ASTM_D_7894_-_D_7894M_-_14.pdf

Designation:D7894/D7894M14Standard 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 case 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 by 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 substrate 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 from 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 determine 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 FrequenciesD1711 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 used 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 conductivesolution,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 ispossible 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.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 electrical 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 breakdown.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 ofSubcommittee 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.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 International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 6.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:6.3.1 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).NOTE1A solution comprised of tap water and a solute such as NaClor NH4Cl has been found to be