ASTM D3638-21e1

Standard Test Method for Comparative Tracking Index of Electrical Insulating Materials

SIGNIFICANCE AND USE
5.1 Electrical equipment can fail as a result of electrical tracking of insulating material that is exposed to various contaminating environments and surface conditions. This method is an accelerated test which at relatively low test voltages, provides a comparison of the performance of insulating materials under wet and contaminated conditions. The comparative tracking index is not related directly to the suitable operating voltage in service.  
5.2 When organic electrical insulating materials are subjected to conduction currents between electrodes on their surfaces, many minute tree-like carbonaceous paths or tracks are developed near the electrodes. These tracks are oriented randomly, but generally propagate between the electrodes under the influence of the applied potential difference. Eventually a series of tracks spans the electrode gap, and failure occurs by shorting of the electrodes.  
5.3 The conditions specified herein are intended to produce a condition conducive to the formation of surface discharges and possible subsequent tracking. Test conditions are chosen to accelerate a process that is reproducible. Consequently, they rarely reproduce the varied conditions found in actual service. Therefore, while tracking tests serve to differentiate materials under given conditions, results of tracking tests cannot be used to infer either direct or comparative service behavior of an application design. Rather, the results provide a tool for judging the suitability of materials for a given application. The suitability can only be verified through testing the design in actual end use or under conditions which simulate end use as closely as possible.  
5.4 The results have been used for insulation coordination of equipment with rated voltage up to 1000 Vac or 1500 Vdc connected to low-voltage supply systems (higher voltages permitted in internal circuits). The complete principles of insulation coordination involve the consideration of the combination of clearanc...
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 are approximate.  
1.3 This test method is technically equivalent to the version of IEC Publication 112 cited in 2.2. However, the 2007 version of IEC 60112 Fourth Edition yields numerical CTI values that are very likely to differ significantly from this test method.  
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Dec-2020
• ICS: 29.035.01 - Insulating materials in general
• Technical Committee: D09 - Electrical and Electronic Insulating Materials
• Drafting Committee: D09.12 - Electrical Tests

Relations

• Refers: ASTM D1711-24 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-Mar-2024
• Refers: ASTM D1711-15 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-Nov-2015
• Refers: ASTM D1711-14a - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-Nov-2014
• Refers: ASTM D1711-14 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-May-2014
• Refers: ASTM D1711-13 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-Nov-2013
• Refers: ASTM D1711-11a - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-Aug-2011
• Refers: ASTM D1711-08 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-May-2008
• Refers: ASTM D1711-02 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 10-Mar-2002
• Refers: ASTM D1711-99 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 10-Oct-1999
• Refers: ASTM D6054-97 - Standard Practice for Conditioning Electrical Insulating Materials for Testing
• Effective Date: 10-Feb-1997

Overview

ASTM D3638-21e1 - Standard Test Method for Comparative Tracking Index of Electrical Insulating Materials - establishes a method for determining the comparative tracking index (CTI) of insulating materials exposed to wet and contaminated conditions. Developed by ASTM, this standard is widely recognized for evaluating the resistance of insulating materials to electrical tracking, a failure mode that can compromise electrical safety and equipment reliability.

The test accelerates conditions that encourage surface discharges and tracking, providing a rapid and reproducible means for comparing different insulating materials. The results help in assessing material suitability for electrical insulation applications, but are not directly predictive of service behavior under operational voltages.

Key Topics

• Comparative Tracking Index (CTI): The CTI is the numerical voltage value at which the material fails under specified conditions when 50 drops of contaminant are applied.
• Electrical Tracking: The phenomenon where conductive paths form on the surface of insulating materials due to electrical discharge, usually under the influence of moisture and contaminants.
• Accelerated Testing: The method applies low voltage stress and contaminant drops at regular intervals to replicate and speed up the tracking process, offering reproducible results for material comparison.
• Applicability: Intended for insulating materials used at voltages up to 600 V, with relevance for insulation coordination in low-voltage electrical systems.
• Test Procedure: Involves subjecting flat specimens to controlled contamination and voltage, recording failure when a conductive track shorts the electrodes.
• Safety and Limitations: Results help differentiate materials, but actual performance in service should be validated under application-specific conditions. All safety, health, and environmental protocols must be followed during testing.

Applications

• Material Selection: The CTI test is used by manufacturers and engineers to screen and select insulating materials for components in electrical and electronic equipment, particularly where exposure to contaminants and moisture is anticipated.
• Insulation Coordination: Results support design decisions related to creepage distances and clearances, essential for preventing electrical tracking in equipment rated up to 1000 Vac or 1500 Vdc connected to low-voltage supply systems.
• Quality Control: This method is valuable for quality assurance and consistency monitoring in the production of insulating materials.
• Regulatory Compliance: Complying with ASTM D3638 is often required to meet international, regional, or industry-specific safety standards for electrical insulation.
• Research and Development: CTI values obtained help in the development and comparison of new insulating compounds and formulations.

Related Standards

• IEC 112 & IEC 60112: Methods for the determination of proof and comparative tracking indices for solid insulating materials. Note that numerical results may differ between IEC 60112 and ASTM D3638 methods.
• IEC 60664-1: Principles and requirements for insulation coordination within low-voltage supply systems, which includes reference to CTI values for determining minimum creepage distances.
• ANSI/UL 840: Standard for insulation coordination, including clearances and creepage distances for electrical equipment.
• ASTM D1711: Terminology relating to electrical insulation, providing definitions critical for understanding tracking and insulation-related phenomena.
• ASTM D6054: Practice for conditioning electrical insulating materials for testing.

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Adhering to ASTM D3638-21e1 ensures robust screening and qualification of electrical insulating materials, contributing to enhanced reliability and safety in electrical and electronic systems. Employing standardized CTI testing is a critical step in material selection, product development, and regulatory compliance for industries reliant on electrical insulation performance.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
´1
Designation: D3638 − 21
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 (´) indicates an editorial change since the last revision or reapproval.
ε NOTE—Subsections 7.1.1, 7.1.2, and 9.4 were corrected editorially in January 2022.
1. Scope* rials for Testing (Withdrawn 2012)
1.1 This test method evaluates in a short period of time the
2.2 IEC Publications:
low-voltage (up to 600 V) track resistance or comparative
IEC 112Method for the determination of the proof and the
tracking index (CTI) of materials in the presence of aqueous
comparative tracking indices of solid insulating materials
contaminants.
IEC60112Methodforthedeterminationoftheproofandthe
comparative tracking indices of solid insulating materials
1.2 The values stated in metric (SI) units are to be regarded
IEC 60664-1Insulation coordination for equipment within
asstandard.Theinch-poundequivalentsofthemetricunitsare
approximate. low-voltage supply systems – Part 1: Principles, require-
ments and tests, 2020 Edition 3.0
1.3 This test method is technically equivalent to the version
ofIECPublication112citedin2.2.However,the2007version
2.3 2.3 ANSI/UL Publication:
of IEC 60112 Fourth Edition yields numerical CTI values that
ANSI/UL 840Standard for Insulation Coordination Includ-
are very likely to differ significantly from this test method.
ing Clearances and Creepage Distances for Electrical
rd
Equipment. January 6, 2005, 3 edition
1.4 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
3. Terminology
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
3.1 Definitions:
mine the applicability of regulatory limitations prior to use.
3.1.1 track, n—a partially conducting path of localized
1.5 This international standard was developed in accor-
deterioration on the surface of an insulating material.
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
3.1.2 tracking, n—the process that produces tracks as a
Development of International Standards, Guides and Recom-
result of the action of electric discharges on or close to an
mendations issued by the World Trade Organization Technical
insulation surface.
Barriers to Trade (TBT) Committee.
3.1.3 tracking, contamination, n—tracking caused by scin-
tillations that result from the increased surface conduction due
2. Referenced Documents
to contamination.
2.1 ASTM Standards:
3.1.4 tracking resistance, n—the quantitative expression of
D1711Terminology Relating to Electrical Insulation
the voltage and the time required to develop a track under the
D6054Practice for Conditioning Electrical Insulating Mate-
specified conditions.
3.1.5 For other terminology, refer to Terminology D1711.
3.2 Definitions of Terms Specific to This Standard:
This test method is under the jurisdiction of ASTM Committee D09 on
Electrical and Electronic Insulating Materials and is the direct responsibility of
Subcommittee D09.12 on Electrical Tests.
Current edition approved Jan. 1, 2021. Published February 2021. Originally The last approved version of this historical standard is referenced on
approved in 1977. Last previous edition approved in 2012 as D3638–12. DOI: www.astm.org.
10.1520/D3638-21E01. Available from International Electrotechnical Commission (IEC), 3, rue de
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Varembé, 1st floor, P.O. Box 131, CH-1211, Geneva 20, Switzerland, https://
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM www.iec.ch.
Standards volume information, refer to the standard’s Document Summary page on Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
the ASTM website. 4th Floor, New York, NY 10036, http://www.ansi.org.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
´1
D3638 − 21
3.2.1 comparative tracking index, n—an index for electrical tually a series of tracks spans the electrode gap, and failure
insulating materials which is arbitrarily defined as the numeri- occurs by shorting of the electrodes.
cal value of that voltage which will cause failure by tracking
5.3 The conditions specified herein are intended to produce
when the number of drops of contaminant required to cause
a condition conducive to the formation of surface discharges
failure is equal to 50.
andpossiblesubsequenttracking.Testconditionsarechosento
3.2.1.1 Discussion—The voltage value is obtained from a
accelerate a process that is reproducible. Consequently, they
plot of the number of drops required to cause failure by
rarely reproduce the varied conditions found in actual service.
tracking versus the applied voltage.
Therefore, while tracking tests serve to differentiate materials
3.2.2 failure, n—an attribute of an electrical circuit contain-
under given conditions, results of tracking tests cannot be used
ing an electrical-current-sensing device that rapidly decreases
to infer either direct or comparative service behavior of an
the applied voltage to zero if the current in the circuit exceeds
application design. Rather, the results provide a tool for
a predetermined limit.
judging the suitability of materials for a given application.The
suitability can only be verified through testing the design in
4. Summary of Test Method
actual end use or under conditions which simulate end use as
4.1 The surface of a specimen of electrical insulating
closely as possible.
material is subjected to a low-voltage alternating stress com-
5.4 The results have been used for insulation coordination
bined with a low current which results from an aqueous
of equipment with rated voltage up to 1000 Vac or 1500 Vdc
contaminant (electrolyte) dropped between two opposing elec-
connected to low-voltage supply systems (higher voltages
trodes every 30 s. The voltage applied across these electrodes
permitted in internal circuits). The complete principles of
is maintained until the current between them exceeds a
insulation coordination involve the consideration of the com-
predetermined value, constituting a failure. Additional speci-
bination of clearances, creepage distances, and the properties
mens are tested at other voltages to establish a relationship
of solid insulation used to constitute the insulation system.
between applied voltage and number of drops to failure
Users of these results need to consider the overvoltage levels
through graphical means. The numerical value of the voltage
and the methods of control which will be utilized and establish
which causes failure with the application of 50 drops of the
the pollution degree to which the product insulation system
electrolyte is arbitrarily called the comparative tracking index.
will be expected to be subjected.
This index provides an indication of the relative track resis-
tance of the material.
NOTE 1—See IEC 60664-1:2020, Table F.5 (Creepage Distances to
Avoid Failure Due to Tracking) and UL 840, Table 9.1 (Minimum
5. Significance and Use Acceptable Creepage Distances) as examples for the use of comparative
tracking index results as part of insulation coordination.
5.1 Electrical equipment can fail as a result of electrical
tracking of insulating material that is exposed to various
6. Apparatus
contaminating environments and surface conditions. This
method is an accelerated test which at relatively low test 6.1 The simplified electrical circuitry used in this test is
voltages, provides a comparison of the performance of insu- illustrated in Fig. 1. For necessary information on the cleanli-
lating materials under wet and contaminated conditions. The nessofapparatus,seeAnnexA1.Theessentialcomponentsare
comparative tracking index is not related directly to the as follows:
suitable operating voltage in service.
6.1.1 Variable Power Source, consisting of a transformer
type supply, such as the combinationT1 andT2 in Fig. 1, with
5.2 When organic electrical insulating materials are sub-
a variable output of 0 to 1000V, 60 Hz capable of maintaining
jected to conduction currents between electrodes on their
a current of 1 A (1 kVA).
surfaces, many minute tree-like carbonaceous paths or tracks
are developed near the electrodes. These tracks are oriented 6.1.2 Voltmeter (V1), capable of measuring the varying a-c
randomly, but generally propagate between the electrodes output of the power source up to 600V with an accuracy of at
under the influence of the applied potential difference. Even- least 62.5% of full scale.
FIG. 1 Electrical Circuit Components
´1
D3638 − 21
6.1.3 Ammeter (A1), with a range of 0 to 1 A a-c and an volumeresistivitynotlessthan0.5MΩ-cmat23°C.Allowthe
accuracy of at least 610% of full scale. solution to stand overnight in a covered, but not sealed,
6.1.4 Current Limiting Resistor(R1),continuouslyvariable, container.
wire wound, rated at greater than 1 A. 7.1.2 Calculate the resistivity of the solution using a con-
6.1.5 Shorting Switch(S1),single-polesingle-throwratedat ductivity cell and an a-c bridge, or meter, following the
1000 V and greater than 1 A. manufacturer’s instructions. If the resistivity is 385 6 5Ω-cm
6.1.5.1 A shorting switch is optional. See Annex A2. at 23 6 0.5°C, the solution is suitable for use in the test. If the
6.1.6 Over-current Relay (R0), which is inserted in the resistivity is outside the above limits, adjust the concentration
circuit shall not trip at currents up to 0.1A. Use a relay having until these limits are observed.Adjustment is accomplished by
a tripping time on short circuit of at least 0.5 s and a current adding water or NH Cl.
limited on short circuit to 1 A with a tolerance of 610%ata 7.1.3 Calibrate the conductivity cell with 0.01 N potassium
power factor of 0.9 to 1.0. chloride calibrating solution which is available from the cell
6.1.6.1 The tripping action can be accomplished with suit- manufacturer.
able electronic circuitry or with a commercial circuit breaker.
8. Test Specimens
6.1.7 Testing Fixture, adjustable platform which supports
8.1 Test specimens shall be any approximately flat surface
the specimen and electrode setup.
providedthattheareaissufficienttoensurethatnoliquidflows
6.1.8 Platinum Electrodes, having a rectangular cross sec-
away from the test electrodes during the test. The thickness of
tion measuring 5.0 6 0.1 by 2.0 6 0.1 mm (0.197 6 0.004 by
the test specimen shall be 2.5 mm or more (see 8.5).
0.079 6 0.004 in.), extending 20 mm (0.8 in.) minimum from
suitable mounting shanks (Fig. 2). Machine the end of each
NOTE2—Testspecimenswithflatsurfacesofnotlessthan20by20mm
(0.8 by 0.8in.) are recommended to reduce the probability of electrolyte
electrode to form a 30 6 2° chisel-point edge, having a radius
flowing away from the test electrodes. Smaller sizes may be used so long
from0.05to
...