1、Designation: D257 14Standard Test Methods forDC Resistance or Conductance of Insulating Materials1This standard is issued under the fixed designation D257; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision.
2、A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the U.S. Department of Defense.1. Scope*1.1 These test methods cover direct-current proced
3、ures forthe measurement of dc insulation resistance, volume resistance,and surface resistance. From such measurements and thegeometric dimensions of specimen and electrodes, both vol-ume and surface resistivity of electrical insulating materialscan be calculated, as well as the corresponding conduct
4、ancesand conductivities.1.2 These test methods are not suitable for use in measuringthe electrical resistance/conductance of moderately conductivematerials. Use Test Method D4496 to evaluate such materials.1.3 This standard describes several general alternativemethodologies for measuring resistance
5、(or conductance).Specific materials can be tested most appropriately by usingstandardASTM test methods applicable to the specific materialthat define both voltage stress limits and finite electrificationtimes as well as specimen configuration and electrode geom-etry. These individual specific test m
6、ethodologies would bebetter able to define the precision and bias for the determina-tion.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 practice
7、s and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D150 Test Methods for AC Loss Characteristics and Permit-tivity (Dielectric Constant) of Solid Electrical InsulationD374 Test Methods for Thickness of Solid Electrical Insu-lation (Me
8、tric) D0374_D0374MD1169 Test Method for Specific Resistance (Resistivity) ofElectrical Insulating LiquidsD1711 Terminology Relating to Electrical InsulationD4496 Test Method for D-C Resistance or Conductance ofModerately Conductive MaterialsD5032 Practice for Maintaining Constant Relative Humidityby
9、 Means of Aqueous Glycerin SolutionsD6054 Practice for Conditioning Electrical Insulating Mate-rials for Testing (Withdrawn 2012)3E104 Practice for Maintaining Constant Relative Humidityby Means of Aqueous Solutions3. Terminology3.1 Definitions:3.1.1 The following definitions are taken from Terminol
10、ogyD1711 and apply to the terms used in the text of this standard.3.1.2 conductance, insulation, nthe ratio of the totalvolume and surface current between two electrodes (on or in aspecimen) to the dc voltage applied to the two electrodes.3.1.2.1 DiscussionInsulation conductance is the recipro-cal o
11、f insulation resistance.3.1.3 conductance, surface, nthe ratio of the currentbetween two electrodes (on the surface of a specimen) to the dcvoltage applied to the electrodes.3.1.3.1 Discussion(Some volume conductance is unavoid-ably included in the actual measurement.) Surface conductanceis the reci
12、procal of surface resistance.3.1.4 conductance, volume, nthe ratio of the current in thevolume of a specimen between two electrodes (on or in thespecimen) to the dc voltage applied to the two electrodes.3.1.4.1 DiscussionVolume conductance is the reciprocalof volume resistance.3.1.5 conductivity, su
13、rface, nthe surface conductancemultiplied by that ratio of specimen surface dimensions (dis-tance between electrodes divided by the width of electrodesdefining the current path) which transforms the measuredconductance to that obtained if the electrodes had formed theopposite sides of a square.1Thes
14、e test methods are under the jurisdiction of ASTM Committee D09 onElectrical and Electronic Insulating Materials and are the direct responsibility ofSubcommittee D09.12 on Electrical Tests.Current edition approved April 1, 2014. Published May 2014. Originallyapproved in 1925. Last previous edition a
15、pproved in 2007 as D257 07. DOI:10.1520/D0257-14.2For 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.3The last ap
16、proved version of this historical standard is referenced onwww.astm.org.*A Summary of Changes section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accor
17、dance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.13.1.5.1 DiscussionSurface conduc
18、tivity is expressed insiemens. It is popularly expressed as siemens/square (the sizeof the square is immaterial). Surface conductivity is thereciprocal of surface resistivity.3.1.6 conductivity, volume, nthe volume conductancemultiplied by that ratio of specimen volume dimensions(distance between el
19、ectrodes divided by the cross-sectionalarea of the electrodes) which transforms the measured conduc-tance to that conductance obtained if the electrodes had formedthe opposite sides of a unit cube.3.1.6.1 DiscussionVolume conductivity is usually ex-pressed in siemens/centimetre or in siemens/metre a
20、nd is thereciprocal of volume resistivity.3.1.7 moderately conductive, adjdescribes a solid materialhaving a volume resistivity between 1 and 10 000 000 -cm.3.1.8 resistance, insulation, (Ri), nthe ratio of the dcvoltage applied to two electrodes (on or in a specimen) to thetotal volume and surface
21、current between them.3.1.8.1 DiscussionInsulation resistance is the reciprocalof insulation conductance.3.1.9 resistance, surface, (Rs), nthe ratio of the dc voltageapplied to two electrodes (on the surface of a specimen) to thecurrent between them.3.1.9.1 Discussion(Some volume resistance is unavoi
22、d-ably included in the actual measurement.) Surface resistance isthe reciprocal of surface conductance.3.1.10 resistance, volume, (Rv), nthe ratio of the dcvoltage applied to two electrodes (on or in a specimen) to thecurrent in the volume of the specimen between the electrodes.3.1.10.1 DiscussionVo
23、lume resistance is the reciprocal ofvolume conductance.3.1.11 resistivity, surface, (s), nthe surface resistancemultiplied by that ratio of specimen surface dimensions (widthof electrodes defining the current path divided by the distancebetween electrodes) which transforms the measured resistanceto
24、that obtained if the electrodes had formed the opposite sidesof a square.3.1.11.1 DiscussionSurface resistivity is expressed inohms. It is popularly expressed also as ohms/square (the size ofthe square is immaterial). Surface resistivity is the reciprocal ofsurface conductivity.3.1.12 resistivity, v
25、olume, (v), nthe volume resistancemultiplied by that ratio of specimen volume dimensions(cross-sectional area of the specimen between the electrodesdivided by the distance between electrodes) which transformsthe measured resistance to that resistance obtained if theelectrodes had formed the opposite
26、 sides of a unit cube.3.1.12.1 DiscussionVolume resistivity is usually ex-pressed in ohm-centimetres (preferred) or in ohm-metres.Volume resistivity is the reciprocal of volume conductivity.4. Summary of Test Methods4.1 The resistance or conductance of a material specimen orof a capacitor is determi
27、ned from a measurement of current orof voltage drop under specified conditions. By using theappropriate electrode systems, surface and volume resistanceor conductance are measured separately. The resistivity orconductivity is calculated with the known specimen andelectrode dimensions are known.5. Si
28、gnificance and Use5.1 Insulating materials are used to isolate components of anelectrical system from each other and from ground, as well asto provide mechanical support for the components. For thispurpose, it is generally desirable to have the insulation resis-tance as high as possible, consistent
29、with acceptablemechanical, chemical, and heat-resisting properties. Sinceinsulation resistance or conductance combines both volumeand surface resistance or conductance, its measured value ismost useful when the test specimen and electrodes have thesame form as is required in actual use. Surface resi
30、stance orconductance changes rapidly with humidity, while volumeresistance or conductance changes slowly with the total changebeing greater in some cases.5.2 Resistivity or conductivity is used to predict, indirectly,the low-frequency dielectric breakdown and dissipation factorproperties of some mat
31、erials. Resistivity or conductivity isoften used as an indirect measure of: moisture content, degreeof cure, mechanical continuity, or deterioration of varioustypes. The usefulness of these indirect measurements is depen-dent on the degree of correlation established by supportingtheoretical or exper
32、imental investigations. A decrease of sur-face resistance results either in an increase of the dielectricbreakdown voltage because the electric field intensity isreduced, or a decrease of the dielectric breakdown voltagebecause the area under stress is increased.5.3 All the dielectric resistances or
33、 conductances depend onthe length of time of electrification and on the value of appliedvoltage (in addition to the usual environmental variables).These must be known and reported to make the measured valueof resistance or conductance meaningful. Within the electricalinsulation materials industry, t
34、he adjective “apparent” is gen-erally applied to resistivity values obtained under conditions ofarbitrarily selected electrification time. See X1.4.5.4 Volume resistivity or conductivity is calculated fromresistance and dimensional data for use as an aid in designingan insulator for a specific appli
35、cation. Studies have shownchanges of resistivity or conductivity with temperature andhumidity (1, 2, 3, 4).4These changes must be known whendesigning for operating conditions. Volume resistivity or con-ductivity determinations are often used in checking the unifor-mity of an insulating material, eit
36、her with regard to processingor to detect conductive impurities that affect the quality of thematerial and that are not readily detectable by other methods.5.5 Volume resistivities above 1021cm (1019m), cal-culated from data obtained on specimens tested under usuallaboratory conditions, are of doubt
37、ful validity, considering thelimitations of commonly used measuring equipment.5.6 Surface resistance or conductance cannot be measuredaccurately, only approximated, because some degree of volume4The boldface numbers in parentheses refer to the list of references appended tothese test methods.D257 14
38、2resistance or conductance is always involved in the measure-ment. The measured value is also affected by the surfacecontamination. Surface contamination, and its rate ofaccumulation, is affected by many factors including electro-static charging and interfacial tension. These, in turn, affect thesur
39、face resistivity. Surface resistivity or conductivity is con-sidered to be related to material properties when contaminationis involved but is not a material property of electrical insulationmaterial in the usual sense.6. Electrode Systems6.1 The electrodes for insulating materials are to allowintim
40、ate contact with the specimen surface, without introduc-ing significant error because of electrode resistance or contami-nation of the specimen (5). The electrode material is to becorrosion-resistant under the conditions of the test. For tests offabricated specimens such as feed-through bushings, ca
41、bles,etc., the electrodes employed are a part of the specimen or itsmounting. In such cases, measurements of insulation resistanceor conductance include the contaminating effects of electrodeor mounting materials and are generally related to the perfor-mance of the specimen in actual use.6.1.1 Bindi
42、ng-Post and Taper-Pin Electrodes, Figs. 1 and 2,provide a means of applying voltage to rigid insulatingmaterials to permit an evaluation of their resistive or conduc-tive properties. These electrodes attempt to simulate the actualconditions of use, such as binding posts on instrument panelsand termi
43、nal strips. In the case of laminated insulating mate-rials having high-resin-content surfaces, lower insulation resis-tance values are obtained with taper-pin than with bindingposts, due to more intimate contact with the body of theinsulating material. Resistance or conductance values obtainedare hi
44、ghly influenced by the individual contact between eachpin and the dielectric material, the surface roughness of thepins, and the smoothness of the hole in the dielectric material.Reproducibility of results on different specimens is difficult toobtain.6.1.2 Metal Bars in the arrangement of Fig. 3 wer
45、e primar-ily devised to evaluate the insulation resistance or conductanceof flexible tapes and thin, solid specimens as a fairly simpleand convenient means of electrical quality control. This ar-rangement is more satisfactory for obtaining approximatevalues of surface resistance or conductance when
46、the width ofthe insulating material is much greater than its thickness.6.1.3 Silver Paint, Figs. 4-6 are available commercially witha high conductivity, either air-drying or low-temperature-baking varieties, which are sufficiently porous to permitdiffusion of moisture through them and thereby allow
47、the testspecimen to be conditioned after the application of the elec-trodes. This is a particularly useful feature in studyingresistance-humidity effects, as well as change with tempera-ture. However, before conductive paint is used as an electrodematerial, it shall be established that the solvent i
48、n the paintdoes not attack the material changing its electrical properties.Smooth edges of guard electrodes are obtained by using afine-bristle brush. However, for circular electrodes, sharperedges are obtained by the use of a ruling compass and silverpaint for drawing the outline circles of the ele
49、ctrodes and fillingin the enclosed areas by brush.6.1.4 Sprayed Metal, Figs. 4-6 are used if satisfactoryadhesion to the test specimen can be obtained. it is possible thatthin sprayed electrodes will have certain advantages in thatthey are ready for use as soon as applied.6.1.5 Evaporated Metal are used under the same conditionsgiven in 6.1.4.6.1.6 Metal Foil, Fig. 4, is applied to specimen surfaces aselectrodes. The thickness of metal foil used for resistance orconductance studies of dielectrics ranges from 6 to 80 m.Lead or tin foil is in most common u
