ASTM D3638-93(1998)
(Test Method)Standard Test Method for Comparative Tracking Index of Electrical Insulating Materials
Standard Test Method for Comparative Tracking Index of Electrical Insulating Materials
SCOPE
1.1 This test method evaluates in a short period of time the low-voltage (up to 600 V) track resistance or comparative tracking index (CTI) of materials in the presence of aqueous contaminants.
1.2 The values stated in metric (SI) units are to be regarded as standard. The inch-pound equivalents of the metric units may be approximate.
1.3 This standard does not purport to address all of the safety problems, 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.
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An American National Standard
Designation:D3638–93 (Reapproved 1998)
Standard Test Method for
Comparative Tracking Index of Electrical Insulating
Materials
This standard is issued under the fixed designation D3638; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope 3.1.2 tracking—the process that produces tracks as a result
of the action of electric discharges on or close to an insulation
1.1 This test method evaluates in a short period of time the
surface.
low-voltage (up to 600 V) track resistance or comparative
3.1.3 tracking, contamination—tracking caused by scintil-
tracking index (CTI) of materials in the presence of aqueous
lations that result from the increased surface conduction due to
contaminants.
contamination.
1.2 The values stated in metric (SI) units are to be regarded
3.1.4 tracking resistance—thequantitativeexpressionofthe
as standard. The inch-pound equivalents of the metric units
voltage and the time required to develop a track under the
may be approximate.
specified conditions.
1.3 This standard does not purport to address all of the
3.2 Definitions of Terms Specific to This Standard:
safety problems, if any, associated with its use. It is the
3.2.1 comparative tracking index—an index for electrical
responsibility of the user of this standard to establish appro-
insulating materials which is arbitrarily defined as the numeri-
priate safety and health practices and determine the applica-
cal value of that voltage which will cause failure by tracking
bility of regulatory limitations prior to use.
when the number of drops of contaminant required to cause
2. Referenced Documents
failure is equal to 50. This value is obtained from a plot of the
number of drops required to cause failure by tracking versus
2.1 ASTM Standards:
the applied voltage.
D618 Practice for Conditioning Plastics and Electrical
Insulating Materials for Testing
4. Summary of Test Method
D647 Practice for Design of Molds for Test Specimens of
4.1 The surface of a specimen of electrical insulating
Plastic Molding Materials
material is subjected to a low-voltage alternating stress com-
D1711 Terminology Relating to Electrical Insulation
2 bined with a low current which results from an aqueous
D1898 Practice for Sampling of Plastics
contaminant (electrolyte) which is dropped between two op-
2.2 IEC Publication:
posing electrodes every 30 s. The voltage applied across these
112, Recommended Method for Determining the Compara-
electrodes is maintained until the current flow between them
tive Track Index of Solid Insulating Materials Under
4 exceeds a predetermined value which constitutes failure. Ad-
Moist Conditions, 1971 Second Edition
ditional specimens are tested at other voltages so that a
3. Terminology relationship between applied voltage and number of drops to
failure can be established through graphical means. The
3.1 Definitions:
numerical value of the voltage which causes failure with the
3.1.1 track—a partially conducting path of localized dete-
application of 50 drops of the electrolyte is arbitrarily called
rioration on the surface of an insulating material.
the comparative tracking index. This value provides an indi-
cation of the relative track resistance of the material.
This test method is under the jurisdiction of ASTM Committee D-9 on 5. Significance and Use
Electrical and Electronic Insulating Materials and is the direct responsibility of
5.1 Electrical equipment may fail as a result of electrical
Subcommittee D09.12 on Electrical Tests.
tracking of insulating material that is exposed to various
Current edition approved Oct. 15, 1993. Published December 1993. Originally
e1
published as D3638–77. Last previous edition D3638–85(1992) .
contaminating environments and surface conditions. There are
Annual Book of ASTM Standards, Vol 08.01.
a number of ASTM and other tests designed to quantify
Annual Book of ASTM Standards, Vol 10.01.
4 behavior of materials, especially at relatively high voltages.
Available from the International Electrotechnical Commission, Geneva, Swit-
zerland. This method is an accelerated test which at relatively low test
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D3638
NOTE 1—The need for a shorting switch can be considered optional. It
voltages, provides a comparison of the performance of insu-
is possible to couple the variable resistor with the autotransformer which
lating materials under wet and contaminated conditions. The
gives an automatic setting of the current throughout the range of the
comparative tracking index is not related directly to the
instrument. Then whenever it is necessary to check the calibration of the
suitable operating voltage in service.
instrument, the shorting action can be accomplished by a jumper wire
5.2 When organic electrical insulating materials are sub-
placedacrosstheelectrodes.Thiscouplingoftheautotransformerwiththe
jected to conduction currents between electrodes on their
variable resistor can be considered another option.
surfaces, many minute tree-like carbonaceous paths or tracks
6.1.6 Over-Current Relay (R0), which has to be inserted in
are developed near the electrodes. These tracks are oriented
the circuit shall not trip at currents up to 0.1Aand the tripping
randomly, but generally propagate between the electrodes
time on short circuit shall be at least 0.5 s (the current shall be
under the influence of the applied potential difference. Even-
limited on short circuit to 1 A with a tolerance of 610%ata
tually a series of tracks spans the electrode gap, and failure
power factor of 0.9 to 1.0).
occurs by shorting of the electrodes.
NOTE 2—Some instruments have used a Heinemann breaker, which is
5.3 The conditions specified herein are intended, as in other
probably the closest standard commercial breaker to that described in the
tracking test methods, to produce a condition conducive to the
IEC Method. This breaker is Heinemann Model Series JA, Curve 2.Also
formation of surface discharges and possible subsequent track-
the tripping action can be accomplished with electronic circuitry.
ing. Test conditions are chosen to reproducibly and conve-
6.1.7 Testing Fixture (J1), adjustable platform which sup-
niently accelerate a process; for this reason, they rarely
ports the specimen and electrode setup.
reproduce the varied conditions found in actual service.There-
6.1.8 Platinum Electrodes, having a rectangular cross sec-
fore, while tracking tests serve to differentiate materials under
tion measuring 5 by 2 mm (0.2 by 0.08 in.), extending 20 mm
given conditions, results of tracking tests cannot be used to
(0.8 in) minimum from suitable mounting shanks (Fig. 2). The
infer either direct or comparative service behavior of an
end of each electrode is machined to form a 30° chisel-point
application design. Rather, tracking test results provide a tool
edge, having a radius from 0.05 to 0.10 mm, extending along
for judging the suitability of materials for a given application.
the 5-mm (0.2-in) side of the electrode. This is the radius that
The suitability can only be verified through testing the design
generally results from polishing a “Omm” radius electrode.
inactualenduseorunderconditionswhichsimulateenduseas
Since the direction of polish may influence the results, all
closely as possible.
electrodes should be polished in a direction perpendicular to
6. Apparatus the long dimension of the electrode face.
6.1.9 Dropping Apparatus, which should drop the electro-
6.1 The simplified electrical circuitry used in this test is
lyte precisely as specified. Included should also be a means of
illustrated in Fig. 1. For necessary information on the cleanli-
electricallystartingandstoppingthedroppingoftheelectrolyte
nessofapparatus,seeAnnexA1.Theessentialcomponentsare
as well as a counting device for monitoring the number of
as follows:
drops. The orifice diameter of the drop mechanism is approxi-
6.1.1 Variable Power Source, consisting of a transformer
mately 1.5 mm. However, it may be necessary to adjust this
type supply, such as the combinationT1 andT2 in Fig. 1, with
diameter somewhat so as to obtain the proper drop size in
a variable output of 0 to 1000V, 60 Hz capable of maintaining
accordance with 9.2.
a current of 1 A (1 kVA).
6.1.2 Voltmeter (V1), capable of measuring the varying a-c
7. Reagents
output of the power source. A 0 to 600-V voltmeter with an
accuracy of at least6 2.5% of full scale. 7.1 Electrolyte Solution of Ammonium Chloride in Water:
6.1.3 Ammeter (A1), with a range of 0 to 1 A a-c and an 7.1.1 Prepare a solution of ammonium chloride at an ap-
accuracy of at least 610% of full scale. proximate concentration of 0.1% by dissolving1gof reagent
6.1.4 Current Limiting Resistor (R1), continuously vari- gradeammoniumchloridein1Lofwater.Thewaterusedshall
able, wire wound, rated at greater than 1 A. have a volume resistivity of no less than 0.5 MV/cm at 23°C.
6.1.5 Shorting Switch (S1), single-pole single-throw rated Allow the solution to stand overnight in a covered, but not
at 1000 V and greater than 1 A. sealed, container.
FIG. 1 Electrical Circuit Components
D3638
FIG. 2 Electrodes (Radius 0.05 to 0.1 mm)
7.1.2 Measuretheresistivityofthesolutionusingaconduc- 9.3 Allow approximately 15 drops of
...
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