Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02439367 2003-09-03
SELF-SEALING ELECTRICAL CABLE HAVING A FINNED OR RIBBED
STRUCTURE BETWEEN PROTECTIVE LAYERS
BACKGROUND OF THE INVENTION
Insulated solid and stranded electrical cables are well known in the art.
Generally
15 stranded cables include a central stranded conductor with a protecting
insulation Jacket
disposed around the conductor.
The most frequent cause of failure of directly buried aluminum secondary
cables is a
cut or puncture in the insulation inflicted during or after installation. This
leads to alternating
20 current corrosion of the aluminum and finally to an open circuit. When a
conductor is
exposed to wet soil, upon damage, leakage current may flow, and cause
localized
electrochemical conversion of aluminum to hydrated aluminum oxide and
eventually to an
open circuit of the conductor.
25 In the U.S., thousands of such instances occur annually and the repair
(location,
excavation, repair, and replacement) can be very costly. As a result of the
failures and in
response to this problem, a tougher insulation system was introduced and-
became an industry
standard. The tougher cable is described as "ruggedized," and generally
consists of nvo
layers: an inner layer of low density weight polyethylene and an outer layer
of high density
30 polyethylene. This design is more resistant to mechanical damage than one
pass low density
polyethylene, but still can result in exposure of the aluminum conductor if
sufficient impact
is involved.
Investigations show that AC electrolysis current can approach half wave
rectificatioli
35 when the current density is high. This accounts for the rapid loss of
aluminum metal
, CA 02439367 2003-09-03
frequently experienced in the field. A caustic solution (pH 10-12) develops at
the aluminum
surface and dissolves the protective oxide film.
The mechanism of aluminum cable failure is the forn~ation of hydrous aluminum
oxide. As the aluminum oxide solids build up, the insulation in the vicinity
'of the l3uncture t:
forced to swell and splits open, making larger areas of the aluminum conductor
surface
available for electrolysis, thus increasing the leakage current and
accelerating the corrosion
process. Rapid loss of aluminum by AC electrolysis continues until ultimately
tl;e ca?bIe is
open-circuited. A caustic environment is created at the aluminum, electrolyte
interface,
which dissolves the protective oxide film.
The ruggedized or abuse resistant type insulation was supposed to protect the
cable
from physical abuse. While it helped this problem, it did not eliminate 600 V
cable failures.
Utilities have recently reported varying numbers of 600 V aluminum underground
1 ~ distribution cable failure rates scattered between 70 and 7000 per year.
Failures are
evidenced by an open circuit condition accompanied by severe corrosion of the
aluminum
conductor.
All the reasons for 600 V failures are unknown, but several have been
postulated by
cable users. These cables seem to experience a high degree of infant
mortality, followed by
failures occurring over decades. The infant mortalities are usually directly
related to damage
caused by adjacent utilities, damage inflicted by landscaping and planting, or
damage to the
cable prior to or during installation. The failures occurring years later are
harder to explain.
There have been postulations of lightning damage, manufacturing defects, or
insulation
~ degradation over the life of the installation.
In order to better understand the insulation characteristics, studies of the
AC
breakdown, and DC impulse breakdown were conducted. AC breakdown studies on
several
different cables showed a high safety margin of performance. Each of these
cables had a
0.080 inch wall thickness. Tests were conducted in water filled conduits. The
AC
breakdown strength of all of these cables was consistently above 20 kV, far in
excess of the
opErating stress.
2
CA 02439367 2003-09-03
Impulse breakdown studies have also been perforn~ed on several 600 V cable
constructions having different insulation formulations. The impulse breakdown
level of these
cables was approximately 150 kV. This exceeds the BIL requirements of a 1 ~ kV
cable
system and should well exceed the impulses on 600 V secondary cables during
operation.
a
The above margins of electrical performance were measured on new cables. They
are
far above what is needed to operate on a 600 V system since most of these
cables operate at
120 V to ground. One of the tests during compound and product development is a
long term
insulation resistance test performed in water at the rated operating
temperature of the
insulation. For crosslinked polyethylene cables the water temperature is
90°C. The
insulation resistance must demonstrate stability and be above minimum values
for a
minimum of twelve weeks. If there is instability indicated, the test is
continued indefinitely.
Relative pelmitivity is measured at 80 vJmil and must meet specific values.
Increase in
capacitance and dissipation factor are also measured in 90°C water over
a 14 day period.
Insulation compounds used in present day cables easily meet these
requirements.
Manufacturing defects in cable insulation are found during production by
either of
two methods. During the extnlsion process, the cable is sent through a spark
tester, where 2S
kV DC, or l7kV AC, is applied to the insulation surface. Any manufacturing
defect resulting
in a hole in the insulation will initiate a discharge, which is detected by
the spark tester. I\~Iost
manufacturers use this method. Another test that is also often employed is a
full reel water
immersion test. In this test 21 kV DC, or 7 kV AC is applied to the cable
after immersion for
1 hour or 6 hours, depending on whether the cable is a plexed assembly or
single conductor,
respectively. The actual voltages used for these tests are dependent on the
wall thickness.
2~ The above values are for an 0.080 inch wall.
The above testing has demonstrated electrical performance that is stable and
far
surpasses the requirements of the installation for G00 V cable. This does not
explain a sudden
cable failure after many years of operation. Such sudden failure can be
explained by a better
s0 understanding of the failure mechanism. A1u11111711111 COr'OS1011 111 the
presence of an
alternating leakage current is a combination of two different mechanisms.
A1111711n11111 IS
normally afforded a great deal of corrosion protection by a relatively thin
barrier layer of
3
CA 02439367 2003-09-03
aluminum oxide, and a more permeable bulk layer of oxide. However, flaws or
cracks exist
in these layers which provides a spot for the corrosion reaction to begin. The
metal in contact
with water undergoes an anodic (positive ions moving into solution) and a
cathodic cycle,
sixty limes per second.
During the anodic half cycle of leakage current, aluminum ions leave the
metallic
surface through these flaws and combine with hydroxyl ions in the water
surrounding the
cable. This reaction results in pitting of the metal and the formation of
aluminum hydroxide,
the whitish powder evident in corroded cables. Another important reaction also
occurs. The
hydroxyl ions are attracted to the metal surface during this half cycle, which
increases the pH,
causing a caustic deterioration of the oxide layer, further exposing more
aluminum.
During the cathodic half cycle another reaction occurs. Hydrogen ions are
driven to
the aluminum surface. Instead of neutralizing the caustic hydroxyl
concentration, the
hydrogen ions combine and forn hydrogen gas, which leaves the cable. The
hydrogen
depletion has the effect of further concentrating the caustic hydroxyl ions,
thus furthering the
deterioration of the surface oxide. No pitting occurs during this half cycle
since the
aluminum ion is attracted to the metal. A caustic solution develops, hydrogen
evolves,
aluminum pitting takes place, and aluminum hydroxide forms during this
reaction.
A critical current density is necessary to sustain the corrosion reaction.
Below this
current density corrosion will be very slight, or almost imperceptible. Once
the current
density is high enough, the reaction can be swift. The necessary cun~ent
density is below
1 mA/in2. The current density of a damaged 600 V cable is influenced by the
voltage, leakage
2~ resistance, and the area of exposed metal. Variables affecting this can
include dampness of
the soil, chemistry of the soil, degree of damage, etc.
4
. CA 02439367 2003-09-03
DESCRIPTION OF THE RELATED ART
The toughest cables on the market today will IlOt always stand up to the
rigors of
handling, installation, and operation. And exposed aluminum will eventually
deteriorate.
The solution, then, is to find a way to economically prevent the corrosion
prdcess.
Attempts have been made to prevent the ingress of moisture by introducing a
sealant
between the strands of the conductor and between the conductor and the
insulation. See U.S.
Pat. Nos. 3,943,271 and 4,130,450. However, it has been found that the mere
introduction of
a sealant into such spaces is not entirely satisfactory. Attempts to prevent
moisture from
reaching the conductor, such as using water swellable material, have not met
with technical
and/or economic success. For example, voids may be formed in the sealant
during the
application thereof or may be formed if the cable is accidentally punctured.
Any such spaces
or voids form locations for the ingress of moisture which can lead to
corrosion of the
1 ~ conductor and conventional sealants used in the cables cannot eliminate
such voids.
A prior art attempt to minimize the flow of moisture or water within the
interstitial
spaces of a stranded conductor came in the form of compacted or compressed
stranded
conductors. The stranded conductor itself was radially crushed in order to
reduce the
diameter of the conductor and to fill the interstitial spacing with metal from
the individual
wires themselves. The drawback to this method is that even though some
deformation of the
individual wires does take place, and some of the interstitial spacing is
filled, there is still the
possibility of cable insulation damage through which moisture can enter the
cable and contact
the conductor.
Another attempt at correcting moisture flowing within interstitial space
consisted of
filling the interstitial space with a foreign substance which physically
prevented the flow of
the moisture or water within the conductor stntcture. These substances
typically comprised
some type of jelly base and a polyethylene filler material. At slightly
elevated temperatures,
this compound becomes fluid and viscous and can be applied as the conductor is
being
formed. The individual wires used to form the conductor are fed into an
extrusion die where
5
CA 02439367 2003-09-03
the moisture blocking compound is extruded onto and around each individual
wire and, as the
wires are stranded into the conductor, the interstitial space is filled with
the jelly-like
material. Upon cooling, the filler becomes very stable and immobile and does
not flow out oC
the interstitial spaces of the stranded conductor. Once the filling compound
is applied ~vithir~
the interstitial spaces of the stranded conductor, it tends to remain in
place. The problems
encountered in applying such a filling substance revolve around precise
metering of the
material into the interstitial spaces as the stranded conductor is being
formed. If too much
material is extruded into the conductor, the outer insulation will not fit
properly. If too little
material is applied, the interstitial spaces will not be filled and therefore
will allow moisriue
to flow within the conductor.
Another drawback to this method of applying a moisture blocking material is
that an
extrusion head and an extrusion pump for applying the material is required for
evcr«
individual layer of wires used to farm the conductor. The problems described
above.
1 ~ regarding the regulation of the volume of material applied through an
extrusion head are.
multiplied every time an additional extrusion pump and extrusion head is
required within the
conductor manufacturing system. Prior art efforts to manufacture an acceptable
moisture
blocked conductor revolved around methods for uniform application of the
moisture blocking
material to the conductor, but did not solve the problems created by handling
and installation
damage.
Applications of moisture blocking material to the spacing of concentric lay
conductors is known within the industry. This can be found in United States
Patents
numbered 3,607,487; 3,889,455; 4,105,485; 4,129,466; 4,435,613; 4,563,540; and
4,273,597.
?~
U.S. Patent 4,273,597 shows a method of strand filling the interstitial
spacing of a
conductor with a powder. This is accomplished by passing the strands through a
fluidized
powder bed, where the interstitial spacing is filled with the powder. The
stranded conductor
then exits the opposite end of the bed where an insulating layer is applied
which prevents the
powder from vacating the interstitial spacing of the conductor.
CA 02439367 2003-09-03
U.S. Patent 4,563,540 describes a conductor which is constructed by flooding a
waterproofing material among the individual conductors which make up the core
of the
stranded conductor. This flooded core is then wrapped with a plurality of
different layers of
shielding material which prevents the influx of moisture into the stranded
conductor.
S 't
U.S. Patent 4,435,613 describes a conductor constructed of a plurality of
layers of
insulating material with the core (or conducting portion) of the conductor
being filled with an
insulating layer of polyethylene. This polyethylene layer is contained by
other rubber and
plastic and epoxy compounds which produce a conductor having a waterproof
construction.
U.S. Patent 4,129,466 deals with a method for the application of the filling
medium
which is applied to a stranded conductor. This method comprises a chamber into
which are
passed individual wires that will be used to form the stranded conductor.
These wires have a
filing medium applied to them in the chamber. After the application of this
filling medium,
the conductor is passed through a chilling chamber where the filling medium is
cooled and
allowed to solidify within the interstitial spaces. This method requires that
the chamber
containing the filling medium and the stranded conductor be both heated and
pressurized.
The heat applied to the chamber reduces the viscosity of the filling material,
while the
pressure assures introduction of the material into the interstitial spaces of
the stranded
conductor.
U.S. Patent 4,105,485 deals with the apparatus utilized in the '466 method
patent
previously discussed.
?~ U.S. Patent 3,889,455 discloses a method and apparatus for filling the
interstitial
spacing of the stranded conductor in a high temperature flooding tank. The
individual wires
are fed into a tank containing the filling material, the material having been
heated to allow it
to become less viscous. The individual wires are stranded and closed within
the confines of
the flooding tank and the finished conductor is withdrawn frOnl the opposite
end of the
flooding tank where it is passed through a cooling means. The disadvantages
experienced
here involve the practice of stranding the conductor beneath tl~e surface of
an elevated
7
CA 02439367 2003-09-03
temperature moisture block pool. No access, either visual or mechanical, to
the conductor
manufacturing process is practical.
U.S. Patent 3,607,487 describes a method whereby individual strands of wire
are fed
into a flooding tank which is supplied with heated filling material by a pump
and an injection
means. The stranded conductor is withdrawn through the opposite end of the
flooding tank,
wiped in a wiping die, wrapped in a core wrapper and then passed through a
binder where it
is bound. The bound, wrapped core is then passed through a cooler which sets
the filling
material. The above described process is repeated through another flooding
tank, another
cooler, another binding machine, another flooding tank, another extruder,
another cooling
trough, and is eventually withdrawn from the end of the manufacturing line as
a product
having a plurality of layers of moisture blocking compound which protects the
conductor
core. The disadvantages here comprise a complex manufacturing line whereby
moisture
blocking material is applied at many different locations, each having to be
meticulously
1 S monitored and controlled in order for a proper conductor construction to
be obtained.
It can be readily seen from the above referenced methods and apparatuses that
moisture blocked conductors are known and it can also be recognized that there
are major
problems concerning the elimination of moisture contacting the conductor as a
result of
handling and installation of a cable.
BRIEr SUMMARY OF THE INVENTION
The present invention relates to improvements in insulated solid and stranded
cables.
2~ An electrical cable and a method for manufacturing the electrical cable are
provided in which
a plurality of insulated conductors have an inner protective layer extended
thereabout, and
outwardly extending ribs, or an exterior ribbed or finned surface, which
includes a pluralit~~
of longitudinally extending ribs or fins between which exist a plurality of
voids- An outer
insulation layer may be formed in the same operation as the inner layer or
ribs or in a
subsequent operation. In a two-pass manufacturing process for the present
cable, the first
pass involves extruding the inner finned layer onto the conductor. The inner
layer can be
8
' CA 02439367 2003-09-03
polyethylene, pvc, or another suitable plastic material. The inner layer can
be cross-linked
while it is being applied or batch cross-linked after it is applied. The
second pass involves
using a hot melt pumping system to apply the sealant layer. This system
advantageously
consists of a Nordson model 550 drum melter which delivers sealant to a CH-440
head
through which the cable passes. Other n1e1110aS Of p11111p111g sealant,
applying sealant, and
sizing the sealant layer can be used depending on process or product
requirements. The
sealant can be applied over a wide range of temperatures. Good results are
obtained by
applying the sealant at about 175 degrees Fahrenheit. The outer encapsulating
layer is then
applied after the sealant layer, downstream from the sealant head. The outer
layer can be
polyethylene, pvc or another suitable plastic material. The outer layer can be
cross-linked
while it is being applied or afterwards in a batch process.
In a single pass manufacturing process, the conductor is fed into a head that
consists
of 3 zones. The inner finned layer is applied in the first zone. In the second
zone the sealant
1 ~ layer is applied. The outer encapsulating layer is applied in the third
zone. This process
requires close control of the sealant temperature. The sealant must be applied
cold enough to
be able to remove enough heat to help set the finned layer. This avoids damage
to the fins
when the outer encapsulating layer is applied in the third zone. The sealant
most not be
applied too cold or it will prevent even distribution of the sealant in the
fins or cause fin
damage. The optimal sealant application temperature is from about 80 to about
150 degrees
Fahrenheit.
In one embodiment of the invention, during manufacture of the self sealing
cable, a
material which provides the cable with puncture, crack, and void self sealing
properties is
2~ included between the ribs or fins and the outer insulation. The voids are
at least partly filled
by the material which will flow into any void, puncture, or crack formed in
the insulation,
thus preventing migration of moisture. The self sealing material is applied in
the voids
bet'veen the ribs or fins and the outer insulation, therefore the self sealing
material does not
contact the conductor.
9
~ CA 02439367 2003-09-03
BRIEF DESCRIPTION OF THE DRAWINGS
The objects and advantages of the invention will be apparent from the
foiiowing
detailed description of the preferred embodiments thereof in conjunction with
the
accompanying drawings in which:
FIG. 1 is a cut-away, perspective view of a cable of the invention showing a
stranded
conductor, the finned inner layer surrounding the conductor, the insulation,
and the area
between the fins containing the material which provides the self sealing
effect;
FIG. 2 is an end view of the embodiment of the cable shown in FIG. 1; a?id
FIG. 3 is a cut-away side view of the cable shown in FIG. 1.
FIG. 4 is a diagrammatic representation showing insulation damage.
FIG. 5 depicts the soil-filled box used to determine current leakage in a
damaged
cable.
FIG. 6 is a graph of sample leakage current measurements.
F1G. 7 is a graph of conductor resistance measurements.
FIG. 8 is a graph of sample temperature measurements.
~J
FIG. 9 is a comparison of samples of the invention and a control after 91 days
in the
test.
FIG. 10 is a close-up of the control sample after 91 days in the test.
10
' CA 02439367 2003-09-03
FIG. 11 is a close-up of the test sample of the present invention after 91
days in the
test.
DETAILED DESCRIPTION IN THE INVENTION
°'
Although the principles of the present invention are applicable to different
types of
electric cables, the invention will be described in connection with a known
cable stmcture,
such as a 600 volt cable, which normally comprises, as a minimum:
( 1 ) A central conductor of stranded wires of a good conductivity metal such
as
copper, aluminum, copper alloys or aluminum alloys; and
(2) A layer of insulation around the stranded conductors which has been
extruded
thereover,
1 ~ FIG. 1 shows a cable 11 comprising a conductor 12 of stranded wires of
copper or
aluminum or alloys thereof. An inner layer 14 encircles cable 11. A plurality
of
longitudinally extending fins or ribs 15 are formed between which extend a
plurality of voids
16. A layer 10 of material which provides the self sealing effect is applied
in and at least
partly fills voids 16 between ribs 15, inner layer 14, and an outer insulation
jacket 13.
Insulation jacket 13 is of known material and is preferably an extntded
polymeric material.
Preferred material l0 comprises a polymer which can be readily pumped at
temperatures at least as low as 25°C. Preferably, the polymer will be a
low molecular weight
polymer such as low molecular weight isomer. Other materials, or combinations
of
materials, with or without such polymers, having such characteristics may also
be useful in
the present invention. A polymer which has been found to be particularly
suitable is
polyisobutene.
The preferred polymer of the present invention has very little or no
significant Shore
A hardness. A test of determining whether or not the polymer has acceptable
properties is the
Penetrometer Test incorporated in ASTM DS Penetration of Bituminous Materials.
The 100
CA 02439367 2003-09-03
grams needle penetration value at 25°C should be greater than about 100
tenths of a
millimeter.
The material used to provide the self sealing effect to the electric cable of
the present
invention has the following properties:
(a) The material is substantially insoluble in water;
(b) The material is a dielectric, i.e., it is non-conductive. and is not a
semi-
conductor;
(c) The material causes the cable to be self sealing, i.e., it will flow, at
ambient
temperature, into insulation voids and/or cracks and prevent contact between
the conductor and moisture which could cause cable failure; and
(d) The material does not absorb moisture or swell upon contact with moisture.
1 ~ In the preferred embodiment of the present invention, the material used to
at least
partly fill voids 16 is a compound of a iow molecular weight isomer or a low
molecular
weight copolymer of an isomer. Preferably, the material is polyisobutene.
Advantageously
there is little or no air present between voids 16 and insulation jacket 13.
The material of the present invention may optionally contain filler material,
but is
essentially free of any solvents or oils.
The cable 11 described in connection with FIG. 1 can be used without further
ladders
encircling the insulation jacket 13.
Also, in other embodiments of the present invention described herein, the
conductor
and layers of insulation can be the same as those described in connection with
FIG. 1.
The cable I1 illustrated in FIG. 2 is an end view of the cable illustrated in
FIG. 1.
12
CA 02439367 2003-09-03
FIG. 3 is a cut-away side view of cable 11 shown in FIG. 1 and illustrates
voids 16
and ribs or fins 15.
The ratio for the height of fins 15 to the width of voids 16 can vary.
Advantageously,
S the height to width ratio ranges from about 0.25 to about 2.00. Preferably
tlfe height to width
ratio ranges from about O.S to about 1.00. The fins do not have to be equally
spaced but it is
generally desirable to equally space the fins to achieve equal distribution of
the medium that
is in the channel regions, voids 16, and improve cable concentricity. The
number of fins can
range from a minimum of 2 up to any practical number that is needed based on
the size of the
cable, structural needs of the cable, the material being used in the voids,
the delivery rate
needed if applicable for the material, or the physical size of the material
being delivered. The
base thickness upon which the fins rest should not be less than about SO
percent of the width
of the fins. The base thickness can vary based on thickness requirements of
industry
specifications, structural needs of the cable, or other specific cable needs.
lS
The retaining mechanism between the outer encapsulating jacket or insulation
and the
fins can be a polymeric bond between the outer extruded layer 13 and the fins
1~, or may be
purely frictional. The frictional mechanism is due to the compressive forces,
surface area,
and frictional coefficient between the two layers. A material can be added
during processing
that increases the frictional coefficient between the two layers. 1f a
polymeric bond is
desired, it should constitute bonding of at least SO% of the exposed surface
area of tins 1s,
i.e., the upper portion of the fins that contact the interior surface of the
outer extended layer
13. Another retaining mechanism is similar to a shaft and a key, i.e., the
upper portion of the
fin is embedded into the outer encapsulating layer which helps prevent
rotation of the inner
2S layer or other movement. Advantageously the fin is embedded to a depth of
at least about
001 inch into the interior of the outer insulation layer, prel-erably from
about .002 inch to
about .00S inch. The embedment can be varied by controlling different
variables of the
process. It is also possible to have combinations of polymeric, frictional,
and embedded fin
retaining mechanisms between the two layers. Fins 15 may be attached to inner
layer 14,
outer layer 13, or both.
13
CA 02439367 2003-09-03
Materials that can be delivered in the channels in addition to sealing
materials are
fiber optics, heat transfer fluids to enhance cable heat transfer properties,
other desirable
materials that would provide a beneficial cable property or use the cable as a
messenger to
connect a beginning and/or end point.
The most desirable materials for use as the inner layer 14, fins I5, and outer
encapsulating layer 13 are plastics that can be either thennoset or
thermoplastic. Known
plastic materials can be used in order to achieve desired cable properties.
1 U The colors of the inner layer 14, fn ~s 15, and outer layer 13 materials
can be the same
or they may differ. Different colors may be used to allow easier
identification of the product
in the field or for other desirable cable properties. The fins or ribs may be
straight, may
spiral, may oscillate about the axis of the cable, or may form different
patterns depending on
the desired cable characteristics and efficiency and flowability of the
sealing material used.
1~
It is to be understood that additional embodiments may include additional
layers of
protective material between the conductor and the insulation jacket,
lIlCludlilg an additional
water ban-ier of a polymer sheet or film, in which case it is not essential
that the jacket tightly
enclose me layers there ~.vithin or enter into the spaces between the wires
and protectivz
20 materials, i.e., the interior size of the jacket can be essentially equal
to the exterior size of the
elongated elements so that compression of the elongated elements, and hence,
indentation of
the layers there within including the insulation, is prevented.
The cable of the present invention is of particular advantage in that not only
does the
- 2~ material fill the space between the inner layer and the insulation as the
cable is manufactured,
but after the cable is placed in service the material will flow into any cuts
or punctures
formed as a result of damage during handling and installation of the cable or
its use in
service. The stresses placed on the conductor and the insulation during
handling and
installation of the cable, such as bending, stretching, reeling and unreeling,
striking with
30 digging and installation equipment can fopo cuts or punctures in the
insulation and between
the insulation and the conductor. Such cuts or punctures can also be formed
after the cable
14
CA 02439367 2003-09-03
has been placed in service as a result of damage from adjacent utilities,
homer owners, or
lightening strikes.
The cable of the present invention can provide acceptable service even after
the
insulation has been cut or punctured, exposing the conductor. In order~~to
determine the
efficiency of using a self sealing material defects were made in the
insulation layer of two
600 V cable samples. On one of the cable samples, a layer of polyisobutene
polymer was
applied before application of the outer insulation layer of the cable. The
other cable sample
did not have the polyisobutene layer. Both cable samples were placed inside
separate 1 liter
glass beakers containing tap water. Each cable sample was energized at 1 l OV
to ground with
AC current. The sample which did not have the polyisobutene layer exhibited
severe
corrosion overnight. The sample containing the polyisobutene layer exhibited
no corrosion
after being energized and submerged for 4 weeks in tap water in the glass
beaker.
1 ~ FXAMP1,F t
This test was designed to evaluate the perfom~ance of the present invention's
self
sealing, 600 V underground cable. The test program was patterned after a
previously
developed procedure to evaluate self sealing or self repairing cable designs.
To conduct the test damaged cables were placed in a specially mixed, moist
soil. The
cables were then energized with 120 V ac to ground. Measurements made included
changes
in leakage current to earth and cable conductor resistance. The temperature of
each cable
near the damage point was also monitored.
Four control sample replicates and eight self sealing sample replicates were
evaluated. All four control samples failed the test relatively early in the
test program. All
eight self sealing samples performed well, with no significant increase in
conductor
resistance and low leakage current values throughout the 60-day test period.
15
CA 02439367 2003-09-03
Conventional and self sealing 600 volt underground cable with a 2/0 AWG
combination unilay aluminum conductor were tested in 10-foot lengths.
The soil used in the test was a mixture of Ottawa Sand, Wyoming Bentonite and
fertilizer. The combination of the three materials provides a sandy-silt type
soil, which is
very conductive. The sand serves as the basic soil structure while the silt
provides small
particles that can work their way into the damaged areas of the cable. The
silt also helps to
keep water evenly dispersed throughout the soil. The fertilizer enhances the
conductivity of
the soil and may enhance corrosion as well. The goal was to achieve a soil
electrical
resistivity of <50 ohmmeters.
16
~ CA 02439367 2003-09-03
Tap water was used to achieve a moisture content near saturation. This
combination
of soil materials provides a worst case condition for the ac corrosion of the
aluminum
conductor in 600 V underground cables and is also repeatable from lab to lab.
The soil mixture was:
100 lbs. Ottawa Sand
3.33 lbs. Bentonite
23.33 lbs. Tap Water
1.26 lbs. of Peters 20-20-20 Plant Fertilizer (mixed with the water before
added to the sand
and clay ingredients)
The amount of water added achieved near saturation conditions. The wet density
was
approximately 127 Ibs./ft.
1 ~ The aging box was made of wood and lined with polyethylene to hold
moisture. The
approximate inside dimensions were 6.5 feet long by 1.3 feet wide by I foot
high. A wide,
copper tape ground electrode covered the bottom and sides of the box on top of
the
polyethylene. A wire connected this electrode to ground.
After moist soil was packed in the bottom of the box (approximately 6 inches),
four
control samples and eight self sealing samples were installed, approximately
six inches apart.
The tivo sample sets were:
Samples I-4: conventional 600 V UD wire (control samples) all with slot damage
at
2~ the center of the sample
Samples S-12: self sealing cable - all with slot damage near the center of the
sample
17
CA 02439367 2003-09-03
Immediately before the samples were placed in the box, they were damaged down
to
the conductor. One damage condition was used. It consisted of a slot cut into
the insulation
down to the conductor, perpendicular to the cable axis. A razor knife and an
angle guide was
used to control the slot size. The size and shape of the damage location is
shown in Figure 4.
The damage locations were staggered so they were not adjacent to each other.
The 10-foot long self sealing samples were first damaged in the middle. After
5
minutes, they were placed in the box with the damage facing up. They were then
covered
with soil.
The control samples were initially 2.5-foot long. They were also damaged in
the
middle, then installed in the box. There was no waiting period before they
were covered with
soil.
As each sample was installed, a type T thermocouple with a welded bead was
attached to the cable surface, approximately one inch from the damage
location. Once all
samples were installed, the soil was compacted. After 24 hours, the ends were
cut off of the
self sealing samples so they were the same length as the control samples. The
test layout is
shown in Figure 5.
s~ ao ate. msk. uo,~:n
vide ~ uLie .Wrbu.6q mn.
wide atowWuctornaAa
25 Figure 4: Insulation damage
18
CA 02439367 2003-09-03
t °' : ~ ~ 6 '7 8 9 iQ 11 7
xwame Lm~a H~r~oaa~ rya
S
Figure 5: Sample layout in soil filled box
Damage Key: ------- slot damage
Samples 1-4 Control
Samples 5-12 Self Sealing
After the installation was complete, the soil was covered with polyethylene to
minimize the evaporation of water from the soil. 120 V ac was applied
continuously to all
sample conductors. The soil was grounded via the copper ground mat lining the
tank. The
data collection was as follows:
1) Measurements (Measured initially, then daily for first 5 workdays, then on
Monday,
Wednesday and Friday of each week thereafter.)
19
CA 02439367 2003-09-03
a) Conductor resistance, each sample individually - Biddle DLRO, CQ # 1010
(Expected accuracy: ~ 3 % of reading)
b) Leakage to ground a 120 V, each sample individually - Fluke 87, CN 4007
(Expected accuracy: t 3% of reading)
c) Sample surface temperature - Yokaggawa DC 100, CN 4015-'
(Expected accuracy: t 2 Deg. C)
2) The test ran for 91 days. When significant degradation occurred on a
sample, it was
disconnected from the voltage source. Significant degradation is defined as:
a) Several days with leakage current greater than 1 amp on an individual
sample
1 S b) Conductor resistance on an individual sample 10 times greater than
starting resistance
3) Final soil electrical resistivity and moisture content was measured when
the test was
completed.
4) All measurements were recorded and resistance, leakage and temperature data
were
plotted using an Excel spreadsheet.
During the first 26 days of the test the conductor resistance and the leakage
current
into the soil increased significantly on all four control samples. They were
each removed
from the test (disconnected from the test voltage) as the conductor resistance
exceeded 1,000
micro-ohms. The conductor resistance and the leakage cun-ent to the soil for
the eight self-
sealing samples did not change significantly during the test.
The soil electrical resistivity was measured at the end of the test by placing
a sample
of the soil in a 17-inch long, 2-inch inside diameter PVC tube. It 4vas packed
to the same
density used in the test tank. Two-inch diameter copper plate electrodes were
pressed against
the soil on each end of the tube. 120 volts ac was applied across the
electrodes and the
resulting current was measured. The current and voltage were used to calculate
the sample
resistance, which was then converted to resistivity.
3~
Moisture content and density were measured at the beginning and end of the
test. To
make the measurement, a soil sample was taken using a 1/30 cubic foot metal
shelb~~ tube.
The sample was then oven dried to calculate moisture and density. The measured
wei«hts
were used to calculate moisture content.
CA 02439367 2003-09-03
Soil resistivity, moisture and density measurements are summarized in Table 1.
Table 1
Time of Electrical ResistivityMoisture ContentVet Density
>\~Ieasurement(ohm-meters) (% by weigh) (lbs./ft3)
Initial 4.3 near saturation~ 126
Final 5.1 15.8 ~ 126
The insulation resistance, conductor resistance and sample temperature
measurements
made during the test are shown in Figures 6-8. The samples are identified as
S1, S2, S3, etc.
The first four are control, the remaining eight are self sealing. In addition,
C = Control, SS =
Self Sealing.
During periods of relatively high leakage cun-ent on the control samples the
temperature of these samples was also relatively high. Photos of the samples
under test are
showm in Figures 5, 6 and 7. From the photos it is obvious that the control
samples
experienced significant corrosion while the self sealing samples experienced
no noticeable
corrosion.
1j
r.v~ram r, ~
A cyclic load test was run on the finned cable of the present invention and
compared with similar non-finned prior art cables. 50 ft. samples were tested.
The samples
had a 50 °C conductor temperature, and were cycled on 8 hours a day and
off 16 hours, 7
days a week. The cables were tern~inated with a mechanical connector. No duct
seal, mastic
tape, electrical tape, or the like was used. The tops of the samples were
approx. 11 ft. above
the floor. The samples gradually droop to the floor.
7j _
Sample 1 (Invention)
\\'eeks of Sbrinkback Sluinkback at Total Sluinl:back
Aging at Top Bottom (in)
Initial .0000 .0000 .0000
1 .3035 .1510 .454
21
CA 02439367 2003-09-03
Sample 2 (Invention)
Weeks of Aging Shrinkback Shrinkback Total Shrinkback
at Top at Bottom (in)
Initial .0000 .0000 .0000
I .1385 .1880 .3265
,x
Sample 1 - Bare
(Prior Art)
\Veeks of AgingShrinkback Shrinl:back Total Stu-inl:back
at Top at Bottom (in)
Initial .8450 .2220 1.0670
1 4.6375 1.2010 5.8385
2 5.5390 .8220 6.3610 I
3 5.9350 .6735 6.60S5
4 6.1110 .6150 6.7260
S 5.9065 .5850 6.4915
6 6.3725 .6020 6.9745
I
7 6.2960 .7320 7.0280
8 6.4500 .5340 6.9840
9 6.6855 .4350 7.1205
~i Samplc 2
- Duct Seal
(Prior Art)
\Veeks of AgingSluinkback Shrinkback Total Sluinhback
i at Top at Bottom (in)
Initial .2205 .25SS 0.4760
1 3.1345 2.7980 5.9325
2 3.7155 2.7255 6.4410
3 4.7570 2.0195 6.7765
I 4 5.1600 I.S31 S 6.6915
5 5.4965 1.2150 6.7115
', 6 5.7300 1. I I 1 S 6.8415
7 5.6915 1.2420 6.9"5
8 6.0065 1.0395 7.0460
9 6.1285 8860 7.0145
22
~ CA 02439367 2003-09-03
Sample 3 - Mastic
'rape (Prior
Art)
\'creeks of Shrinkback Shrinlcback Total Siirinhback
Aging at Top at Bottom (in)
'
Initial .2270 .2195 0.4465
1 3.6490 1.6500 X5.2990
2 3.5330 2.0550 5.5580
3 4.0990 1.6900 5.7890
4 4.3685 1.5315 5.9000
4.4675 1.4650 5.9325
6 4.6870 1.3660 6.05 30
7 4.6605 1.3435 6.00:10
8 4.7635 1.2190 5.9825 I
9 4.9370 1.0500 5.9870
Over 80% of the total shrinkback of the prior art cable occurred in the first
week of
testing.
l0
Comparative results with the present invention show a dramatic reduction in
shrinkback after 1 week of testing. The reduction is more than 92% when
compared with the
pnor art.
1 ~ Although preferred embodiments of the present invention have been
described and
illustrated, it will be apparent to those skilled in the art that various
modifications may be
made without departing from the principles of the invention.
30
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