CA1164064A - Compositely insulated conductor cable for use in buildings - Google Patents

Abstract

COMPOSITELY INSULATED CONDUCTOR CABLE
FOR USE IN BUILDINGS

Abstract of the Disclosure A cable which is suitable for use as a riser cable in buildings comprises a plurality of conductors each of which is covered with a composite insulation. The composite insulation comprises an inner layer of an expanded polyethylene material and an outer layer of a polyvinyl chloride material. This construction results in a cable which is not only fire retardant but in one which has an unusually low fuel content. Advantageously, this same cable lends itself to color coding for inside wiring and is capable of having a relatively high pair count density.

Description

D()IJGIIERT~
" 1~ B40~4 CO~PCSITELY I~SULATED CONDUCTOR CABLE
FOR USE IN BUILDINCS

Technical Eield S This invention relates to a compositely insulated conductor cable suitable for use in buildings, and, more particularly, to a riser cable which includes an increased number of conductor pairs within a smaller cross-sectional area than prior cables, which has a decreased fuel content and which is capable of being color-coded.
Background of the Invention Telephone service within buildings is provided by riser cables which generally extend from vaults in basements to the floors above. Typically, these cables include a plurality of twisted pairs of conductors which are individually insulated with a dual layer of insulation which comprises a polyvinyl chloride skin that is extruded over a solid polyethylene inner insulation cover. This structure combines the acceptable fire retardant characteristics of polyvinyl chloride and the superior dielectric constant of polyethylene with the latter allowing the required mutual capacitance of 52 nf/kilometer mutual capacitance to be achieved with reasonable cable diameters. However, this insulation contains approximately 50~ polyethylene by weight which is relatively high in fuel content. The term "fire retardancy" is designed to mean the capability of an insulation or jacketing material to retard the beginning of a fire, whereas the term "fuel content" is intended to mean that quantity of fuel which is released by the materials comprising the insulation and the jacketing after a fire starts.
There has been a desire to improve the fire-retardance and fuel content properties of the conductor insulation in riser cables notwithstanding the current construction in which the above-described insulated conductors are enclosed in a sheath which is designated as ~ ' , ' DOUGtlE~ rY - 1 1 1 ~ 64~4 an ALVY~ sheath. The ALVYN sheath comprises a polyvinyl chloride jacket which is bonded to a corrugated aluminum shield. This jacketing arrangement is substantially fire retardant; however, the fire retardancy of the insulated conductors which are enclosed by the ,acket requires improvement.
Another consideration is the number of conductors in -a qiven cable cross-sectional size within a building riser system. With the trend toward larger and larger buildings and the increased use of the telephone for various methods of communication, a pair count density, which is the number of insulated conductor pairs in a given cable cross-section, must be greater than that achieved in the past.
Clearly then, there is a need for a riser cable which optimizes several requirements. These requirements are that the conductors must be fire retardant and have a relatively low fuel content, that the conductor must have a relatively small diameter-over-dielectric in order to reduce the outside diameter of the cable, and that the outer insulation must lend itself to a color coding scheme in order to facilitate inside wirin~ and splicing.
What is needed is an insulation and jacketing system which minimizes the oppGrtunity for the beginning of a fire along a riser cable, and should such a flame be initiated, one which minimizes the propagation of the flame and the total heat which is released by the cable system.
A number of jacket and insulation systems are well known in the art, but none that are known meet all of the above-mentioned requirements. For example, it is known that polyvinyl chloride is a fire retardant material, and it is well known that polyethylene has an excellent dielectric constant which is helpful to the transmission qualities of the cable. Expanded polyethylene has a lower dielectric constant which leads to the optimization of the 1 ~ 64064 cable size and which is somewhat thermally insulating, that is, it limits fire spread. Pulp insulation has acceptable fire-retardant properties and lends itself well to high pair density cable systems, but it does not lend itself to the color coding scheme which is desired for inside wiring and splicing. With these qualities of the various insulations and jacketing materials in mind, the problem that must be overcome and that is not overcome by the prior art is a cable system which meets all the above-mentioned requirements of being fire retardant, of having optimum size and of being capable of facilitating a color coding scheme with positive pair identification.
Summary of the Invention In accordance with an aspect of the invention there is provided a cable which is particularly suited for use within a building, said cable comprising: a core which comprises: a plurality of individually insulated conductors, each of said conductors being insulated with a composite insulation which comprises: an inner layer of cellular polyolefin plastic material; and an outer layer of relatively fire-retardant plastic material; and a metallic shield which encloses said core; and an outer jacket which encloses said shield and which is made of fire-retardant plastic material.
The foregoing problems which are not solved by the prior art and which must be overcome in order to meet various building codes in today's environment as well as to provide economy in meeting those requirements are met by the cable of this invention. One embodiment of this invention provides a cable which includes a core which includes a plurality of individually insulated conductors with each of the conductors being insulated with a composite insulation. The composite insulation includes an inner cover which comprises an expanded polyolefin plastic material and an outer insulation cover which - 3a -comprises a relatively fire retardant plastic material.
Thle core is enclosed in a corrugated metallic shield and a~ outer jacket which is made of a fire retardant material.
In a preferred embodiment, the composite insulation com-prises an inner layer of expanded polyethylene material and an outer layer of polyvinyl chloride.
The composite insulation which is used to cover the conductors is relatively fire retardant and has a relatively low fuel content. Moreover, the composite insulation provides the capability of having an optimum pair count density within a given cross-sectional cable size as well as provides the capability for color coding the individually insulated conductors.

DOUGIIER'I`Y- I 1 ~ 1 6~06~

Brief Description of the Drawingts Other features of the present invention will be ~ore readily understood from the following detailed description of specific embodiments thereof when read in conjunction with the accompanying drawings, in which:
FIG. 1 is an elevational view partially in section of a building showing a cable distribution network to various floors of the building;
FIG. 2 is a perspective view of the cable of this invention which includes a plurality of individually insulated conductors having a low fuel content and being fire retardant;
FIGS. 3A and 3B are cross-sectional views of a conductor of this invention showing a composite insulation cover for the conductor and a prior art conductor which has been used for riser cable;
FIG. 4 is a graph of diameter-over-dielectric versus gauge size for a conductor of this invention and for prior art conductors;
FIG. 5 is a graph of insulation fuel content for a conductor of this invention and for prior art conductors;
and FIG. 6 is a graph showing limiting oxygen index for conductors made in accordance with this invention and for a priorly used riser cable conductor.
Detailed Description Referring now to FIG. 1, there is shown a cross-sectional view of a building 20 which includes a cable vault 21 in a basement portion 22 thereof. An exchange cable 23 is routed into the cable vault 21 with individual riser cables 25-25 of this invention being ro~ted vertically upwardly. At each of selected floors 26-25, distribution cables 27-27 which are also made in accordance with this invention are fed off the riser cables 25-25 in order to provide telephone service. A typical lnstallation may include a 2700 conductor pair riser cable with 300 pair distribution cables being fed therefrom.

DOUGHER'I`Y- 1 1 1 1 6406-~

Referring now to FIG. 2, there is shown a cable 25 of this invention which includes a core 3n having a plurality of individually insulated conductors 31-31 that are enclosed in a core wrap 32, and a corrugated aluminum shield 33 having copolymer material coated on outwardly facing surface thereof. The copolymer material on the corrugated aluminum shield causes the shield 33 to be bonded to a jacket 36 which is made of a polyvinyl chloride ~PVC) material that is substantially fire retardant.
Each of the individually insulated conductors 31-31 in the cable 25 shown in FIG. 2 includes a composite insulation designated generally by the numeral 40 (see FIG. 3A). The composite insulation 40 replaces an insulation 51 which is shown in FIG. 3B and which includes an ir.ner layer of solid polyethylene covering a copper conductor 42 and an outer layer of polyvinyl chloride.
The composite insulation 40 of this invention includes an inner layer 41 of expanded polyethylene which surrounds the copper conductor 42 in order to provide a concentrically disposed insulation cover. The composite insulation 40 also includes an outer layer or skin 43 which is comprised of a plasticized polyvinyl chloride material.
In a preferred embodiment, the plasticized polyvinyl chloride may be one which includes for example at least about 72 parts by weight of a polyvinyl chloride with the other 28 parts by weight including ingredients such as for example a plasticizer and other materials which strengthen and add particular properties to the insulation. Such a polyvinyl chloride insulation is well known.
Advantageously, the plastic skin 43 lends itself to color coding to facilitate inside wiring and splicing. As will be recalled, the lack of this capability is one of the drawbacks of pulp insulation.
Turning now to the graphs shown in FIGS. 4-~, it will become obvious that the composite insulation 40 of this invention offers numerous advantages. FIGS. 4-h include plots of the insulation 51 which comprises solid DOU(',llF.R'r~
1 1 640~

~olyvinyl chloride over solid polyethylene and which is the currently used insulation, an insulation 52 comprising a solid 0.005 cm skin of solid polyethylene over expanded polyethylene, a polyethylene insulation 53, the composite 5 insulation 40 of this invention, which includes, for example, a polyvinyl chloride skin over an expanded polyethylene inner layer, and finally an insulation 54 of a cable designated DUCTPIC* cable. In DUCTPIC cable, the term DUCTPIC being a trademark of the Western Electric 10 Company Incorporated, the diameter-over-dielectric (DOD) in a given cable cross-section is optimizecl with respect to transmission characteristics. The last mentionec?
insulation syste~ 54 is described in U. S. patent 4,05~,69 which issued on November 15, 1977 in the names of 15 W. G. Nutt and G. H. Webster.
Referring to FIG. 4, there is shown a graph of diameter-over-dielectric ~DOD) in centimeters versus the gauge size of the conductor 42 for each of the five above-identified insulation types. The DOD of an insulated 20 conductor is determinative of cable diameter and the number of pairs is directly proportional to the insulation area.
[Jsing the insulation 51 as a reference, the composite insulation 40 of this invention has a diameter-over-dielectric of about 0.103 cm for 22 gauge which is about 25 80% and a cross-sectional area which is about 64% of that of the insulation 51 having a DOD of about 0.130 cm. Only the insulation 54 of DUCTPIC cable surpasses the size reduction with a DOD of 7~% and an area of 55% of that of the reference insulation. Because the dielectric constant 30 of the com~osite insulation 40 of this invention is slightly larger than that of the DUCTPIC cable, all polyethylene insulation 54, the diameter-over-dielectric of this insulation is slightly larger than that of the DUCTPIC
cable conductor. However, while the dielectric 35 constant, o is somewhat larger than that of DUCTPIC cable insulation because of the presence of the PVC skin, the cable of this invention has better than acceptable 1 ~ 6~06~

transmission characteristics.
The decrease in pair count density over that afforded by DUCTPIC cable is not that significant when the other advantages of the cable 25 of this invention are considered. Not only is the pair density of a cable 25 of this invention higher than any except that of DUCTPIC
cable, it optimizes the fuel content and limiting oxygen index (LOI) while at the same time providing the capability of being color coded for positive pair identification. The LOI is a parameter which is used to rank order various materials as to their flame retardance properties.
Fuel content and limiting oxygen index are properties which are important with respect to flame spread and smoke evolution. During a fire, the intumescent process of carbonacious charring of insulation along its outwardly facing surface acts to inhibit further degradation of the insulation by blocking internal convective air movements, and hence prevent the longitudinal travel of heated air which decomposes the insulation and causes smoke evolution. This is accomplished by the charred insulation effectively blocking off a section of the length of cable 25 to localize further decomposition to the portion of the cable adjacent to the flame.
If the jacket coat retained its integrity, it could function to insulate the substrate, but in a simple jacketed cable, it is ruptured by the expanding insulation char, exposing the virgin interior of the jacket and insulation to elevated temperatures. The jacket as well as the restricted insulation char begin to pyrolize and emit flammable gases. These gases ignite and by convection burn beyond the area of flame impingement, propagating flane and evolving smoke.
Tests have shown that heat is principally transferred into the cable core 3~ by thermal radiation, secondly by conduction and finally by convection. The amount of heat transferred by conduction depends upon the DOU('.IIF.RTY- 1 1 ~ 16406~

thermal conductivity of the materials through which the heat is passing and the area and the thickness of the conducting path. The most important physical properties of a material so far as conduction of heat are concerned are thermal conductivity, density and specific heat.
Advanta~eously, the metallic barrier 33 functions not only to conduct heat away from the point of conflagration, but also functions to reflect heat.
Different materials give off different amounts of heat when subjected to flame and this is measured by fuel content. Polyethylene, for example, has a higher fuel content than polyvinyl chloride and therefore adds more fuel to a fire than the PVC. Since cellular polyethylene such as that which compriSes the layer 41 includes air cells, there is less material to fuel a fire. Not only is the cellular polyethylene less dense than its solid polyethylene predecessor, but the DOD is less than that of the PVC over polyethylene and this decreases the amount of material which could fuel the fire.
Turning now to FIG. 5, there is shown a plot of the insulation fuel content in kilo calories per 30 centimeters versus the gauge size of the conductor 42.
The plots 51-54 in the graph shown in FIG. 5 are those of conductors having the same insulation arrange~ent as those de~icted in FIG. 4. As can be seen in FIG. 5, the insulation fuel content for the insulation 40 of this invention compares favorably with that of the insulation 54 of the so-called DUCTPIC cable and notwithstanding the use of the ALVYN sheath is lo~er by far than the fuel content of the insulation 51 which contains approximately 50 polyethylene by weight.
Turning now to FIG. 6, there is shown a graph ot the limiting oxygen index (LOI) versus the content of the precent of polyvinyl chloride in the insulation. At the left hand side of the graph in FIG. ~, the percent PVC in the insulation is zero which would correspond to a standard PIC, polyethylene, insulation, for example. FIG. 6 chow5 DOlJ(`.l`~ERl`~
1 ~ ~406~

plots for the various gauges such as 22, 24 and 26 gauge size in a plot representing polyethylene surrounded by polyvinyl chloride and with a plot 40 representing expanded polyethylene covered by polyvinyl chloride.
It will be recalled that the higher the oxygen index the less susceptible is the insulation material to burning. For example, since there is 20~ oxygen in the atmosphere, an insulation material having a limiting oxygen index of 22~, cannot burn under ambient conditions.
However, this should not be ta~en to be an absolute situation since air could be drawn in to the vicinity of the cable which could help to fuel a fire.
FIG. 6 taken together with FIGS. 4 and 5 would seem to indicate that the limiting oxygen index of the insulation of this invention is optimized with respect to the fuel content as well as the size of composite insulation which provides a maximum number of pairs within a given cable cross-section. Of course, it should be realized that the DUCTPIC cable would optimize the number

2~ of pairs still further, but for other reasons it would not be suitable for riser cable. For example, since DUCTPIC
cable includes conductors which are polyethylene insulated, the limiting oxygen index is less than 22 which is not preferred for use in buildings.
Viewing again FIG. 6, it can be seen that the insulation 40 of this invention has an LOI which is somewhat less than that of the insulation 51 because it has less mass and is less dense. Of course, the interpretation of the limiting oxygen index (LOI) is subject to the particular construction of an insulated conductor and to the method of testing in which the burning is caused to precede downwardly. As should be apparent, the method of test used to arrive at the LOI does not precisely conincide with the method of burning which might be anticipated for a riser cable 25 of this invention.
Also, the LOI is not in and of itself determinative of the fire resistance capability of the ~OUGI~ERTY-ll ~ ~ ~4~S.l - lC -cable 25 of this invention since it can vary depending on the thickness of the sample under test. The less the volume of the material of the specimen under test, the less the heat input which is required to initiate a fire. Since the conductors 31-31 of this invention have less volume, the heat input required is less than that for the insulation 51.
While the compositely insulated conductor 31 of this invention exhibits a slightly reduced LOI over that of the priorly used insulation, the inner layer ~1 of expanded polyethylene inhibits the preheating of the copper conductor 42. If uninhibited, this preheating would assist the propagation of the flame along the insulated conductor 31. Since air acts as an insulator and since it can only be heated by conduction, the cellular insulation 41 effectively decreases the heat transfer by the copper conductors 42-42 which otherwise would contribute to the propagation of the fire within a building. This property of the composite insulation ~0 of this invention is important because in building fires, the preheating of combustibles is one of the principal modes by which flames spread.
Also, the lower LOI of the insulation 40 of this invention may be deceptively low. This may be so because in a solid insulation test, the solid insulation tends to conduct heat away from the flame thereby requiring more oxygen to support the fire. Since the layer 41 of cellular insulation would be less apt to do this, its LOI should be expected to be slightly lower. However, this mechanism works against the solid insulation 51 in the riser space since it may tend to fuel sections of the cable above the point of the flame moreso than the composite, partially cellular insulation of this invention.
In a preferred embodiment of this invention for 22 gauge copper conductors, the diameter-over-dielectric of the total insulation is about 0.103 cm and the thickness of the outer layer of polyvinyl chloride is about 0.C05 cm.

DOU~.IIE.RTY-ll 1 ~ 64~)B`~

The expansion of the polyethylene which comprises the inner layer 44 of the composite insulation is about 45% with the percent polyvinyl chloride in the composite insulation being about 53%. In each one hundred kilometers, there are approximately 18 kilograms of poleythylene with each 7.62 cm dia~eter cross-section of cable being capable of including 12C0 pairs of conductors.
~ `or a 26-gauge size cable, the diameter-over-dielectric of the total insulation is about 0.069 cm while the skin thickness is the same as for the 22 gauge cable described above. The percent expansion is about 40%, the percent PVC in the insulation increases to about 63% and the kilograms of polyethylene per one hundred kilometers decreases to about 8. The 7.62 cm diameter cable cross-section is capable of including 3000 conductor pairs. Itshould be apparent that the riser cable of this invention optimizes all three requirements in a particularly advantageous ~ashion.
It is to be understood that the above-described arrangements are simply illustrative of the invention.
Other arrangements may be devised by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.

Claims (10)

Claims

1. A cable which is particularly suited for use within a building, said cable comprising:
a core which comprises:
a plurality of individually insulated conductors, each of said conductors being insulated with a composite insulation which comprises:
an inner layer of cellular polyolefin plastic material; and an outer layer of relatively fire-retardant plastic material; and a metallic shield which encloses said core;
and an outer jacket which encloses said shield and which is made of fire-retardant plastic material.

2. The cable of claim 1, wherein said inner layer of plastic material of said composite insulation is an expanded polyethylene material.

3. The cable of claim 2, wherein said polyethylene material is expanded to have a percent expansion in the range of about 35 to 50%.

4. The cable of claim 3, wherein said polyethylene material is expanded to have a percent expansion of about 45%.

5. The cable of claim 1, wherein said outer layer of fire-retardant plastic material of said composite insulation is a polyvinyl chloride material.

6. The cable of claim 1, wherein said outer jacket which encloses said metallic shield comprises a polyvinyl chloride material.

7. The cable of claim 1, wherein the limiting oxygen index of said composite insulation is in the range of about 23 to about 25%.

8. The cable of claim 1, wherein said inner layer of said composite insulation has a thickness of about 0.015 cm.

9. The cable of claim 8, wherein said outer layer of said composite insulation has a thickness of about 0.005 cm.

10. The cable of claim 1, wherein said composite insulation has a fuel content in the range of about 0.5 to 1 K Cal for each 30 cm of each said conductor.