1、Designation: D7894/D7894M 14Standard Test Method forThermal Endurance of Coating Powders Used for IntegralBus Bar Insulation Systems1This standard is issued under the fixed designation D7894/D7894M; the number immediately following the designation indicates theyear of original adoption or, in the ca
2、se of revision, the year of last revision. A number in parentheses indicates the year of lastreapproval. A superscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method provides a procedure for evaluatingthermal endurance of coating powders b
3、y determining thelength of aging time at selected elevated temperatures requiredto achieve dielectric breakdown at room temperature at apre-determined proof voltage. Thermal endurance is expressedin terms of a temperature index.1.2 This test method is applicable to insulating powdersused over a subs
4、trate material of copper or aluminum.1.3 UnitsThe values stated in either SI units or inch-pound units are to be regarded separately as standard. Thevalues stated in each system are not necessarily exact equiva-lents; therefore, each system shall be used independently of theother. Combining values f
5、rom the two systems is likely toresult in non-conformance with the standard.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determi
6、ne the applica-bility of regulatory limitations prior to use. Specific precau-tionary statements are given in Section 7.2. Referenced Documents2.1 ASTM Standards:2D149 Test Method for Dielectric Breakdown Voltage andDielectric Strength of Solid Electrical Insulating Materialsat Commercial Power Freq
7、uenciesD1711 Terminology Relating to Electrical InsulationD5423 Specification for Forced-Convection Laboratory Ov-ens for Evaluation of Electrical Insulation2.2 IEEE Document:3IEEE 101A Simplified Method for Calculation of the Re-gression Line3. Terminology3.1 DefinitionsFor definitions of terms use
8、d in this testmethod refer to Terminology D1711.4. Summary of Test Method4.1 Specimens are aged in air at a minimum of threetemperatures above the expected use temperature of the mate-rial. Exposure to ambient stress conditions, followed bydielectric proof voltage tests in an electrically conductive
9、solution, are periodically conducted to determine the time ofaging at each elevated temperature required to reduce thebreakdown voltage to a pre-selected percentage of the originalbreakdown value. These thermal life values are used toconstruct a thermal endurance graph by means of which it ispossibl
10、e to estimate a temperature index, corresponding to athermal life as specified in the material specification or asagreed upon between the user and the supplier. It is possiblethat a material will have multiple indices, each correspondingto a different thermal life as required by a given application.
11、5. Significance and Use5.1 Amajor factor affecting the life of insulating materials isthermal degradation. It is possible that other factors, such asmoisture and vibration, will cause failures after the materialhas been weakened by thermal degradation.5.2 Electrical insulation is effective in electr
12、ical equipmentonly as long as it retains its physical and electrical integrity.The following are potential indicators of thermal degradation:weight change, porosity, crazing, and generally a reduction inflexibility. Thermal degradation is usually accompanied by anultimate reduction in dielectric bre
13、akdown.5.3 This test method is useful in determining the thermalendurance of coating powders applied over a copper oraluminum substrate material.1This test method is under the jurisdiction of ASTM Committee D09 onElectrical and Electronic Insulating Materials and is the direct responsibility ofSubco
14、mmittee D09.01 on Electrical Insulating Varnishes, Powders and Encapsulat-ing Compounds.Current edition approved Nov. 1, 2014. Published November 2014. DOI:10.1520/D7894_D7894M-14.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org
15、. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Available from Institute of Electrical and Electronics Engineers, Inc. (IEEE),445 Hoes Ln., P.O. Box 1331, Piscataway, NJ 08854-1331, http:/www.ieee.org.Copyright ASTM Internationa
16、l, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States16. Apparatus6.1 Dielectric Breakdown Test SetThe set shall meet therequirements of Test Method D149.6.2 OvensOvens shall meet the requirements of Specifi-cation D5423, Type I.6.3 Electrically Conductive Solution:6
17、.3.1 An appropriate container shall be selected for theelectrically conductive solution.6.3.2 Prepare a solution having a conductivity of at least2.5 mS (millisiemens).NOTE 1A solution comprised of tap water and a solute such as NaClor NH4Cl has been found to be acceptable. Since the water solution
18、is usedto detect (not cause) material decomposition/breakdown, the solution isacceptable for use as long as it meets the minimum conductivityrequirements. It is recommended to change the solution when it becomesdifficult to observe breakdown due to cloudiness or other contamination.6.3.3 During test
19、ing, the conductive solution is at groundpotential. The ground connection, using an immersedelectrode, is made using Nichrome, stainless steel, or othernon-corrosive metal.6.3.4 Fit the container with a cover to prevent evaporationof the solution when not in use.7. Hazards7.1 WarningLethal voltages
20、are a potential hazard duringthe performance of this test. It is essential that the testapparatus, and all associated equipment electrically connectedto it, be properly designed and installed for safe operation.Solidly ground all electrically conductive parts which arepossible for a person to contac
21、t during the test. Provide meansfor use at the completion of any test to ground any parts whichwere at high voltage during the test, or have the potential foracquiring an induced charge during the test, or retain a chargeeven after disconnection of the voltage source. Thoroughlyinstruct all operator
22、s as to the correct procedures for perform-ing tests safely. When making high voltage tests, particularly incompressed gas or in oil, it is possible for the energy releasedat breakdown to be sufficient to result in fire, explosion, orrupture of the test chamber. Design test equipment, testchambers,
23、and test specimens so as to minimize the possibilityof such occurrences and to eliminate the possibility of personalinjury. If the potential for fire exists, have fire suppressionequipment available.8. Test Specimens8.1 The substrate material to be used for evaluating integralbus-bar insulation syst
24、ems shall be copper or aluminum, orboth. It is preferred that the configuration of the substratematerial be representative of the end application. One configu-ration that has been found to be useful is shown in Fig. 1.Alternative configurations are acceptable if agreed to by allinterested parties.8.
25、2 The substrate shall be coated with the powder and curedusing the manufacturers standard process. One end of thespecimens shall be left uncoated for making an electricalconnection during the dielectric strength tests.9. Number of Test Specimens9.1 A set of specimens for thermal aging shall consist
26、of atleast ten specimens. Prepare one set for each aging temperaturechosen.9.2 The initial value (unaged) set shall consist of at leasttwenty specimens.9.3 The minimum number of test specimens is 50. It isrecommended to prepare extra specimens for situations such asthe need to add additional aging t
27、emperatures, or to havereplacement specimens available for any specimens that arefound during the screening test to be unsuitable for use inthermal aging.10. Calibration and Standardization10.1 Expose the test material to at least three aging tem-peratures. The preferred number of aging temperatures
28、 is four.10.2 Minimum thermal life target values for the high andlow temperatures shall be selected based on application.FIG. 1 Example Specimen DimensionsD7894/D7894M 14210.3 Aging temperatures shall differ by at least 10C.NOTE 2Experience has shown that the suggested target thermal lifevalues list
29、ed in Table 1 are generally satisfactory.11. Selection of Aging Cycles11.1 To provide approximately equal exposures to the otherconditionings, and to more accurately determine the propertyendpoint, the heat aging time per cycle shall be shorter for thehigher aging temperatures and longer for the low
30、er agingtemperatures.11.2 Cycle times shall be selected such that each setundergoes an adequate (but not excessive) number of cyclesbefore completion. In order to obtain an appropriate number ofcycles, the following adjustments to the cycle time are recom-mended:11.2.1 If no specimens in a set fail
31、by the end of the fourthcycle, double the heat aging period of the test cycle.11.2.2 If three or more specimens in a set fail by the end ofthe fourth cycle, halve the heat aging period of the test cycle.11.2.3 Use observations in the highest temperature set toselect appropriate adjustments to the ot
32、her temperature sets.NOTE 3Experience has shown that the suggested cycle times in Table1 are generally satisfactory.12. Initial Value Test12.1 To determine an initial dielectric strength level, at leasttwenty specimens shall be selected at random from the sampleand subjected to a voltage breakdown t
33、est.12.2 Connect one electrode of the tester to the uncoated endof the specimen. Suspend the coated end of the specimen in theelectrically conductive solution described in 6.3.2 (see Fig. 2).The total surface area to be tested shall be sufficient to berepresentative of the intended application.12.3
34、Apply voltage uniformly to the test electrode in accor-dance with the guidelines of the Short-Time Test described in12.2.1 of Test Method D149, unless another rate is specified,until breakdown occurs.12.4 Make one measurement on each specimen and com-pute the average dielectric breakdown voltage for
35、 the set.Record this as the initial value.13. Screening Test13.1 Prior to thermal aging, all specimens shall be subjectedto a screening test in order to remove defective units. Anyspecimen that cannot pass the screening test shall be discardedand a replacement specimen selected for thermal aging.13.
36、2 Select a screening test voltage level that is a percentageof the average initial value.13.3 Select a screening test period of time that is expected tocause failure in weak specimens without causing damage to theremaining specimens.NOTE 4Experience has shown that a screening test voltage level of75
37、 % of the initial value for a period of 10 s is generally satisfactory.13.4 Suspend each specimen in the electrically conductivesolution in the same manner as described for the initial valuetest. Apply voltage uniformly to the test electrode in accor-dance with the guidelines of the Short-Time Test
38、described in12.2.1 of Test Method D149, unless another rate is specified,until the screening voltage level is achieved. Maintain thevoltage for the screening test period before removing thevoltage from the specimen.13.5 Discard any specimen that breaks down. Specimensthat do not break down during th
39、e screening test are acceptablefor use in the thermal aging.14. Procedure14.1 Thermal Aging of SpecimensTag four sets of speci-mens by any reliably permanent means and expose the sets inthe oven at each of the selected temperatures. Position the setsso that free movement of air exists across the ent
40、ire specimen.14.2 Stress Cycling:14.2.1 Following the completion of each thermal exposure,all specimens in the set shall be removed from the oven andallowed to cool to 23 6 2C.14.2.2 All specimens shall then be subjected to a coldexposure and allowed to reach equilibrium with the coldenvironment.14.
41、2.2.1 If the coating resin is intended for outdoorapplications, the cold exposure temperature shall be minus 20.06 2.0C minus 4.0 6 3.6F.14.2.2.2 If the coating resin is intended for indoorapplications, the cold exposure temperature shall be 0.0 62.0C 32.0 6 3.6F.14.2.3 Following the cold exposure,
42、all specimens shall beallowed to return to 23 6 2C.14.2.4 All specimens shall then be mounted on a shake tableand operated for a period of 10 000 cycles with a 60 Hzoscillating motion and an acceleration of 7 Gs.14.2.4.1 The specimens shall be mounted such that themotion occurs at right angles to th
43、e largest flat surface of thespecimen and parallel to the smallest flat surface of thespecimen (see Fig. 3).14.2.5 Following the vibration exposure, all specimens shallbe placed into a humidity exposure for a minimum of 24 h at25 to 30C 77 to 86F, with the relative humidity adjusted to95 to 100 %.NO
44、TE 5The humidity exposure is intended to minimize the occur-rence of trapped air in any potential fractures developed in the coatingduring the previous cycle.14.2.6 The total time required for each stress exposure willbe dependent on the configuration of the specimens used. Theminimum time for each
45、exposure environment shall be the timerequired for the specimens to reach equilibrium with theexposure environment.TABLE 1 Suggested Cycle Times and Target LivesTemperature Set Cycle Time Target LifeHigh Temperature 4872 h 400700 hMiddle High Temperature 168 h 10001500 hMiddle Low Temperature 336 h
46、20003000 hLow Temperature 504 h 5000+ hD7894/D7894M 143NOTE 6Experience has shown that exposures longer than the timerequired to reach equilibrium do not influence the test results.14.3 Testing of Specimens:14.3.1 Select a proof test voltage level that is a percentageof the average initial value. Th
47、e same proof test voltage levelshall be used in all subsequent proof testing.14.3.2 Select a proof test period of time that is expected todetect the onset of thermal decomposition without causingdamage to intact specimens. The same proof test period of timeshall be used in all subsequent proof testi
48、ng.NOTE 7Experience has shown that a proof test voltage level of 50 %of the initial value for a period of 10 s is generally satisfactory.14.3.3 Within1hofremoval from the humidity exposure,subject each specimen to a proof voltage test. Suspend eachspecimen in the electrically conductive solution in
49、the samemanner as described for the initial value test. Apply voltageuniformly to the test electrode in accordance with the guide-lines of the Short-Time Test described in 12.2.1 of Test MethodD149, unless another rate is specified, until the proof voltagelevel is achieved. Maintain the voltage for the proof test periodbefore removing the voltage from the specimen.NOTE 8Testing is permissible on one or more specimens at a time, andis limited only by the physical capacity of the test container. Failures willbe more easily identified if the specimen
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