Note: Descriptions are shown in the official language in which they were submitted.
CA 02220368 1999-09-24
SINGLE-JACKETED PLENUM CABLE
FIELD OF THE INVENTION
This invention relates to a communications cable suitable for plenum, riser,
and other
applications in building structures. More particularly, the present invention
relates to an
improved construction for a high-frequency communications cable that is
capable of meeting
rigorous burn requirements and is electrically stable during operation at
substantially higher
temperatures than prior art cables.
BACKGROUND OF THE INVENTION
It is common practice to route communication cables and the like for
computers, data
devices, and alarm systems through plenums in building constructions. If a
fire occurs in a
building which includes plenums or risers, however, the non-fire retardant
plenum
construction would enable the fire to spread very rapidly throughout the
entire building. Fire
could travel along cables installed in the plenum, and smoke originating in
the plenum could
be conveyed to adjacent areas of the building.
A non-plenum rated cable sheath system, which encloses a core of insulated
copper
conductors, and which comprises only a conventional plastic jacket, may not
exhibit
acceptable flame spread and smoke generation properties. As the temperature in
such a cable
rises due to a fire, charring of the jacket material may occur. If the jacket
ruptures, the
CA 02220368 1997-11-06
interior of the jacket and the insulation are exposed to elevated
temperatures. Flammable
gases can be generated, propagating flame and generating smoke.
Generally, the National Electrical Code requires that power-limited cables in
plenums
be enclosed in metal conduits. This is obviously a very expensive construction
due to the
cost of materials and labor involved in running conduit or the like through
plenums. The
National Electrical Code does, however, permit certain exceptions to the
requirements so
long as such cables for plenum use are tested and approved by an independent
testing
laboratory, such as the Underwriters Laboratory, as having suitably low flame
spread and
smoke-producing characteristics. The flame spread and smoke production
characteristics of
cable are measured per specification UL-910 plenum burn analysis.
With plenum cables, in addition to concerns about flammability and smoke
production, the cables must also, of course, have suitable electrical
characteristics for the
signals intended to be carried by the cables. There are various categories of
cable, such as
Category 3, Category 4, Category 5, etc., with increasing numbers referring to
enhanced or
1 S higher frequency electrical transmission capabilities. With Category 5,
for example,
extremely good electrical parameters are required, including low attenuation,
structural return
loss, and cross-talk values for frequencies up to 100 MHz. Unfortunately,
cable materials
which generally have the requisite resistance to flammability and smoke
production also
result in electrical parameters for the cable generally not suitable for the
higher transmission
rates, such as a Category 5 cable. Specifically, in the case of cables
intended for Category
5, the cable core, in addition to passing the plenum burn test UL-910, must
also pass physical
property testing provided by the specification requirements UL-444, as well as
meet
Category 5 electrical requirements such as provided in Electronic Industries
Association
specification TIAIEIA-568A.
Currently, a cable construction which is available and which meets these
requirements
is provided in a configuration which includes fluorinated ethylene propylene
(FEP) as
insulation, with a low-smoke polyvinyl chloride (PVC) jacket. Such a cable
construction
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meets the 100 MHz frequency operation requirements, and it has been
demonstrated that
such a cable construction can be suitable for operation at 155 Megabits or 150
MHz.
Unfortunately, FEP at times may be in short supply. Given the manufacturing
capacity of
FEP producers, only enough FEP is currently produced to meet approximately 50
percent of
the demand for the volume of material required to construct high-category
cables. Although
it could be expected that the supply of FEP will continue to increase, it is
apparent that the
available quantity of FEP may not meet the demand for the material for use in
plenum cables
as the market is projected to increase at a rate of approximately 25 percent
per year through
1999, particularly in anticipation of European and Scandinavian market demands
for plenum
cables.
Current riser cables utilize a foam/skin insulation. The insulation material
construction is a foamed, high density polyethylene and PVC skin composite. A
jacketed
and shielded cable of these insulation cores meets Category 3 electrical and
the CMP burn
requirements. However, developing a Category 5 cable is very difficult due to
the extreme
electrical parameters necessary, i.e., attenuation, structural return loss,
and cross-talk values
to 100 MHz. Furthermore, this core must pass elevated temperature attenuation
requirements
at 40 °C and 60 °C. The above-mentioned insulation composite
with a PVC skin will not pass
the elevated temperature attenuation requirements because the dielectric
constant of PVC
increases with temperature.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of this invention to provide a cable construction suitable for
high
frequency electrical applications while at the same time being resistant to
burning.
It is a more specific object of this invention to provide a cable design that
meets
Category 5 or higher electrical parameters, including elevated temperature
attenuation
requirements, while at the same time satisfying the burn rating standards for
plenum cable.
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It is an additional object of this invention to provide a cable construction
which meets
the electrical and burn rating requirements and additionally meets various
physical
requirements such as cold bend, room temperature and aged tensile strength,
elongation, and
the like, required for plenum cables.
It is another object of this invention to provide such a cable construction
meeting the
above requirements, which does not utilize FEP, and which is suitable for
operation up to
155 Megabits or 150 MHz.
A further object of the present invention is that it provides a cable
construction having
an outer jacket construction that exhibits electrically stable characteristics
at substantially
high temperatures, relative to the temperature requirements of currently
available plenum
cables.
Briefly, in accordance with one embodiment of the invention, a riser and
plenum rated
cable construction includes a plurality of twisted wire pairs utilizing a
polyolefin primary
insulation material and a single outer jacket for the cable construction
formed of a
thermoplastic halogenated polymer. To improve the electrical characteristics
of the cable,
the outer jacket is of a foamed construction.
BRIEF DESCRIPTION OF THE DRAB
A more complete understanding of the present invention may be derived by
referring
to the detailed descriprion and claims when considered in connection with the
Figures, where
like reference numbers refer to similar elements throughout the Figures, and:
FIG. 1 is an elevation of a cable construction in accordance with the present
invention
with a portion of the outer jacket broken away for illustrative purposes;
FIG. 2 is a cross sectional view of a cable construction in accordance with
the present
invention in which a plurality of cable cores are enclosed as a composite in
an outer jacket;
and
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FIG. 3 is a cross-section of one of the conductors in a twisted wire pair of
the cable
shown in FIG. 2.
DETAILED DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS
As noted, FEP insulation with a low-smoke PVC jacket meets Category S
electrical
requirements and the applicable physical and burn property tests for plenum
rated cable.
While the electrical and physical property requirements for Category 5 and
higher cable
could be met with other plastics such as polyolefins or modified polyolefins,
the plenum burn
requirements, such as UL-910, could not be met since polyolefins burn readily.
If a
polyolefin material was smoke suppressed and flame retarded, the ingredients
necessary for
flame protection would detract from the necessary electrical values of the
polyolefin
material, and would also detract from the physical property attributes of the
material.
The CMP or plenum burn test is a severe test. The test takes place in a closed
1 S horizontal fixture or tunnel, with the ignition flame source being a
300,000 BTU/hour
methane flame with a high heat flux, and a 240 foot/minute air draft. The test
lasts 20
minutes, and the cable is stretched side to side across a 12 inch wide, 25
foot long wire mesh
rack in the tunnel. To pass this test, flame spread must not exceed 5.0 feet
after the initial
4.5 foot flame source; smoke generarion must not exceed a peak optical density
of 0.5 (33%
light transmission); and the average optical density must not exceed 0.15 (70%
light
transmission). The purpose of this optical smoke density parameter is to allow
a person
trapped in a fire the ability to see exit signs as well as visually discern a
route or means of
escape.
FIG. 1 shows an elevation of a cable construction in accordance with a
preferred
embodiment of the present invention for providing a cable meeting Category 5
electrical
requirements and the applicable burn and smoke generation requirements, as
well as the
physical property requirements, for plenum-rated cable without the use of FEP.
Referring
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now to FIG. l, there is shown a cable which is designated gener211v by the
reference numeral
5, which is suitable for use in building plenums and the like. Lz the specific
e~cample shown
in FIG. l, the cable ~ is illustrated as having four twisted pair of
~ansmission media, referred
to as twisted pairs and indicated by reference numerals 6, 7, 8 and 9, forming
what is
generally referred to as the cable core. Ln accordance with this embodiment of
the invention,
the twisted pairs 6-9 have a polyolefin primary insulation, which has good
electrical
characteristics even though it readily burns. In a specific embodiment of the
present
invention, a foam/skin high density polyethylene (APE) is used for the primary
insulation,
which has the requisite electrical characteristics for high frequency cable
applications.
In order to enable having the required resistance to burning, the cable
construction in
accordance with this invention is provided with an outer jacket 11 which is
highly resistant
to burning. Thermoplastic halogenated polymers have been found to be suitable
materials,
particularly thermoplastic fluorocarbon polymers. In a specific embodiment of
the invention,
polyvinylidene fluoride (PVDF) has been found to be quite suitable in terms of
providing
adequate flame and burn resistance to meet the applicable standards.
A cable construction consisting of only the core of twisted pairs with
polyolefin
insulation surrounded by a jacket of conventionally extruded thermoplastic
fluorocarbon
polymer (such as PVDF) meets the applicable burn standards, but does not meet
the high
frequency electrical standards for cable. Specifically, the less than optimal
electrical
characteristics of a conventionally manufactured fluorocarbon polymer jacket,
and its
proximity to the twisted pairs, degrade the cable's electrical
characteristics.
In accordance with the present invention, a single outer foamed PVDF jacket 11
may
be employed by cable 5 without any intermediate material between the cable
core and the
outer PVDF jacket 11. The particular foam construction of the outer PVDF
jacket 11
suitably enhances the electrical characteristics of the PVDF material, which
typically exhibits
very poor dielectric constant and dissipation factor values in a substantially
solid or
unfoamed state.
6
CA 02220368 1997-11-06
Although not shown in FIG. l, cable 5 may include a shield located within
outer
jacket 11. Preferably, such a shield substantially surrounds the cable core
and is configured
to enhance the electrical performance of the cable core. For example, the
shield may be
configured to protect the cable core from extraneous RF or electromagnetic
fields and
S signals. The shield may be formed from a metallic foil, such as aluminum or
copper, and
may be constructed according to any number of conventional methodologies. Such
shields
are known to those skilled in the art, and need not be described in detail
herein.
Referring now to FIG. 2, there is shown a construction of a cable 10 in
accordance
with this invention, suitable for use in building plenums, and the like, i.e.,
indoor/outdoor
rated cable, in which a plurality of cable cores are enclosed within a single
foamed PVDF
outer jacket. In FIG. 2, the cable 10 comprises one or more wrapped cables 20,
each of
which may include a core 22. The core 22 may be one which is suitable for use
in data,
computer, alarm, and other signaling networks as well as communications. The
core 22 is
the transmission medium and is shown in FIG. 2 as comprising one or more
twisted wire
1 S pairs, the pairs of which are referred to in FIG. 2 by reference numerals
24, 26, 28 and 30.
Cables which are used in plenums may include 25 or more conductor pairs,
although some
cables include as few as six, four, two or even a single conductor pair such
as shown in FIG.
1. In the exemplary embodiment shown in FIG. 2, each of the cores 22 comprise
four twisted
conductor pairs, identified in FIG. 2 with reference numerals 24, 26, 28 and
30.
As shown in FIG. 2, each of the cables 20 preferably utilizes a foamed PVDF
inner
jacket configured identified by reference numeral 23. The inner jacket 23 may
be configured
as described more fully hereafter. Those skilled in the art will appreciate
that the inner jacket
23 is not a requirement of the present invention, and that any suitable
wrapping element
known to those skilled in the art may be employed by cable 10. Furthermore,
the particular
material utilized as the inner jacket 23 may be selected to enhance the
electrical and/or
physical properties of cable 10.
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As also shown in FIG. 2, a plurality of the cables 20 are disposed within an
outer
jacket 34 in this embodiment. In FIG. 2, three cables 20 are shown as enclosed
in an outer
jacket 34, although the invention is equally applicable to there only being
one cable enclosed
by an outer jacket (as shown in FIG. 1 ) and for there being more or less than
three cables 20
disposed within the outer jacket 34.
FIG. 3 is a cross-section of one of the conductors in one of the twisted
pairs, such as
twisted pair 24. The conductor or transmission medium 24 includes a conductor
36
surrounded by an insulating material 38. The insulating material 38 may have a
skin portion
indicated by reference numeral 40.
In accordance with a preferred embodiment of the invention, the primary
insulation
38 surrounding conductor 36 in each wire in the twisted wire pairs, such as
wire pair 24, is
a foam/skin polyolefin dual extruded insulation, which is acceptable for
Category 5 electrical
characteristics. The reasons for using a foam/skin insulation such as foam 38
with skin 40
(FIG. 2), in addition to achieving improved electrical properties, is to
effectively decrease
1 S the amount of polyolefin material available to burn.
It is important to keep the foam/skin pure, with no fillers, such that this
insulation can
match or exceed the electrical properties of FEP. For example, FEP has a
dielectric constant
of 2. I , with a dissipation factor of 0.0001; in accordance with a specific
embodiment of the
invention described herein, the insulation is a pure foam/skin HDPE having a
dielectric
constant of 1.8, with an equivalent dissipation factor of 0.0001. With this
configuration, the
velocity of propagation is even improved with the foam/skin at approximately
78% as
opposed to approximately 75% for FEP. By comparison, a flame retardant
polyolefin with
fillers would have a velocity of propagation of 67%. Also, a 2x2 cable (two
pairs of flame
retardant polyolefins plus two pairs of FEP) would encounter velocity of
propagation skew
problems, which is the difference in the distribution of electrical flow
between the two
insulation types. There are no skew problems with the pure foam/skin HDPE.
Velocity of
propagation considerations and skew factors are discussed more fully
hereafter.
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In accordance with one specific embodiment of the present invention, the
primary
insulation is dual extruded, with foam insulation 38 being a HDPE. A suitable
material is
one produced and available from Union Carbide Corporation identified as DGDB-
1351NT,
although an equivalent suitable for mechanical foaming may be used. In
accordance with
S the specific embodiment of the invention, the skin portion 40 of wire 24 is
also a HDPE
produced by Union Carbide Corporation and available therefrom and identified
as DGDM-
3364 NT. In such an insulation construction, the polyolefin skin 40 has to be
of adequate
thickness to protect the overall foam/skin primary insulation from crushing
during twist. The
degree of foaming, the foam thickness, and the skin thickness are dependent
upon
compliance with UL-444 physical property testing requirements.
In accordance with a specific embodiment of the invention, the conductor 36 in
each
wire 24 had a diameter range from 0.0194 inches to 0.0215 inches. In
accordance with this
specific embodiment, the insulating material 38 had a thickness of 0.0060
inches, and the
skin 40 had a thickness of 0.0022 inches.
I S In accordance with one embodiment of this invention, each of the cables 20
may be
provided with a substantially flame retardant core wrap rather than inner PVDF
jacket 23.
Such a construction may be desirable for a cable arrangement having a large
number of
insulated pairs, e.g., more than 12. A flame retardant core wrap may be
employed to ensure
that the cable arrangement satisfies the associated plenum burn requirements.
As previously mentioned, preferably the primary insulation of the transmission
media
is a foamed/skin construction of HDPE. One material which was found to be
quite suitable
in accordance with the invention is a polyethylene material known as DGDB-
1351NT, and
available under that designation from Union Carbide. When this material is
foamed and dual
extruded with a skin, DGDM 3364 NT also produced by Union Carbide Corporation,
it has
a dielectric constant at 1 MHz of I .80, a dissipation factor at 1 MHz of
0.0001, and an LOI
of 17 percent. LOI refers to the limiting oxygen index, the percent of oxygen
in air at which
the sample burns completely. The specific gravity of this material is 0.945,
but this material
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does not char, and hence needs to be protected by additional materials to meet
the burn test,
in accordance with and as provided by this invention.
As described above, the outer jacket 11 or 34 in accordance with this
invention is
made of a foamed halogenated polymer, and can be a foamed PVDF material. One
PVDF
material which has proved to be extremely suitable is known as SOLEF 31508-
0009,
available from Solvay Polymers, Inc. In an unfoamed state, this material has a
dielectric
constant of 8.40 at 1 MHz, a dissipation factor of 0.1850 at 1 MHz, and an LOI
of 100
percent (the ideal LOI). The specific gravity of the unfoamed material is
1.78, and it exhibits
excellent char formation.
It should be appreciated that other materials, such as a PVDF alloy, may also
be
suitable for outer jackets 11 or 34. One such alloy that has been employed in
a dual jacket
embodiment is available from Solvay and identified as SOLEF 70109-X003. The
dielectric
constant of this material at 1 MHz is 5.20, the dissipation factor at 1 MHz is
0.1250, and the
LOI is 85 percent. The specific gravity of this material is 1.64, and its char
formation is
1 S excellent. The inventors contemplate that this and other PVDF alloys,
including other
suitable PVDF materials available from other commercial suppliers, may be
foamed in
accordance with the present invention.
During manufacturing of the preferred cable construction, an extrusion tool
may be
employed to ensure that outer jackets 11 and 34 are properly formed to meet
physical and
electrical requirements. With the exception of the extrusion tool having a
die/core tube Land
length of one to two inches, such extrusion tools and related processes are
known to those
skilled in the art and, therefore, need not be described in detail herein. In
accordance with
an exemplary manufacturing technique, a quench water trough is placed within
approximately three inches from the extruder head to thereby quench the tube
extruded jacket
during draw-down. In addition, air (or another suitable gas) may be injected
through the
extruder head during draw-down to expand the jackets 11 and 34 and maintain
their
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substantially round cross sectional shape throughout the extrusion process.
The use of such
air injection prevents the foamed PVDF from collapsing during manufacturing.
In accordance with a first embodiment of the present invention, outer jackets
11 and
34 are formed by a chemical foaming process that utilizes a chemical foaming
agent. In one
exemplary embodiment, the outer jacket material is formed by introducing a
chemical
foaming agent to the PVDF (or other suitable material). Such chemical foaming
techniques
are known to those skilled in the material sciences and cable manufacturing
arts. Of course,
the specific amount of foaming agent may be varied depending upon the desired
electrical
and physical characteristics of the end product, the particular manufacturing
processes and
equipment used, the particular outer jacket material, or other application-
specific variables.
In accordance with a second embodiment of the present invention, outer jackets
11
and 34 are formed by gas injection, where the gas injected during the foaming
process is
preferably nitrogen. Such gas injection processes are known to those skilled
in the art and,
therefore, are not described in detail herein. In accordance with one
exemplary embodiment,
the amount of foaming agent/plastic carrier employed to electrically enhance
the PVDF
jacket material falls within the range of approximately 1 to 10 percent by
weight, and within
a preferred range of about 3 to 8 percent by weight.
In accordance with another exemplary embodiment, outer jackets 11 and 34 are
foamed to an expansion within the range of 5 to 30 percent, and within a
preferred range of
about 10 to 20 percent. In the context of this specification, the percent of
expansion refers
to the change in the specific gravity of the solid versus the foamed outer
jacket material. The
percent of expansion may be calculated by physically measuring the weight and
dimensions
of a sample portion of the foamed PVDF outer jacket and comparing the weight
to a
comparably sized amount of solid PVDF.
In the preferred embodiment, outer jacket 11 or 34 has a thickness within the
range
of 15 to 40 mils. The foamed PVDF outer jacket 11 or 34 is preferably about 25
mils thick.
In the preferred embodiment, the PVDF outer jacket 11, 34 is foamed from its
inner surface
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to its outer surface with small, discrete cells. The uniformity and size of
the foam cells
suitably enhances the electrical characteristics of cables 5, 11. It should be
noted that
extrusion tools may be configured to impart a smooth (but not a skin) outer
surface to cables
5, I 1. For example, the die tip of an exemplary extrusion tool may be heated
to smooth the
outer surface of the jacket after it has been foamed. In addition, the die
Land length may be
configured to suitably impose a higher pressure drop (and correspondingly
higher foaming)
as the PVDF material exits the die tip. In a preferred tooling embodiment, a
die Land length
of greater than one inch is utilized.
Those skilled in the art will appreciate that the specific thickness and
surface texture
of outer jacket 11 or 34 may vary depending upon the particular electrical
and/or physical
requirements of the cable. For example, the preferred embodiment of the
present invention
incorporates cores 22 and outer jacket 11 or 34 configured such that
electrical performance
of the cable is in compliance with TIA/EIA ~68A Category 5 cable standards.
The particular
amount of foaming and the specific composition of outer jacket may be suitably
selected to
ensure that the electrical, physical, and burn characteristics of the cable
meet all of the
relevant requirements.
It should be appreciated that the use of a single outer jacket may reduce the
manufacturing time and costs associated with a Category S cable, e.g., cable
5. The foamed
PVDF construction of outer jacket 11 enables cable 5 to pass the required UL
bum tests and
the Category 5 electrical tests without the need for an inner or intermediate
jacket or a core
wrap. Nonetheless, as previously described, in accordance with one aspect of
the invention
the core can be wrapped with an inner jacket of foamed PVDF material to
provide further
burn and smoke protection andlor to enhance the electrical performance of the
cable.
A number of experimental cables were fabricated utilizing the materials set
forth
previously for insulation construction and outer cable jackets. The
experimental cables
which passed the UL-910 plenum burn test at an independent laboratory along
with the
relevant test data, are set forth in Table 1 below:
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TABLE 1
UL-910 Steiner Tunnel Burn Results
Foamed PVDF Single Jacket Cable
Jacket
Cable Thickness Peak Average Flame
Construction (mils) Optical Densitx Qptical DensitX read ft
(Requirements) (s0.5) (s0.15) (s5 ft)
Cable #1 - 4 Pairs 24
Burn 1 0.19 0.07 2.5
Burn 2 0.25 0.07 3.5
Cable #2 - 4 Pairs 22
Burn 1 0.17 0.05 3.5
Burn 2 0.20 0.06 4.0
All of the above listed cables passed the plenum burn test as indicated, and
also passed
the Category 5 electrical requirements, as well as the UL-444 physical
property test
requirements.
Although an initial objective in accordance with the present invention focused
on
developing a cable construction that met the performance of existing cable
using FEP
insulation, it has been unexpectedly found that cable constructed in
accordance with the
principles of this invention actually exceeds the performance of FEP insulated
cable. In the
prior art, in addition to cables utilizing, for example, four twisted pair,
all having FEP
insulation, there have been constructions using a combination of insulation
materials. These
combination insulation constructions have been aimed at dealing with the
shortage of FEP
material relative to the demand for high category cables. For example, one
prior art
construction utilized a cable containing three twisted pair of FEP insulated
conductors with
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one twisted pair of olefin insulated conductors. Another prior art
construction utilized a
cable containing two twisted pair of FEP insulated conductors, and two twisted
pair of olefin
conductors.
When plenum cables are subjected to increased temperatures, the electrical
characteristics of the cable (e.g., attenuation, structural return loss, and
cross-talk) may drift
by an undesirable amount. Indeed, Category 5 cables must pass elevated
temperature
attenuation requirements at 40 ° C and at 60 ° C; in accordance
with current standards, the
attenuation of Category 5 cables must be less than about 67.0 dB at room
temperature, less
than about 72.3 dB at 40°C, and less than about 77.7 dB at 60°C.
Although a cable utilizing
FEP insulation and a low-smoke PVC jacket may meet these elevated temperature
attenuation requirements, it may not remain electrically stable at much higher
temperatures,
e.g., greater than 100°C.
In accordance with the present invention, outer jackets 11 and 34 enable
cables 5 and
10 to exhibit electrical stability (for purposes of performance tests) from
room temperature
to a temperature exceeding 60°C. In an exemplary embodiment, cables S
and 10 are
electrically stable to at least about 121 °C, which is approximately
the highest temperature
that may be reached within a plenum. For example, although the attenuation of
Category 5
cables must be less than about 94 dB at 121 °C, a prototype cable
constructed in accordance
with the present invention exhibited attenuation less than 70.0 dB at 121
°C. In addition to
the enhanced attenuation performance, cables 5 and 10 also meet or exceed the
electrical
performance requirements associated with structural return loss and cross-talk
from room
temperature to 121 °C.
In all cables intended for high frequency transmission applications, the
velocity of
signal propagation (which should be as high as possible) is extremely
important, as is the
allowable skew. Skew refers to variations among twisted pair in a single cable
of the
velocity of propagation or other characteristics, and should be as small as
possible to
minimize data distortion. Table 2 represents the results of measurements of
characteristics
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of 4 pair FEP, 3 pair FEP + 1 pair flame-retardant olefin, 2 pair FEP + 2 pair
flame-retardant
olefin, and 4 pair foam/skin HDPE in accordance with the present invention. In
Table 2, the
velocity of propagation is expressed in percent of the speed of light, and the
delay is
expressed in nanoseconds over a 100 meter cable run. The skew percent is
determined by
the ratio between the worst twisted pair characteristics and the best twisted
pair
characteristics. The references to BRN, GRN, BLU and ORN, are simply
references to
particular colors of twisted pair in a standard 4 twisted pair color standard.
TABLE 2
Conductor Characteristics
Cable Dielectric Velocity of
Construction Insulation olor Constant Propagation (%1 ela ns
4 pr. FEP
FEP BRN 1.74 75.80 1.35
FEP GRN 1.76 75.40 1.36
FEP BLU 1.81 74.30 1.36
FEP ORN 1.83 73.90 1.39
Average 1.79 74.90 1.37
Skew 5.20% 2.80% 3.00%
3 pr. FEP
1 pr. Olefin
Olefin BRN 1.99 70.90 1.43
FEP GRN 1.84 73.70 1.37
FEP BLU 1.90 72.50 1.39
FEP ORN 1.92 72.20 1.40
Average 1.91 72.30 1.40
Skew 8.20% 3.10% 4.40%
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Cable Dielectric Velocity of
Construction Insulation olor Constant Propagation (%1 Dela ns
2 pr. Olefin
Olefin BRN 2.20 67.40 1.52
FEP GRN 1.79 74.70 1.3 8
FEP BLU 1.79 74.70 1.38
Olefin ORN 2.20 67.40 1.52
Average 2.00 71.05 1.45
Skew 22.90% 10.80% 10.10%
4 pr.
foam/skin
Z O F/S BRN 1.59 79.20 1.30
F/S GRN 1.61 78.80 1.31
F/S BLU 1.64 77.90 1.32
F/S ORN 1.66 77.50 1.33
Average 1.63 78.35 1.32
Skew 4.40% 2.20% 2.30%
As shown by the above table, the dielectric constant, velocity of propagation,
and
delay time for cable constructed with foam/skin insulation in accordance with
the present
invention are all significantly better than FEP-only insulated cable, and
vastly superior to
those for composite FEP/olefin insulated cables. The skew for the cable of
this invention is
also significantly better than for FEP-only insulated cable. Such a cable
construction is
indeed suitable for operation at signal frequencies of 150 MHz or 155
Megabits.
In accordance with the present invention, an improved cable construction is
achieved,
which is a result of a novel combination of electrical and burn properties of
materials.
Specifically, primary insulation of polyolefin, which in a specific example is
foamed, such
326 t 69.6/29144.0800 16
CA 02220368 1997-11-06
as HDPE surrounded by a HDPE skin, is surrounded by a jacket of thermoplastic
halogenated polymer, which in a specific example is a foamed PVDF material.
Although the specific examples discussed herein have, for purposes of
completeness,
included identification of specific suitable materials available from various
manufacturers,
S equivalent materials available now or hereafter can obviously be substituted
with satisfactory
results. It is intended, therefore, in the appended claims, to cover not only
the specific
materials and constructions which have been discussed herein, but also
substitution of
equivalent materials in the overall cable construction. For example, rather
than the HDPE
foamlskin insulation, a polypropylene foam/skin insulation may be utilized to
improve the
crush resistance and the overall physical robustness of the cable. In
addition, the present
invention may employ an HDPE skin/foam/skin triple extruded insulation or a
polypropylene
skin/foam/skin insulation for improved velocity of propagation values.
326169.6/29144.0800 17
