ASTM D2275-01(2008)e1
(Test Method)Standard Test Method for Voltage Endurance of Solid Electrical Insulating Materials Subjected to Partial Discharges (Corona) on the Surface
Standard Test Method for Voltage Endurance of Solid Electrical Insulating Materials Subjected to Partial Discharges (Corona) on the Surface
SIGNIFICANCE AND USE
This test method is used to compare the endurance of different materials to the action of corona on the external surfaces. A poor result on this test does not indicate that the material is a poor selection for use at high voltage or at high voltage stress in the absence of surface corona. Surface corona should be distinguished from corona that occurs in internal cavities for which no standardized test has been developed. Evaluation of endurance by comparison of data on specimens of different thickness is not valid.
The processing of the material may affect the results obtained. For instance, residual strains produced by quenching, or high levels of crystallinity caused by slow cooling may affect the result. Also, the type of molding process, injection or compression, may be important especially if the mixing of fillers or the concentration and sizes of gas-filled cavities are controlled in any degree by the process. Indeed, this test method may be used to examine the effects of processing.
The data are generated in the form of a set of values of lifetimes at a voltage. The dispersion of failure times can be analyzed using Weibull or extreme value statistics to yield an estimate of the central value of the distribution and its standard deviation. This is particularly recommended when the dispersion of failure times is large, and a comparison of lifetimes of two materials must be made at a specified level of confidence.
This test is often used to demonstrate the differences between different classes of materials, and to illustrate the importance of eliminating corona in any application of a particular material. When the test is used for such purposes or other similar ones, the need for precision is reduced, and certain time saving techniques, such as truncating a test at the time of the fifth failure of a set of nine, and using that time as the measure of the central tendency, are recommended. Two such techniques are described in 10.2. Both techniques remove th...
SCOPE
1.1 This test method differentiates among solid electrical insulating materials for use at commercial power frequencies with respect to their voltage endurance under the action of corona (see Note 1). In general, this test method is more meaningful for rating materials with respect to their resistance to prolonged a-c stress under corona conditions than is dielectric strength.
Note 1—The term “corona” is used almost exclusively in this test method instead of “partial discharge”, because it is a visible glow at the edge of the smaller electrode. This is a difference in location, not in kind. Partial discharges also occur at the edges of electrodes, and in general corona describes an electrical discharge irrespective of its location.
1.2 The values stated in SI units are to be regarded as standard. The values given in parentheses are mathematical conversions to inch-pound units that are provided for information only and are not considered 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. For specific hazard statements, see Section 7.
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´1
Designation: D2275 − 01(Reapproved 2008)
Standard Test Method for
Voltage Endurance of Solid Electrical Insulating Materials
Subjected to Partial Discharges (Corona) on the Surface
This standard is issued under the fixed designation D2275; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
ε NOTE—The units statement in subsection 1.2 was corrected and old footnote ten deleted to conform to ASTM guide-
lines on sole source editorially in July 2008.
1. Scope D1868Test Method for Detection and Measurement of
Partial Discharge (Corona) Pulses in Evaluation of Insu-
1.1 This test method differentiates among solid electrical
lation Systems
insulating materials for use at commercial power frequencies
D5032PracticeforMaintainingConstantRelativeHumidity
with respect to their voltage endurance under the action of
by Means of Aqueous Glycerin Solutions
corona (see Note 1). In general, this test method is more
D6054Practice for Conditioning Electrical Insulating Mate-
meaningful for rating materials with respect to their resistance
rials for Testing (Withdrawn 2012)
to prolonged a-c stress under corona conditions than is dielec-
E41Terminology Relating To Conditioning
tric strength.
E104Practice for Maintaining Constant Relative Humidity
NOTE 1—The term “corona” is used almost exclusively in this test
by Means of Aqueous Solutions
method instead of “partial discharge”, because it is a visible glow at the
E171Practice for Conditioning and Testing Flexible Barrier
edge of the smaller electrode. This is a difference in location, not in kind.
Packaging
Partial discharges also occur at the edges of electrodes, and in general
2.2 Special Technical Publications:
corona describes an electrical discharge irrespective of its location.
1.2 The values stated in SI units are to be regarded as
Symposium on Corona, STP 198,ASTM, 1956.
standard. The values given in parentheses are mathematical
Corona Measurement and Interpretation, Engineering
conversions to inch-pound units that are provided for informa-
Dielectrics, Vol 1, STP 669, ASTM, 1979.
tion only and are not considered standard.
2.3 International Electrotechnical Commission (IEC) Docu-
1.3 This standard does not purport to address all of the
ments:
safety concerns, if any, associated with its use. It is the
IEC Publication60343Recommended test methods for de-
responsibility of the user of this standard to establish appro-
termining the relative resistance of insulating materials to
priate safety and health practices and determine the applica-
breakdown by surface discharges
bility of regulatory limitations prior to use. For specific hazard
2.4 Institute of Electrical and Electronic Engineers (IEEE)
statements, see Section 7.
Document:
IEEE SS 11205-TBR Guide for the Statistical Analysis of
2. Referenced Documents
Electrical Insulation Voltage Endurance Data, 1987
2.1 ASTM Standards:
3. Terminology
D149Test Method for Dielectric Breakdown Voltage and
DielectricStrengthofSolidElectricalInsulatingMaterials
3.1 Fordefinitionsofothertermsusedinthisstandard,refer
at Commercial Power Frequencies
to Terminology D1711 and Test Method D1868.
D1711Terminology Relating to Electrical Insulation
3.2 Definitions of Terms Specific to This Standard:
3.2.1 threshold voltage—That voltage below which failure
This test method is under the jurisdiction of ASTM Committee D09 on will not occur under the test conditions irrespective of the
Electrical and Electronic Insulating Materials and is the direct responsibility of
duration of the test.
Subcommittee D09.12 on Electrical Tests.
CurrenteditionapprovedMay1,2008.PublishedJuly2008.Originallyapproved
in 1964. Last previous edition approved in 2001 as D2275–01. DOI: 10.1520/ The last approved version of this historical standard is referenced on
D2275-01R08E01. www.astm.org.
2 4
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM 4th Floor, New York, NY 10036, http://www.ansi.org.
Standards volume information, refer to the standard’s Document Summary page on Available from Institute of Electrical and Electronics Engineers, Inc. (IEEE),
the ASTM website. 445 Hoes Ln., P.O. Box 1331, Piscataway, NJ 08854-1331, http://www.ieee.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
´1
D2275 − 01 (2008)
3.2.1.1 Discussion—Demonstration of a threshold is diffi- Test Method D1868. Also, comparative measurements of
cult when the slope of a volt-time curve is small, and failure corona power or energy by bridge and oscilloscope techniques
times are long. High frequency tests are often an aid in can be informative (see ASTM STP 198 and STP 669).
demonstration, by reducing the time required to reach a
4.7 If elevated frequencies are used to accelerate the test, it
necessary number of voltage cycles.
is recommended that the corona-discharge pulse heights and
3.2.2 voltage endurance, n—The time that an insulating
energy per cycle at the test frequency be compared with these
material can withstand a prolonged alternating voltage stress
values at rated power frequency. If the energy per cycle is the
under the action of surface corona.
same, it can be concluded that failure time is inversely
proportional to frequency.
3.2.3 voltage stress-time curve, n—A plot of the logarithm
of the mean or median time to failure of a material against
5. Significance and Use
voltage stress (or the logarithm of voltage stress) for a
5.1 This test method is used to compare the endurance of
particular set of test conditions.
different materials to the action of corona on the external
3.2.3.1 Discussion—Theplotisthequantitativedepictionof
the voltage stress endurance over a range of voltage stress for surfaces. A poor result on this test does not indicate that the
material is a poor selection for use at high voltage or at high
the conditions of test, and for the thickness tested. The curves
of a material obtained at two thicknesses are different. voltage stress in the absence of surface corona. Surface corona
should be distinguished from corona that occurs in internal
3.2.4 volt-time curve, n—A plot of the logarithm of the
cavities for which no standardized test has been developed.
meanormediantimetofailureofamaterialagainstvoltage(or
Evaluation of endurance by comparison of data on specimens
the logarithm of voltage) for a particular set of test conditions.
of different thickness is not valid.
3.2.4.1 Discussion—Theplotisthequantitativedepictionof
the voltage endurance over a range of voltage for the condi- 5.2 The processing of the material may affect the results
tions of the test, which includes the particular thickness tested. obtained.Forinstance,residualstrainsproducedbyquenching,
or high levels of crystallinity caused by slow cooling may
4. Summary of Test Method affecttheresult.Also,thetypeofmoldingprocess,injectionor
compression, may be important especially if the mixing of
4.1 In this test method, voltage sufficient to produce corona
fillers or the concentration and sizes of gas-filled cavities are
is applied to the specimen until failure occurs. Comparative
controlled in any degree by the process. Indeed, this test
voltageenduranceistherelativetimetofailureoftwodifferent
method may be used to examine the effects of processing.
materials of the same thickness when tested with similar
electrodes at the same voltage. Comparison is also possible in
5.3 The data are generated in the form of a set of values of
terms of the magnitude of voltage stress (kV/mm or kV/in.) lifetimes at a voltage. The dispersion of failure times can be
required to produce failure in a specified number of hours.
analyzed using Weibull or extreme value statistics to yield an
estimateofthecentralvalueofthedistributionanditsstandard
4.2 Surface corona exists in the electrically stressed gas
deviation. This is particularly recommended when the disper-
where electrodes are near insulation surfaces.
sion of failure times is large, and a comparison of lifetimes of
4.3 As with most tests at constant stress, there may be a
two materials must be made at a specified level of confidence.
large dispersion of times to failure for a given sample. The
5.4 This test is often used to demonstrate the differences
median time of nine specimens (time of fifth failure) may be
between different classes of materials, and to illustrate the
used as the failure time for the sample. This removes the
importance of eliminating corona in any application of a
necessityofwaitingforthelastfewtofail.Themeanmayalso
particular material. When the test is used for such purposes or
be determined statistically (see IEEESS 11205-TBR for addi-
other similar ones, the need for precision is reduced, and
tional information).
certain time saving techniques, such as truncating a test at the
4.4 Undertheproperconditions,thetestmaybeaccelerated
time of the fifth failure of a set of nine, and using that time as
by increasing the frequency of the applied voltage (see
the measure of the central tendency, are recommended. Two
Appendix X1).
suchtechniquesaredescribedin10.2.Bothtechniquesremove
4.5 Standardized test conditions and conditioning prior to the necessity of testing beyond median failure, and reduce the
testing are important. In particular, tests with specified air flow required testing time to approximately half of that required to
at both low and moderate humidities may be informative. In obtain failures on all specimens.
special cases, where a service condition is thought to alter the
5.5 Insulating materials operating in a gaseous medium are
corona endurance, this factor should be introduced as part of
subjected to corona attack at operating voltage on some types
the test and reported. Such conditions might include
of electrical apparatus in those regions where the voltage
elongation, elevated temperature, high humidity, other gases
gradientinthegasexceedsthecoronainceptionlevel.Onother
besides air, pollution, etc.
typesofequipment,wheredetectablecoronaisabsentinitially,
4.6 Additional information from the test may be obtained if itmayappearlaterduetotransientover-voltagesorchangesin
corona-voltage levels and corona intensity are measured at the insulation properties attending aging. Certain inorganic mate-
startofthetestandmonitoredatvariousstagesofdeterioration rials can tolerate corona for a long time. Many organic
of the insulation. The voltage levels include corona-inception materials are damaged quickly by corona, and for these,
voltage, corona-extinction voltage, and corona intensity using operation with no detectable corona is imperative. This test
´1
D2275 − 01 (2008)
methodintensifiessomeofthemorecommonlymetconditions procedures of this test for void-free materials such as polyeth-
of corona attack so that materials may be evaluated in a time ylenesheetgenerallyhasacontinuouscurvaturethatisslightly
that is relatively short compared to the life of the equipment.
concave upward. The low voltage end of the curve tends
As with most accelerated life tests, caution is necessary in
toward the horizontal and approaches a threshold voltage
extrapolationfromtheindicatedlifetoactuallifeundervarious
below which the curve does not go.Asimilar threshold would
operating conditions in the field.
be expected for many materials in an application involving
surface corona. Moreover, if the material possesses a low
5.6 The failure produced by corona may be due to one of
electric strength (as measured by Test Method D149), or
several possible factors. The corona may erode the insulation
until the remaining insulation can no longer withstand the especiallyifinservicethereisanothermechanismoffailurein
appliedvoltage.Thecoronamaycausetheinsulationsurfaceto the short time range of this test, the shape of the left hand end
become conducting. For instance, carbonization may occur, so
of the curve would be affected and would not reach the same
thatfailureoccursquickly.Ontheotherhand,compoundssuch
high levels of stress as are exhibited by polyethylene either on
asoxalicacidcrystalsmaybeformed,aswithpolyethylene,in
this test or in many service applications, including surface
which case the surface conductance will vary with ambient
corona.Insummary,voltagestress-timecurvesareusefultools
humidity, and at moderate humidities the conductance may be
for examining modes and mechanisms of failure, but must be
at the proper level to reduce the potential gradient at the
used with care.
electrodeedge,andthuscauseeitherareductionintheamount
5.9 For materials that possess a basic resistance to corona,
of corona, or its cessation, thus retarding failure. The corona
such as mica, or, to a smaller degree, silicone rubber, the time
may cause a “treeing” within the insulation, which may
progress to failure. It may release gases within the insulation required for the curve to reach the threshold produced by
thatchangeitsphysicaldimensions.Itmaychangethephysical corona may be greater by many orders of magnitude than the
properties of an insulating material; for instance, it may cause
time required for materials such as polyethylene, polyethylene
the material to embrittle or crack, and thus make it useless.
terephthalate, or polytetrafluoroethylene.
5.7 Tests are often made in open air, at 50% relative
5.10 Thevariabilityofthetimetofailureisafunctionofthe
humidity.Itmaybeimportantforsomematerialstomaketests
constancy of the parameters of the test, such as the test
in circulating air at 20% relative humidity or less (see
voltages, which should be monitored. It is also a significant
AppendixX1).Iftestsaremadeinanenclosure,therestriction
materialproperty.TheWeibullslopefactor,β,isrecommended
in the flow of air or other gas may influence the results (see
as a measure of variability. β is the slope obtained when
Appendix X2).
percent failure is plotted against failure time on Weibull
5.8 The shape of the (voltage stress)-(time-to-failure) curve
probability paper. Such a plot is called a “Weibull probability
is sometimes useful as an indicator of the useable electric
plot” (see Fig. 1).
strength of a material in an application involving surface
5.11 The shape of the Weibull probability plot can provide
corona and its variation with time of application of voltage,
additional information. A non-straight-line plot may indicate
though such comp
...
This document is not anASTM standard and is intended only to provide the user of anASTM 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.
An American National Standard
e1
Designation:D2275–95 Designation:D2275–01(Reapproved 2008)
Standard Test Method for
Voltage Endurance of Solid Electrical Insulating Materials
Subjected to Partial Discharges (Corona) on the Surface
This standard is issued under the fixed designation D2275; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
e NOTE—The units statement in subsection 1.2 was corrected and old footnote ten deleted to conform toASTM guidelines on
sole source editorially in July 2008.
1. Scope
1.1 This test method differentiates among solid electrical insulating materials for use at commercial power frequencies with
respecttotheirvoltageenduranceundertheactionofcorona(seeNote1).Ingeneral,thistestmethodismoremeaningfulforrating
materials with respect to their resistance to prolonged a-c stress under corona conditions than is dielectric strength.
NOTE 1—The term “corona” is used almost exclusively in this test method instead of “partial discharge”, because it is a visible glow at the edge of
thesmallerelectrode.Thisisadifferenceinlocation,notinkind.Partialdischargesalsooccurattheedgesofelectrodes,andingeneralcoronadescribes
an electrical discharge irrespective of its location.
1.2The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.2 The values stated in SI units are to be regarded as standard. The values given in parentheses are mathematical conversions
to inch-pound units that are provided for information only and are not considered 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. For specific hazard statements, see Section 7.
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
D618PracticeforConditioningPlasticsandElectricalInsulatingMaterialsforTesting 1711 TerminologyRelatingtoElectrical
Insulation
D1711Terminology Relating to Electrical Insulation 1868 Test Method for Detection and Measurement of Partial Discharge
(Corona) Pulses in Evaluation of Insulation Systems
D1868Test Method for Detection and Measurement of Partial Discharge (Corona) Pulses in Evaluation of Insulation Systems
D5032 Practice for Maintaining Constant Relative Humidity by Means of Aqueous Glycerin Solutions
D6054 Practice for Conditioning Electrical Insulating Materials for Testing
E41 Terminology Relating toTo Conditioning
E104 Practice for Maintaining Constant Relative Humidity by Means of Aqueous Solutions
E171 Specification for Standard Atmospheres for Conditioning and Testing Flexible Barrier Materials
2.2 Special Technical Publications:
Symposium on Corona, STP 198,ASTM, 1956.
, ASTM, 1956.
Corona Measurement and Interpretation, Engineering Dielectrics, Vol 1,STP STP 669, ASTM, 1979.
2.3 International Electrotechnical Commission (IEC) Documents:
IEC Publication343RecommendedTest Methods for Determining the Relative Resistance of Insulating Materials to Breakdown
by Surface Discharges
ThistestmethodisunderthejurisdictionofASTMCommitteeD-9D09onElectricalandElectronicInsulatingMaterialsandisthedirectresponsibilityofSubcommittee
D09.12 on Electrical Tests.
CurrenteditionapprovedSept.10,1995.May1,2008.PublishedNovember1995.July2008.OriginallypublishedasD2275–64T.approvedin1964.Lastpreviousedition
´1
D2275–89(1994) .approved in 2001 as D2275–01.
ForreferencedASTMstandards,visittheASTMwebsite,www.astm.org,orcontactASTMCustomerServiceatservice@astm.org.For Annual Book of ASTM Standards
, Vol 10.01.volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
e1
D2275–01 (2008)
IECPublication60343 Recommendedtestmethodsfordeterminingtherelativeresistanceofinsulatingmaterialstobreakdown
by surface discharges
2.4 Institute of Electrical and Electronic Engineers (IEEE) Document:
IEEEP930.7-1987“GuidefortheStatisticalAnalysisofVoltageEnduranceData”IEEESS11205-TBR GuidefortheStatistical
Analysis of Electrical Insulation Voltage Endurance Data, 1987
3. Terminology
3.1Descriptions of Terms:
3.1.1threshold voltage—That voltage below which failure will not occur under the test conditions irrespective of the duration
of the test. (Demonstration of a threshold is difficult when the slope of a volt-time curve is small, and failure times are long. High
frequency tests are often an aid in demonstration, by reducing the time required to reach a necessary number of voltage cycles.)
3.1.2voltage endurance—the time that an insulating material can withstand a prolonged alternating voltage stress under the
action of surface corona.
3.1.3voltage stress-time curve—Aplotofthelogarithmofthemeanormediantimetofailureofamaterialagainstvoltagestress
(or the logarithm of voltage stress) for a particular set of test conditions.The plot is the quantitative depiction of the voltage stress
endurance over a range of voltage stress for the conditions of test, and for the thickness tested. The curves of a material obtained
at two thicknesses are different.
3.1.4volt-time curve—A plot of the logarithm of the mean or median time to failure of a material against voltage (or the
logarithm of voltage) for a particular set of test conditions. The plot is the quantitative depiction of the voltage endurance over a
range of voltage for the conditions of the test, which includes the particular thickness tested.
3.2Definitions—See Terminology D1711 and Test Method D1868
3.1 For definitions of other terms used in this standard, refer to Terminology D1711 and Test Method D1868.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 threshold voltage—That voltage below which failure will not occur under the test conditions irrespective of the duration
of the test.
3.2.1.1 Discussion—Demonstration of a threshold is difficult when the slope of a volt-time curve is small, and failure times are
long.Highfrequencytestsareoftenanaidindemonstration,byreducingthetimerequiredtoreachanecessarynumberofvoltage
cycles.
3.2.2 voltage endurance, n—Thetimethataninsulatingmaterialcanwithstandaprolongedalternatingvoltagestressunderthe
action of surface corona.
3.2.3 voltage stress-time curve, n—Aplot of the logarithm of the mean or median time to failure of a material against voltage
stress (or the logarithm of voltage stress) for a particular set of test conditions.
3.2.3.1 Discussion—The plot is the quantitative depiction of the voltage stress endurance over a range of voltage stress for the
conditions of test, and for the thickness tested. The curves of a material obtained at two thicknesses are different.
3.2.4 volt-time curve, n—A plot of the logarithm of the mean or median time to failure of a material against voltage (or the
logarithm of voltage) for a particular set of test conditions.
3.2.4.1 Discussion—The plot is the quantitative depiction of the voltage endurance over a range of voltage for the conditions
of the test, which includes the particular thickness tested.
4. Summary of Test Method
4.1 Inthistestmethod,voltagesufficienttoproducecoronaisappliedtothespecimenuntilfailureoccurs.Comparativevoltage
endurance is the relative time to failure of two different materials of the same thickness when tested with similar electrodes at the
same voltage. Comparison is also possible in terms of the magnitude of voltage stress (kV/mm or kV/in.) required to produce
failure in a specified number of hours.
4.2 Surface corona exists in the electrically stressed gas where electrodes are near insulation surfaces.
4.3 Aswithmosttestsatconstantstress,theremaybealargedispersionoftimestofailureforagivensample.Themediantime
of nine specimens (time of fifth failure) may be used as the failure time for the sample. This removes the necessity of waiting for
the last few to fail. The mean may also be determined statistically (see IEEEP930.7IEEESS 11205-TBR for additional
information).
4.4 Under the proper conditions, the test may be accelerated by increasing the frequency of the applied voltage (seeAppendix
X1).
4.5 Standardizedtestconditionsandconditioningpriortotestingareimportant.Inparticular,testswithspecifiedairflowatboth
low and moderate humidities may be informative. In special cases, where a service condition is thought to alter the corona
Annual Book of ASTM Standards, Vol 08.01.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
Annual Book of ASTM Standards, Vol 10.02.
Available from Institute of Electrical and Electronics Engineers, Inc. (IEEE), 445 Hoes Ln., P.O. Box 1331, Piscataway, NJ 08854-1331, http://www.ieee.org.
e1
D2275–01 (2008)
endurance, this factor should be introduced as part of the test and reported. Such conditions might include elongation, elevated
temperature, high humidity, other gases besides air, pollution, etc.
4.6 Additionalinformationfromthetestmaybeobtainedifcorona-voltagelevelsandcoronaintensityaremeasuredatthestart
of the test and monitored at various stages of deterioration of the insulation. The voltage levels include corona-inception voltage,
corona-extinction voltage, and corona intensity using Test Method D1868.Also, comparative measurements of corona power or
energy by bridge and oscilloscope techniques can be informative (see ASTM STP 198 and STP 669).
4.7 Ifelevatedfrequenciesareusedtoacceleratethetest,itisrecommendedthatthecorona-dischargepulseheightsandenergy
per cycle at the test frequency be compared with these values at rated power frequency. If the energy per cycle is the same, it can
be concluded that failure time is inversely proportional to frequency.
5. Significance and Use
5.1 This test method is used to compare the endurance of different materials to the action of corona on the external surfaces.
Apoor result on this test does not indicate that the material is a poor selection for use at high voltage or at high voltage stress in
the absence of surface corona. Surface corona should be distinguished from corona that occurs in internal cavities for which no
standardized test has been developed. Evaluation of endurance by comparison of data on specimens of different thickness is not
valid.
5.2 Theprocessingofthematerialmayaffecttheresultsobtained.Forinstance,residualstrainsproducedbyquenching,orhigh
levels of crystallinity caused by slow cooling may affect the result.Also, the type of molding process, injection or compression,
maybeimportantespeciallyifthemixingoffillersortheconcentrationandsizesofgas-filledcavitiesarecontrolledinanydegree
by the process. Indeed, this test method may be used to examine the effects of processing.
5.3 Thedataaregeneratedintheformofasetofvaluesoflifetimesatavoltage.Thedispersionoffailuretimescanbeanalyzed
usingWeibullorextremevaluestatisticstoyieldanestimateofthecentralvalueofthedistributionanditsstandarddeviation.This
is particularly recommended when the dispersion of failure times is large, and a comparison of lifetimes of two materials must be
made at a specified level of confidence.
5.4 This test is often used to demonstrate the differences between different classes of materials, and to illustrate the importance
of eliminating corona in any application of a particular material.When the test is used for such purposes or other similar ones, the
need for precision is reduced, and certain time saving techniques, such as truncating a test at the time of the fifth failure of a set
of nine, and using that time as the measure of the central tendency, are recommended.Two such techniques are described in 10.2.
Bothtechniquesremovethenecessityoftestingbeyondmedianfailure,andreducetherequiredtestingtimetoapproximatelyhalf
of that required to obtain failures on all specimens.
5.5 Insulating materials operating in a gaseous medium are subjected to corona attack at operating voltage on some types of
electrical apparatus in those regions where the voltage gradient in the gas exceeds the corona inception level. On other types of
equipment, where detectable corona is absent initially, it may appear later due to transient over-voltages or changes in insulation
properties attending aging. Certain inorganic materials can tolerate corona for a long time. Many organic materials are damaged
quickly by corona, and for these, operation with no detectable corona is imperative.This test method intensifies some of the more
commonly met conditions of corona attack so that materials may be evaluated in a time that is relatively short compared to the
lifeoftheequipment.Aswithmostacceleratedlifetests,cautionisnecessaryinextrapolationfromtheindicatedlifetoactuallife
under various operating conditions in the field.
5.6 The failure produced by corona may be due to one of several possible factors. The corona may erode the insulation until
the remaining insulation can no longer withstand the applied voltage. The corona may cause the insulation surface to become
conducting. For instance, carbonization may occur, so that failure occurs quickly. On the other hand, compounds such as oxalic
acid crystals may be formed, as with polyethylene, in which case the surface conductance will vary with ambient humidity, and
at moderate humidities the conductance may be at the proper level to reduce the potential gradient at the electrode edge, and thus
cause either a reduction in the amount of corona, or its cessation, thus retarding failure. The corona may cause a “treeing” within
theinsulation,whichmayprogresstofailure.Itmayreleasegaseswithintheinsulationthatchangeitsphysicaldimensions.Itmay
changethephysicalpropertiesofaninsulatingmaterial;forinstance,itmaycausethematerialtoembrittleorcrack,andthusmake
it useless.
5.7 Tests are often made in open air, at 50% relative humidity. It may be important for some materials to make tests in
circulating air at 20% relative humidity or less (see Appendix X1)
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