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An American National Standard

Designation: D 257 – 99

Standard Test Methods for

DC Resistance or Conductance of Insulating Materials1
This standard is issued under the fixed designation D 257; 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 (e) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.

1. Scope 2. Referenced Documents

1.1 These test methods cover direct-current procedures for 2.1 ASTM Standards:
the determination of dc insulation resistance, volume resis- D 150 Test Methods for AC Loss Characteristics and Per-
tance, volume resistivity, surface resistance, and surface resis- mittivity Dielectric Contant of Solid Electrical Insulation2
tivity of electrical insulating materials, or the corresponding D 374 Test Methods for Thickness of Solid Electrical Insu-
conductances and conductivities. lation2
1.2 These test methods are not suitable for use in measuring D 618 Practice for Conditioning Plastics for Testing3
the electrical resistivity/conductivity of moderately conductive D 1169 Test Method for Specific Resistance (Resistivity) of
materials. Use Test Method D 4496 to evaluate such materials. Electrical Insulating Liquids4
1.3 The test methods and procedures appear in the follow- D 1711 Terminology Relating to Electrical Insulation2
ing sections: D 4496 Test Method for DC Resistance or Conductance of
Moderately Conductive Materials5
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Test Method or Procedure Section
Calculation 13 D 5032 Practice for Maintaining Constant Relative Humid-
Choice of Apparatus and Test Method 7
ity by Means of Aqueous Glycerin Solutions5
(https://standards.iteh.ai)
Cleaning Solid Specimens 10.1
Conditioning of Specimens 11 E 104 Practice for Maintaining Constant Relative Humidity
Effective Area of Guarded Electrode X2 by Means of Aqueous Solutions6
Electrode Systems 6

Measurements
Humidity Control
Document3.Preview
Factors Affecting Insulation Resistance or Conductance
Terminology X1

3.1 Definitions—The following definitions are taken from

11.2
Liquid Specimens and Cells 9.4
Precision and Bias 15
Terminology D 1711 and apply to the terms used in these test
Procedure for the Measurement of Resist- ASTM D257-99
methods. 12
ance or Conductance 3.1.1 conductance, insulation, n—the ratio of the total
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Referenced Documents 2
Report 14
volume and surface current between two electrodes (on or in a
Sampling 8 specimen) to the dc voltage applied to the two electrodes.
Significance and Use 5 3.1.1.1 Discussion—Insulation conductance is the recipro-
Specimen Mounting 10
Summary of Test Methods 4 cal of insulation resistance.
Terminology 3 3.1.2 conductance, surface, n—the ratio of the current
Test Specimens for Insulation, Volume, and Surface 9 between two electrodes (on the surface of a specimen) to the dc
Resistance or Conductance Determination
Typical Measurement Methods X3 voltage applied to the electrodes.
3.1.2.1 Discussion—(Some volume conductance is un-
1.4 This standard does not purport to address all of the avoidably included in the actual measurement.) Surface con-
safety concerns, if any, associated with its use. It is the ductance is the reciprocal of surface resistance.
responsibility of the user of this standard to establish appro- 3.1.3 conductance, volume, n—the ratio of the current in the
priate safety and health practices and determine the applica- volume of a specimen between two electrodes (on or in the
bility of regulatory limitations prior to use. For a specific specimen) to the dc voltage applied to the two electrodes.
hazard statement, see 6.1.8.

1 2
These test methods are under the jurisdiction of ASTM Committee D-9 on Annual Book of ASTM Standards, Vol 10.01.
3
Electrical and Electronic Insulating Materials and are the direct responsibility of Annual Book of ASTM Standards, Vol 08.01.
4
Subcommittee D09.12 on Electrical Tests. Annual Book of ASTM Standards, Vol 10.03.
5
Current edition approved Oct. 10, 1999. Published November 1999. Originally Annual Book of ASTM Standards, Vol 10.02.
6
published as D 257 – 25 T. Last previous edition D 257 – 93 (1998). Annual Book of ASTM Standards, Vol 11.03.

Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.

1
 D 257
3.1.3.1 Discussion—Volume conductance is the reciprocal 3.1.11.1 Discussion—Volume resistivity is usually ex-
of volume resistance. pressed in ohm-centimetres (preferred) or in ohm-metres.
3.1.4 conductivity, surface, n—the surface conductance Volume resistivity is the reciprocal of volume conductivity.
multiplied by that ratio of specimen surface dimensions (dis-
tance between electrodes divided by the width of electrodes 4. Summary of Test Methods
defining the current path) which transforms the measured 4.1 The resistance or conductance of a material specimen or
conductance to that obtained if the electrodes had formed the of a capacitor is determined from a measurement of current or
opposite sides of a square. of voltage drop under specified conditions. By using the
3.1.4.1 Discussion—Surface conductivity is expressed in appropriate electrode systems, surface and volume resistance
siemens. It is popularly expressed as siemens/square (the size or conductance may be measured separately. The resistivity or
of the square is immaterial). Surface conductivity is the conductivity can then be calculated when the required speci-
reciprocal of surface resistivity. men and electrode dimensions are known.
3.1.5 conductivity, volume, n—the volume conductance 5. Significance and Use
multiplied by that ratio of specimen volume dimensions
5.1 Insulating materials are used to isolate components of an
(distance between electrodes divided by the cross-sectional
electrical system from each other and from ground, as well as
area of the electrodes) which transforms the measured conduc-
to provide mechanical support for the components. For this
tance to that conductance obtained if the electrodes had formed purpose, it is generally desirable to have the insulation resis-
the opposite sides of a unit cube. tance as high as possible, consistent with acceptable mechani-
3.1.5.1 Discussion—Volume conductivity is usually ex- cal, chemical, and heat-resisting properties. Since insulation
pressed in siemens/centimetre or in siemens/metre and is the resistance or conductance combines both volume and surface
reciprocal of volume resistivity. resistance or conductance, its measured value is most useful
3.1.6 moderately conductive, adj—describes a solid mate- when the test specimen and electrodes have the same form as
rial having a volume resistivity between 1 and 10 000 000 is required in actual use. Surface resistance or conductance

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V-cm. changes rapidly with humidity, while volume resistance or
3.1.7 resistance, insulation, (Ri), n—the ratio of the dc conductance changes slowly although the final change may
voltage applied to two electrodes (on or in a specimen) to the eventually be greater.
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total volume and surface current between them.
3.1.7.1 Discussion—Insulation resistance is the reciprocal
5.2 Resistivity or conductivity may be used to predict,
indirectly, the low-frequency dielectric breakdown and dissi-
of insulation conductance.
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3.1.8 resistance, surface, (Rs), n—the ratio of the dc voltage
pation factor properties of some materials. Resistivity or
contivity is often used as an indirect measure of moisture
content, degree of cure, mechanical continuity, and deteriora-
applied to two electrodes (on the surface of a specimen) to the
tion of various types. The usefulness of these indirect measure-
current between them. ASTM D257-99
ments is dependent on the degree of correlation established by
3.1.8.1 Discussion—(Some volume resistance is unavoid-
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supporting theoretical or experimental investigations. A de-
ably included in the actual measurement.) Surface resistance is crease of surface resistance may result either in an increase of
the reciprocal of surface conductance. the dielectric breakdown voltage because the electric field
3.1.9 resistance, volume, (Rv), n—the ratio of the dc voltage intensity is reduced, or a decrease of the dielectric breakdown
applied to two electrodes (on or in a specimen) to the current voltage because the area under stress is increased.
in the volume of the specimen between the electrodes. 5.3 All the dielectric resistances or conductances depend on
3.1.9.1 Discussion—Volume resistance is the reciprocal of the length of time of electrification and on the value of applied
volume conductance. voltage (in addition to the usual environmental variables).
3.1.10 resistivity, surface, (rs), n—the surface resistance These must be known to make the measured value of resistance
multiplied by that ratio of specimen surface dimensions (width or conductance meaningful.
of electrodes defining the current path divided by the distance 5.4 Volume resistivity or conductivity can be used as an aid
between electrodes) which transforms the measured resistance in designing an insulator for a specific application. The change
to that obtained if the electrodes had formed the opposite sides of resistivity or conductivity with temperature and humidity
of a square. may be great (1, 2, 3, 4),7 and must be known when designing
for operating conditions. Volume resistivity or conductivity
3.1.10.1 Discussion—Surface resistivity is expressed in
determinations are often used in checking the uniformity of an
ohms. It is popularly expressed also as ohms/square (the size of insulating material, either with regard to processing or to detect
the square is immaterial). Surface resistivity is the reciprocal of conductive impurities that affect the quality of the material and
surface conductivity. that may not be readily detectable by other methods.
3.1.11 resistivity, volume, (rv), n—the volume resistance 5.5 Volume resistivities above 1021 V·cm (1019 V·m), ob-
multiplied by that ratio of specimen volume dimensions tained on specimens under usual laboratory conditions, are of
(cross-sectional area of the specimen between the electrodes
divided by the distance between electrodes) which transforms
the measured resistance to that resistance obtained if the 7
The boldface numbers in parentheses refer to the list of references appended to
electrodes had formed the opposite sides of a unit cube. these test methods.

2
 D 257
doubtful validity, considering the limitations of commonly
used measuring equipment.
5.6 Surface resistance or conductance cannot be measured
accurately, only approximated, because some degree of volume
resistance or conductance is always involved in the measure-
ment. The measured value is also affected by the surface
contamination. Surface contamination, and its rate of accumu-
lation, is affected by many factors including electrostatic
charging and interfacial tension. These, in turn, may affect the
surface resistivity. Surface resistivity or conductivity can be
considered to be related to material properties when contami-
nation is involved but is not a material property in the usual
sense.
6. Electrode Systems
6.1 The electrodes for insulating materials should be of a
material that is readily applied, allows intimate contact with the
specimen surface, and introduces no appreciable error because
of electrode resistance or contamination of the specimen (5).
The electrode material should be corrosion-resistant under the
conditions of test. For tests of fabricated specimens such as
feed-through bushings, cables, etc., the electrodes employed
are a part of the specimen or its mounting. Measurements of
insulation resistance or conductance, then, include the contami-
nating effects of electrode or mounting materials and are
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generally related to the performance of the specimen in actual
use.
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6.1.1 Binding-Post and Taper-Pin Electrodes, Fig. 1 and
Fig. 2, provide a means of applying voltage to rigid insulating

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materials to permit an evaluation of their resistive or conduc- FIG. 2 Taper-Pin Electrodes
tive properties. These electrodes simulate to some degree the
actual conditions of use, such as binding posts on instrument
panels and terminal strips. In the case of laminated insulating
materials having high-resin-content surfaces, somewhatASTM lower D257-99
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insulation resistance values may be obtained with taper-pin
than with binding posts, due to more intimate contact with the
body of the insulating material. Resistance or conductance
values obtained are highly influenced by the individual contact
between each pin and the dielectric material, the surface
roughness of the pins, and the smoothness of the hole in the
dielectric material. Reproducibility of results on different
specimens is difficult to obtain.
6.1.2 Metal Bars in the arrangement of Fig. 3 were prima-
rily devised to evaluate the insulation resistance or conduc-

FIG. 3 Strip Electrodes for Tapes and Flat, Solid Specimens

tance of flexible tapes and thin, solid specimens as a fairly

simple and convenient means of electrical quality control. This
arrangement is somewhat more satisfactory for obtaining
approximate values of surface resistance or conductance when
the width of the insulating material is much greater than its
FIG. 1 Binding-Post Electrodes for Flat, Solid Specimens thickness.

3
 D 257
6.1.3 Silver Paint, Fig. 4, Fig. 5, and Fig. 6, is available
commercially with a high conductivity, either air-drying or
low-temperature-baking varieties, which are sufficiently po-
rous to permit diffusion of moisture through them and thereby
allow the test specimen to be conditioned after the application
of the electrodes. This is a particularly useful feature in
studying resistance-humidity effects, as well as change with
temperature. However, before conductive paint is used as an
electrode material, it should be established that the solvent in
the paint does not attack the material so as to change its
electrical properties. Reasonably smooth edges of guard elec-
trodes may be obtained with a fine-bristle brush. However, for
circular electrodes, sharper edges can be obtained by the use of
a ruling compass and silver paint for drawing the outline circles
of the electrodes and filling in the enclosed areas by brush. A
narrow strip of masking tape may be used, provided the
pressure-sensitive adhesive used does not contaminate the
surface of the specimen. Clamp-on masks also may be used if
the electrode paint is sprayed on.
6.1.4 Sprayed Metal, Fig. 4, Fig. 5, and Fig. 6, may be used
if satisfactory adhesion to the test specimen can be obtained. D0 = (D1 + D2)/2 L > 4t g |La 2t Volume Resistivity g |Ls 2t Surface Resistivity
Thin sprayed electrodes may have certain advantages in that FIG. 5 Tubular Specimen for Measuring Volume and Surface
they are ready for use as soon as applied. They may be Resistances or Conductances
sufficiently porous to allow the specimen to be conditioned, but
6.1.6 Metal Foil, Fig. 4, may be applied to specimen
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this should be verified. Narrow strips of masking tape or
clamp-on masks must be used to produce a gap between the surfaces as electrodes. The usual thickness of metal foil used
for resistance or conductance studies of dielectrics ranges from
guarded and the guard electrodes. The tape shall be such as not
to contaminate the gap surface.
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6.1.5 Evaporated Metal may be used under the same con-
6 to 80 µm. Lead or tin foil is in most common use, and is
usually attached to the test specimen by a minimum quantity of

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ditions given in 6.1.4. petrolatum, silicone grease, oil, or other suitable material, as an
adhesive. Such electrodes shall be applied under a smoothing
pressure sufficient to eliminate all wrinkles, and to work excess
adhesive toward the edge of the foil where it can be wiped off
ASTM D257-99
with a cleansing tissue. One very effective method is to use a
hard narrow roller (10 to 15 mm wide), and to roll outward on
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the surface until no visible imprint can be made on the foil with
the roller. This technique can be used satisfactorily only on
specimens that have very flat surfaces. With care, the adhesive
film can be reduced to 2.5 µm. As this film is in series with the
specimen, it will always cause the measured resistance to be
too high. This error may become excessive for the lower-
resistivity specimens of thickness less than 250 µm. Also the
hard roller can force sharp particles into or through thin films
(50 µm). Foil electrodes are not porous and will not allow the
test specimen to condition after the electrodes have been
applied. The adhesive may lose its effectiveness at elevated
temperatures necessitating the use of flat metal back-up plates
under pressure. It is possible, with the aid of a suitable cutting
device, to cut a proper width strip from one electrode to form
a guarded and guard electrode. Such a three-terminal specimen
normally cannot be used for surface resistance or conductance
measurements because of the grease remaining on the gap
surface. It may be very difficult to clean the entire gap surface
without disturbing the adjacent edges of the electrode.
6.1.7 Colloidal Graphite, Fig. 4, dispersed in water or other
suitable vehicle, may be brushed on nonporous, sheet insulat-
Volume Resistivity g |Ls 2t Surface Resistivity ing materials to form an air-drying electrode. Masking tapes or
FIG. 4 Flat Specimen for Measuring Volume and Surface clamp-on masks may be used (6.1.4). This electrode material is
Resistances or Conductances recommended only if all of the following conditions are met:

4
 D 257

FIG. 6 Conducting-Paint Electrodes

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6.1.7.1 The material to be tested must accept a graphite
coating that will not flake before testing,
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6.1.7.2 The material being tested must not absorb water
readily, and

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6.1.7.3 Conditioning must be in a dry atmosphere (Proce-
dure B, Methods D 618), and measurements made in this same
atmosphere.
6.1.8 Mercury or other liquid metal electrodes give satisfac-
tory results. Mercury is not recommended for continuous ASTM use D257-99
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or at elevated temperatures due to toxic effects. (Warning—
Mercury metal vapor poisoning has long been recognized as a
hazard in industry. The maximum exposure limits are set by the
American Conference of Governmental Industrial Hygienists.8
The concentration of mercury vapor over spills from broken
thermometers, barometers, or other instruments using mercury
can easily exceed these exposure limits. Mercury, being a
liquid and quite heavy, will disintegrate into small droplets and
seep into cracks and crevices in the floor. The use of a
commercially available emergency spill kit is recommended
whenever a spill occurs. The increased area of exposure adds
significantly to the mercury vapor concentration in air. Mer-
cury vapor concentration is easily monitored using commer-
cially available sniffers. Spot checks should be made periodi- NOTE 1—Warning: See 6.1.8
cally around operations where mercury is exposed to the FIG. 7 Mercury Electrodes for Flat, Solid Specimens
atmosphere. Thorough checks should be made after spills.) The
metal forming the upper electrodes should be confined by
stainless steel rings, each of which should have its lower rim 6.1.9 Flat Metal Plates, Fig. 4, (preferably guarded) may be
reduced to a sharp edge by beveling on the side away from the used for testing flexible and compressible materials, both at
liquid metal. Fig. 7A and Fig. 7B show two electrode arrange- room temperature and at elevated temperatures. They may be
ments. circular or rectangular (for tapes). To ensure intimate contact
with the specimen, considerable pressure is usually required.
8
American Conference of Governmental and Industrial Hygienists, 6500 Glen- Pressures of 140 to 700 kPa have been found satisfactory (see
way Ave., Building D-7, Cincinnati, OH, 45211. material specifications).

5
 D 257
tested is either added to the cell between fixed electrodes or the
electrodes are forced into the material to a predetermined
electrode spacing. Because the configuration of the electrodes
in these cells is such that the effective electrode area and the
distance between them is difficult to measure, each cell
constant, K, (equivalent to the A/t factor from Table 1) can be
derived from the following equation:
K 5 3.6 p C 5 11.3 C (1)
where:
K has units of centimetres, and
C has units of picofarads and is the capacitance of the electrode system with
air as the dielectric. See Test Methods D 150 for methods of measurement
for C.
6.1.10 Conducting Rubber has been used as electrode ma-
terial, as in Fig. 4, and has the advantage that it can quickly and
easily be applied and removed from the specimen. As the
electrodes are applied only during the time of measurement,
they do not interfere with the conditioning of the specimen.
The conductive-rubber material must be backed by proper
plates and be soft enough so that effective contact with the
specimen is obtained when a reasonable pressure is applied.
NOTE 1—There is evidence that values of conductivity obtained using
conductive-rubber electrodes are always smaller (20 to 70 %) than values
obtained with tinfoil electrodes (6). When only order-of-magnitude
NOTE 1—Warning: See 6.1.8 accuracies are required, and these contact errors can be neglected, a

iTeh Standards
FIG. 7 Mercury Cell for Thin Sheet Material (continued) properly designed set of conductive-rubber electrodes can provide a rapid
means for making conductivity and resistivity determinations.
6.1.9.1 A variation of flat metal plate electrode systems is
6.1.11 Water is widely employed as one electrode in testing
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found in certain cell designs used to measure greases or filling
insulation
compounds. Such cells are preassembled and the material to be
on wires and cables. Both ends of the specimen must
be out of the water and of such length that leakage along the
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TABLE 1 Calculation of Resistivity or ConductivityA
Type of Electrodes or Specimen Volume Resistivity, V-cm Volume Conductivity, S/cm

ASTM D257-99
rv 5
A
R
t v
g 5
t
A
G v v

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Circular (Fig. 4) p~D 1 g! 2
1
A 5
4
Rectangular A = (a + g) (b + g)
Square A = (a + g) 2
Tubes (Fig. 5) A = pD0(L + g)
Cables 2pLRv
rv 5
D2
ln
D1
D2
ln
D1
gv 5
2pLRv

Surface Resistivity, V (per square) Surface Conductivity, S (per square)

P g
ps 5 Rs gs 5 G
g P s
Circular (Fig. 4) P = pD0
Rectangular P = 2(a + b + 2g)
Square P = 4(a + g)
Tubes (Figs. 5 and 6) P = 2p D2
Nomenclature:
A = the effective area of the measuring electrode for the particular arrangement employed,
P = the effective perimeter of the guarded electrode for the particular arrangement employed,
Rv = measured volume resistance in ohms,
Gv = measured volume conductance in siemens,
Rs = measured surface resistance in ohms,
Gs = measured surface conductance in siemens,
t = average thickness of the specimen,
D0, D1, D2, g, L = dimensions indicated in Figs. 4 and 6 (see Appendix X2 for correction to g),
a, b, = lengths of the sides of rectangular electrodes, and
ln = natural logarithm.
A
All dimensions are in centimetres.