ASTM D6097-97a
(Test Method)Standard Test Method for Relative Resistance to Vented Water-Tree Growth in Solid Dielectric Insulating Materials
Standard Test Method for Relative Resistance to Vented Water-Tree Growth in Solid Dielectric Insulating Materials
SCOPE
1.1 This test method covers the relative resistance to vented water-tree growth in solid translucent thermoplastic or cross-linked electrical insulating materials. This test method is especially applicable to extruded polymeric insulation materials used in medium-voltage cables.
1.2 The values given in SI units are to be regarded as the standard.
1.3 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 and health practices and determine the applicability of regulatory limitation prior to use. For specific hazard statements see 8.1.
1.3 There is no similar or equivalent IEC standard.
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Designation: D 6097 – 97a An American National Standard
Standard Test Method for
Relative Resistance to Vented Water-Tree Growth in Solid
Dielectric Insulating Materials
This standard is issued under the fixed designation D 6097; 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.
1. Scope 3.1.2 resistance to water-tree growth (RWTG)— a dimen-
sionless value which is L divided by the LWT.
1.1 This test method covers the relative resistance to vented
3.1.3 thickness of point-to-plane specimen (L), n—the ver-
water-tree growth in solid non-black thermoplastic or
tical distance in millimetres from the tip of the conical defect
crosslinked electrical insulating materials, especially those
to the opposite surface of the solid dielectric material.
materials used to insulate medium-voltage power cables. The
water-tree is initiated and vented from the tip of a controlled
4. Summary of Test Method
conical defect under a high electric field at specific wet test
4.1 Ten compression-molded disc specimens, each contain-
conditions. The treed specimen is stained and sliced for tree
ing a conical-shaped defect, are subjected to an applied voltage
measurement under a microscope.
of 5 kV at 1 kHz and 2362°C in an aqueous conductive
1.2 The values given in SI units are to be regarded as the
solution of 0.01 N NaCl for 30 days. This controlled conical
standard.
defect is created by a sharp needle with an included angle of
1.3 This standard does not purport to address all of the
60° and a tip radius of 3 μm. The electrical stress at the defect
safety concerns, if any, associated with its use. It is the
tip is enhanced and can be estimated by the Mason’s Hyper-
responsibility of the user of this standard to establish appro-
bolic point-to-plane stress enhancement equation. This en-
priate safety and health practices and determine the applica-
hanced electrical stress initiates the formation of a vented
bility of regulatory limitation prior to use. For specific hazard
water-tree grown from the defect tip. Each treed specimen is
statements see 8.1.
stained and sliced. The water-tree length and point-to-plane
1.4 There is no similar or equivalent IEC standard.
specimen thickness measured under microscope are used to
2. Referenced Documents calculate a ratio that is defined as the resistance to water-tree
growth.
2.1 ASTM Standards:
D 1898 Practice for Sampling of Plastics
5. Significance and Use
D 1928 Practice for Preparation of Compression-Molded
2 5.1 This is a laboratory test designed to simulate the growth
Test Sheets and Test Specimens
of vented water-trees in the solid dielectric insulating material
D 2275 Test Method for Voltage Endurance of Solid Elec-
initiated by a sharp protrusion at the insulating and conductive
trical Insulating Materials Subjected to Partial Discharges
interface under a wet environment in a high electrical field.
(Corona) on the Surface
Water-treeing is the phenomenon which describes the appear-
D 3756 Test Method for Evaluation of Resistance to Elec-
ance of tree-like growth in organic dielectrics under an ac field
trical Breakdown by Treeing in Solid Dielectric Materials
4 when exposed to moist environments. Two types of water-trees
Using Diverging Fields
are formed. Bow tie trees (within the dielectric) and vented
3. Terminology water-trees formed from conductive/insulating material inter-
face into the insulating material. The water-trees referred to in
3.1 Definitions of Terms Specific to This Standard:
this test are the vented type. The insulating material is the solid
3.1.1 length of water tree (LWT), n—the maximum length
dielectric organic material. The conductive material is the salt
of tree-like micro-channel path in millimetres measured from
solution. This salt solution is used on both sides of the
the axis of the tip of the defect, parallel to the electrical field.
insulating material to simulate the same inner and outer
semiconductive shields saturated with moisture between the
This test method is under the jurisdiction of ASTM Committee D-9 on
Electrical and Electronic Insulating Materials and is the direct responsibility of
Subcommittee D09.12 on Electrical Tests. The sole source of supply of the base, Dow Corning 3110RTV, the catalyst,
Current edition approved Sept. 10, 1997. Published April 1998. Originally Dow Corning RTV Catalyst S, and the sealant, Dow Corning Multipurpose Silicone
published as D 6097–97. Last previous edition D 6097–97. Sealant 732, known to the committee at this time is Dow Corning, Inc., Midland, MI
Annual Book of ASTM Standards, Vol 08.01. 48686. If you are aware of alternative suppliers, please provide this information to
Annual Book of ASTM Standards, Vol 10.01. ASTM Headquarters. Your comments will receive careful consideration at a meeting
Annual Book of ASTM Standards, Vol 10.02. of the responsible technical committe, which you may attend.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
D 6097
insulation layer used in a medium-voltage underground power (2) the time under test conditions of 30 days is long enough to
cable. establish the difference in water-tree growth. If there is a doubt,
5.2 This test method provides comparative data. The degree at least three different testing times (such as 30, 90, and 180
of correlation with the performance in service has not been days) should be used to verify their comparative performance
established. and disclose their kinetic nature of water-tree growth. Of
5.3 The standard test conditions are designed to grow a course, it is also assumed that all water-treed regions are
sufficient water-tree length for most solid dielectric insulating oxidized regions that can be stained for optical observation.
materials of interest before electrical breakdown occurs. Ma- Different materials may also have different temperatures and
terials with a very high resistance to water-tree growth may times to stain the oxidized (treed) regions due to their different
require a longer time under test conditions (such as 180 days) softening temperature.
or higher voltage (such as 10 or 15 kV) in order to differentiate
6. Apparatus
their performance. For materials with a very low resistance to
water-tree growth, electrical breakdown may occur during the 6.1 Power Supply— A high-voltage supply with a sinusoidal
30-day testing time. A shorter testing time (such as one or ten voltage output of at least 5 kV at a frequency of 1 kHz and an
days) is recommended to prevent electrical breakdown during output power of 3 kVA.
6.2 Conical Needles— Conical needles are made from steel
testing for those low water-tree resistant materials.
5.4 Other voltages, frequencies, temperatures, aqueous so- or tungsten carbide. Their dimensions are 14.5 6 0.5 mm long,
4 6 0.2 mm in diameter, point radius of 3 6 1 μm for the
lutions, and defects can be used to evaluate specific materials
for specific applications. Temperatures should not exceed the needle tip radius, and 60 6 1° point angle.
softening or melting point of the material or 10 to 15 °C below 6.3 Test-Specimen Mold—The test-specimen mold is a
the boiling point of the salt solution. Any non-standard three-layer metal mold. The top metal plate is flat. The center
conditions should be reported along with the results. plate has at least ten holes to make ten test specimens for each
5.5 Tree-growth rates generally increase with the test fre- material. Each hole has a 25.4-mm diameter and at least
quency. An acceleration factor due to frequency is given by ( 31.75-mm spacing from center to center of each hole. The
k
f/60) where f is the test frequency and k is between 0.6 and 0.7. center plate also has the guide holes about 8 mm in diameter at
The test frequency of 1 kHz is selected to accelerate the two corners to mate with pins in the bottom plate section. The
water-tree growth. However, the chemical nature of oxidized bottom plate section consists of two metal plates bolted
products from water-treeing may be different at different together. The first bottom plate has the same number of holes
frequency ranges. as the center plate. Each hole has the inside diameter of 4 mm
5.6 Two assumptions for this test method are: (1) all tested to accommodate needles. The second bottom plate has the
materials grow trees in the same power law kinetic manner and holes with an inside diameter of 10 mm. The center points of
FIG. 1 Test Specimen Mold Cavity
D 6097
all the holes in the bottom and center plates are matched and elevated temperatures.
aligned. These holes at the second bottom plate are threaded to
NOTE 1—Circulation of the solution in the bath even at room tempera-
accommodate the needle support member. The needle support
ture is necessary to remove gas bubbles formed at the interface of the
member is fabricated from threaded stainless steel rod drilled at
solution and the test specimens caused by electrolysis.
one end to provide a snug fit for needles, and at the other end
6.7 Microscope—A microscope equipped for 20 and 1003
to accommodate an hexagonal head driver. Needles are
magnification.
threaded into the support member. The needle and needle
support assembly is carefully screwed into the base until the 7. Reagents
needle point extends 3.2 6 0.1 mm above the surface. Fig. 1 is
7.1 Salt—Reagent-grade sodium chloride.
an example of the mold cavity.
7.2 Sealants—The material used for sealing in this test
6.4 Specimen Holder— The specimen holder, designed to
method is a two-part silicone rubber sealant consisting of a
5 5
hold at least ten specimens, is made from a solid block of clear
base and a catalyst .
polymethyl methacrylate (PMMA). PMMA is used because of
7.3 Multipurpose Silicone Sealant—One-part silicone rub-
the ease of machining and its good electrical properties. The
ber sealant.
inside is machined to a depth of 50.8 mm with a 12.7-mm wall
7.4 Staining Dye— The staining dye is a mixture of the
thickness. The outside bottom has the same number of holes
methylene blue and sodium hydroxide.
with an inside diameter of 25.4 mm with a depth of 6.35 mm,
7.5 Deionized Water, or distilled water.
drilled with a spacing of 38.1 mm from center to center of the
8. Hazards
holes. The inside bottom has the same holes with inside
diameter of 12.7 mm and a depth of 6.35 mm in line with the
NOTE 2—Warning—Lethal voltages are a potential hazard during the
centers of the holes drilled at the outside bottom. Fig. 2 is an
performance of this test method. It is essential that the test apparatus and
example of the specimen holder. all associated equipment that may be electrically connected to it be
properly designed and installed for safe operation.
6.5 Electrodes—The electrode is made from a 1-m length of
24 AWG nickel-chromium wire or other suitable conductive,
8.1 Solidly ground all electrically conductive parts that may
noncorrosive metal wire formed, on one end, into a closed loop be possible for a person to contact during the test. Provide
about 50 mm smaller in diameter than the inside diameter of means for use at the completion of any test to ground any parts
the specimen holder with the remainder bent perpendicular to which were at high voltage during the test or have the potential
the loop so that it can be connected to the transformer to for acquiring an induced charge during the test or retaining a
conduct the voltage into the electrolyte (the salt solution). charge even after disconnection of the voltage source. Thor-
6.6 Water Bath—A circulating water bath; provided with oughly instruct all operators as to the correct procedures for
heaters and temperature controls if tests are to be made at performing tests safely. When making high-voltage tests,
FIG. 2 A PMMA Specimen Holder
D 6097
particularly in compressed gas, oil, water, or aqueous solution, pressure of 0.30 MPa, 2 min at 120 6 5°C at a high pressure
it is possible for the energy released at breakdown to be of 3 MPa, and 15 min at 175 6 5°C at the same high pressure.
sufficient to result in fire, explosion, or rupture of the test Cool the mold in the press at 15°C/min to ambient temperature.
chamber. Design test equipment, test chambers, and test See Practice D 1928. Different materials may have different
specimens so as to minimize the possibility of such occur- conditions for molding. For materials other than polyethylene,
rences and to eliminate the possibility of personal injury. If the obtain molding conditions from the material supplier.
potential for fire exists, have fire suppression equipment 10.3.1 Remove the mold assembly from the press and take
available. off the top plate. Slowly lift the center section of the mold,
containing specimens, away from needles. Be careful not to
NOTE 3—Warning—Water in the test tank is gradually evaporated.
drag material across needle tips. Remove the test specimens
Keeping the water level constant is important to prevent an electrical
from the mold using a 25.4-mm diameter punch.
hazard.
10.4 Peroxide Crosslinked Materials—Heat in a vacuum
9. Sampling
oven at a temperature of 80 6 3°C and an absolute pressure of
133 Pa (1 mm Hg) or less for 160 to 168 h. When releasing the
9.1 Sample in accordance with Practice D 1898.
vacuum do so with nitrogen instead of air. This step is
10. Test Specimen
necessary to remove by-products of the peroxide decomposi-
tion, some of which are known to be tree-retardants.
10.1 Geometry of Test Specimens—The test specimen is a
10.5 Do not use test specimens that have visible bubbles or
disk containing a conical defect at the center of one side. The
cracks. It is also desirable that the flat surfaces on both sides of
disk has a diameter of 25.4 mm and a thickness of 6.35 mm.
each disk are smooth. A dummy or trial sample could be used
This conical defect has a diameter of 3.2 mm and height of 3.2
to measure and report the actual dimensions of the built-in
mm with an included angle of 60°. The radius of the cone tip
protrusion. This step will take into account the difference in
is 3 6 1 μm. Fig. 3 is the geometry of the test specimen.
shrinkage response between different polymeric materials.
10.2 Preparation of Test Specimens—Compression mold
ten specimens for each solid dielectric material using the
11. Procedure
preparation method described in Practice D 1928. Use a
pre-drilled polyethylene terephthalate sheet over needles to 11.1 Sealant Preparation—Prepare a m
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