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. Specific precaution statements are given in Section 9.
1.6 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:
D149 Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at
Commercial Power Frequencies
D374 Test Methods for Thickness of Solid Electrical Insulation (Metric) D0374_D0374M
D2413 Practice for Preparation of Insulating Paper and Board Impregnated with a Liquid Dielectric
2.2 American National Standard:
C 68.1C68.1 Techniques for Dielectric Tests (IEEE Standard No. 4)
2.3 IEC Standard:
Pub 243-3 Methods of Test for Electric Strength of Solid Insulating Materials—Part 3: Additional Requirements for Impulse
Tests
3. Terminology
3.1 Definitions:
3.1.1 Reference should be made to Fig. 1 for the symbols mentioned.
3.1.2 full-impulse-voltage wave, n—an aperiodic transient voltage that rises rapidly to a maximum value, then falls less rapidly
to zero.
3.1.3 peak value of an impulse voltage wave, n—the maximum value of voltage.
This test method is under the jurisdiction of ASTM Committee D09 on Electrical and Electronic Insulating Materials and is the direct responsibility of Subcommittee
D09.12 on Electrical Tests.
Current edition approved Nov. 1, 2012March 1, 2019. Published November 2012March 2019. Originally approved in 1975. Last previous edition approved in 20042012
as D3426 – 97 (2012).(2004). DOI: 10.1520/D3426-97R12.10.1520/D3426-19.
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
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D3426 − 19
FIG. 1 Full-Impulse Voltage Wave
3.1.4 virtual-peak value of an impulse voltage wave, n—a value derived from a recording of an impulse wave on which
high-frequency oscillations or overshoot of limited magnitude may be present. If the oscillations have a magnitude of no more than
5 % of the peak value and a frequency of at least 0.5 MHz, a mean curve may be drawn, the maximum amplitude of which is the
virtual-peak value. If the oscillations are of greater magnitude, the voltage wave is not acceptable for standard tests.
3.1.5 virtual-front time of an impulse voltage wave, n—equal to 1.67 times the interval t between the instants when the voltage
f
is 0.3 and 0.9 times the peak value (t , Fig. 1).
3.1.6 virtual origin of an impulse voltage wave, n—the point of intersection O with the line of zero voltage of a line drawn
through the points of 0.3 and 0.9 times the peak voltage on the front of an impulse voltage wave.
3.1.7 virtual time to half-value of an impulse voltage wave, n—the time interval t between the virtual origin O and the instant
2 1
on the tail when the voltage has decreased to half the peak value.
4. Summary of Test Method
4.1 A series of sets-of-three voltage waves of a specified shape (see 5.3) is applied to the test specimen. The voltage of
successive sets is increased in magnitude until breakdown of the test specimen occurs.
4.2 The procedures for sampling and specimen preparation are as specified in the material specification or other document
calling for the use of this test method. The surrounding medium (air or other gas, or oil or other liquid) is also as specified if it
differs from the medium in which the specimens are finally conditioned for test.
5. Significance and Use
5.1 Insulating It is possible for insulating materials used in high-voltage equipment mayto be subjected to transient voltage
stresses, resulting from such causes as nearby lightning strokes. This is particularly true of apparatus such as transformers and
switchgear used in electrical-power transmission and distribution systems. The ability of insulating materials to withstand these
transient voltages is important in establishing the reliability of apparatus insulated with these materials.
D3426 − 19
5.2 Transient voltages caused by lightning maywill be of either positive or negative polarity. In a symmetrical field between
identical electrodes, the polarity has no effect on the breakdown strength. However, with dissimilar electrodes there maycan be a
pronounced polarity effect. It is common practice when using dissimilar electrodes, to make negative that electrode at which the
higher gradient will appear. When asymmetrical electrodes are used for testing materials with which the tester has no previous
experience or knowledge, it is recommended that he make comparative tests with positive polarity and negative polarity applied
to the higher gradient, or smaller electrode, to determine which polarity produces the lower breakdown voltage.
5.3 The standard wave shape is a 1.2 by 50-μs wave, reaching peak voltage in approximately 1.2 μs and decaying to 50 % of
peak voltage in approximately 50 μs after the beginning of the wave. This wave is intended to simulate a lightning stroke that may
strike strikes a system without causing failure on the system.
5.4 For most materials, the impulse dielectric strength will be higher than either its power frequency alternating voltage or its
direct voltage dielectric strengths. Because of the short time involved, dielectric heating and other thermal effects are largely
eliminated during impulse testing. Thus, the impulse test gives values closer to the intrinsic breakdown strength than do longer time
tests. From comparisons of the impulse dielectric strength with the values obtained from longer time tests, inferences may be drawn
it is possible to draw inferences as to the modes of failures under the various tests for a given material. Refer to Appendix X1 of
Test Method D149 should be referred to for further information on this subject.
6. Apparatus
6.1 Impulse Generator, capable of applying to the test specimen a standard 1.2 by 50-μs wave of either positive or negative
polarity. The virtual front time shall be 1.2 μs 6 30 % and the virtual time to half value 50 μs 6 20 %. The maximum voltage and
the energy-storage capability must be sufficient to provide impulse waves of the proper shape to any specimen to be tested up to
the breakdown voltage (or specified proof voltage) of the material. The electrical characteristics (particularly capacitance) of the
test spe
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