CA1155940A - Electrical conductor for telecommunications cable - Google Patents
Electrical conductor for telecommunications cableInfo
- Publication number
- CA1155940A CA1155940A CA000375110A CA375110A CA1155940A CA 1155940 A CA1155940 A CA 1155940A CA 000375110 A CA000375110 A CA 000375110A CA 375110 A CA375110 A CA 375110A CA 1155940 A CA1155940 A CA 1155940A
- Authority
- CA
- Canada
- Prior art keywords
- conductor
- insulated
- cellular
- insulation
- conductors
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B7/00—Insulated conductors or cables characterised by their form
- H01B7/02—Disposition of insulation
- H01B7/0233—Cables with a predominant gas dielectric
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B11/00—Communication cables or conductors
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- Communication Cables (AREA)
Abstract
ELECTRICAL CONDUCTOR FOR TELECOMMUNICATIONS CABLE
Abstract of the Disclosure Insulated electrical conductor for telecommunications cable in which two layers of insulation are provided. The inner of the two layers is a solid non-cellular construction and the outer layer is cellular. The nominal mutual capacitance between the conductor and an identical conductor is at a desired value with a dielectric breakdown value between conductors above a desired minimum value while having an outside diameter across the insulation which is less than for a conductor of the same gauge which provides the same mutual capacitance and has a solid insulation of the same material as the inner layer.
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Abstract of the Disclosure Insulated electrical conductor for telecommunications cable in which two layers of insulation are provided. The inner of the two layers is a solid non-cellular construction and the outer layer is cellular. The nominal mutual capacitance between the conductor and an identical conductor is at a desired value with a dielectric breakdown value between conductors above a desired minimum value while having an outside diameter across the insulation which is less than for a conductor of the same gauge which provides the same mutual capacitance and has a solid insulation of the same material as the inner layer.
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Description
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This invention relates to an insulated electrical conductor for telecommunications cable.
; Telecommunications cables conventionally comprise d plurality of individually insulated conductors, usually twisted together in pairs, the conductors forming a core encased in a cable sheath. In "air core" polyolefin insulated cables, i.e. those not filled, the usual practice in some countries is to use an insulation consisting of solid non-cellular polymeric material.
Interstices exist between the insulated conduc~ors. If perforations are present or are otherwise formed in the sheath e.g. due to lightning or ~echanical damage, it is possible in certain applications for moisture entering into the cable to reach these ; interstices and to Fill them for long distances along the cable by migration. The presence of this moisture degrades the electrical performance of the cable and may even cause short circuits between conductors when pinholes or other defects are present in the individual insulation of the conductors. The moisture acts as an electrolyte ~o lead to corrosion of exposed metal surfaces directly or by facilitating galvanic action.
In view of all these problems, for instance for buried cable, the interstices between conductors in cable cores have been filled with a water repellent and water impermeable medium such as grease or petrola~:um iel 1YD
Known filling materials all have a permittivity greater than 1 which is the permittivity of air. Hence, displacement of the air from between the insulated conductors by these filling materials affects the electrical characteristics and thus telecommunication .. ~ . . . . - .
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characteristics compared to air-core cable. For instance, where grease is used as filling material, these changes are in some respects deleterious in that the filling materials increase the capacitance between adjacent conductors, but it is also found that the grease advantageously increases the dielectric strPngth of the insulation.
Qriginally9 the problem of increase in capacitance with grease filled cable was overcome by an increase in the thickness of the individual solid insulation on the conductors, but this resulted in an increase in the amount of insulation material required over that for air-core cable and hence an increase in cable diameter which, is undesirable for cost and installation reasons.
The above further problem o~ increase in the amount of insulation material and cable diameter has been overcome by an invention described in Canadian Patent No. 952,991. In this patent, there is described a communication cable having a filled core of a plurality of insulated conductors, the insulation on each conductor comprising an inner layer of cellular polymeric material and a relatively thin outer layer of solid polymeric material. The cellular polymeric material has the advantage that it has a lower permittivity than solid non-cellular materials and is adjacent to the conductor to retain the capacitance down to commerc;ally acceptable levels. This also results in a saving in materials in replacing solid material with cellular material and the overall diameter of each insulated conductor is reduced9 thereby advantageously reducing the outside cable diameter for filled cable. In an example given in the copending applications, the inner layer of cellular insulation, on 22AWG aluminum conductor, has a thickness of 9 mils with 40% of its volume being air, and the . ,;, .
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outer solid layer has a thickness of 2.5 mils, the overall diameter of the insulated conductor being approximately 48 mils. The dielectric strength between conductors is held at acceptable levels mainly by the combined dielectric properties of the outer solid layer and the surrounding filling material in the core.
Unfortunately, in this described construction, the dielectric strength between conductors would be lower and possibly may not be acceptable if this cable was air-core cable. In addition, it should be real k ed that these results would be obtained with an outside diameter of 48 mils for 22 AWG which is greater than a conventionally insulated conductor of less than 45 mils and which provides cornmercially acceptable levels of nominal capacitance and dielectric strength. However, it is extremely important that cable diameters should be as small as possible as the spaces for accepting cable are very restricted. Of course, cable diameter is governed by outside diameter of insulated conductor.
The present invention is concerned with the provision of an electrically insulated conductor for telecommunications cable which is useful for air-core and filled cable particularly when filled with particulate material and when included in air-core cable exhibits d~electric strength and capacitance properties which are within commercially acceptable levels while having a smaller outside diameter ~han comparable insulated conduc~ors for air-core cable and which have a conventional solid insula~ion.
Accordingly, the present invention provides an insulated electrical conductor for telecommunications cable comprising a telecornmunications conductor and an insulation comprising an inner ."',d~
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layer and an outer layer of electrically insulating material, the inner layer of solid non~cellular construction and the outer layer of cellular polymeric material, and wherein the nomiral mutual capacitance between the conductor and an identically insulated conductor in a pair is at a desired value and a dielectric breakdown value above an acceptable minimum is obtained between the conductor and an identically insulated conduc~or while having an outside diameter across the insulation which is less than an insulated electrical conductor of the same gauge which provides the same mutual capacitance and which is insulated with ~olid non-cellular material only, that material being the same material as said inner layer. The inner layer may be pigmented but there should normally be no reason for this. In any case, avoidance of pigmentation provides the better dielectric strength.
The desired mutual capacitance is dependent upon requirements laid down by any particular authority. For instance, in some cases, a nominal mutual capacitance value of 83 nanofarads/mile is the requirement. This of course may vary between acceptable ~anufacturing limits, say between 79 and 87 nanofarads/mile.
From the above defined invention, it is clear that w;th ~he materials arranged in the layers as specified, the desired values of nominal mutual capacitance and of dielectric strength are achievable with an outside diameter across the insulation which is less than with a conductor having a single layer of insulation.
It is found tha~ desirable values are acnievable with suitable combinations of two parameters, i.e. the thickness of the inner layer and the percentage blow of the foam. For instance, where ., ~I ~ '.
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the thickness of ~he inner layer is 54creased, this has an undesirable effect on the capacitance and, to counteract this, a higher percentage of air space needs to be provided in the ou~er cellular layer. The situation is, however, that the inner layer is located at the position of greatest field intensity and is sufticient to provide the required dielectric properties in an air-core cable whil still being sufficiently thin to enable the cellular outer layer to be disposed as close to the conductor as possible and provide the required capacitance value.
The invention also includes a telecommunlcations cable having an air-core or a core filled with particulate material in which a plurality of insulated electrical conductors are provided, each of which comprises a conductor having insulation comprising an inner layer of solid non-cellular material and an outer layer of cellular polymeric material and wherein the nominal mutual capacitance between conductors is at a desired value and a dielectric breakdown value above an acceptable minimum is obtained between conductors wi~h each conductor having an outside diameter across the insulation which is less than an electrical insulated conductor of the same conductor gauge which ~`
provides the same mutual capacitance and breakdown values and which is insulated with solid material only, the material being the same material of said inner layer.
In preferred constructions, the air space volume in the ; total volume of the~cellular layer in the construction according to the invention is at least 20% whereby signi~icant savings in materials may ; be obtained over materials required for conventionally insulated conductors in air-core cable.
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One embodiment of the invention will now be described, by way of example, wikh reference to the accornpanying drawings in which:-Figure 1 is a cross-sectional view through a telecommuni-cations cable; and Figure 2 is a cross-sectional view of an insulated conductor incorporated in the cable.
In the embodiment now to be described~ specific dimensions of conductor and insulation layers will not be referred to.
Dimensions will be discussed at the end o~ the description for different gauges of conductor to enable comparisons to be made between the dielectric strength and capacity values of constructions of the embodiment and other insulated conductors not within the scope of this ~, invention.
In the embodimen~, a telecommunications cable 10 comprises a core having a plurality of pairs of insulated conduc~ors 11. The core is wrapped in a composite wrap comprising an inner layer 12 of plas~ic tape, e.g. 3 mils thick, such as "Mylar" tape. The inner layer may comprise other materials such as paper or polyethylene or combinations of these materials. Around this is another layer 13 o~
aluminum tape, e.g. 8 mils thick which has been coated on both sides with polyethylene, followed by a medium density block polyethylene outer layer 14 of about 80 mils thickness.
The core, commonly referred to as an air-core, has each insulated conductor 11 of each pair constructed in the manner shown in ; Figure 2. Each insulated conductor comprises a conductor 15 covered by an inner layer 16 of solid non-cellular insulating material which in line with this invention has a maximum thickness of 4 mils. This may `
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be made from any suitable electrically insulating plastics material such as polypropylene or medium density polyethylene. An outer layer 17 enclosing the inner layer is cellular polypropylene which is preferably closed cell but may be of open cell struc~ure.
Alternatively, the inner layer and outer layer are both formed from high density polyethylene with the outer layer, of course, being cellular.
The insulated conductor is manufactured by passing conductor through a two stage extruder (not shown), the first stage providing the inner non-cellular insulating layer 13 and the second stage extruding the cellular layer. The cellular layer is formed by `
normal foam extrusion techniques.
It is found that while the cells expand directly after extrusion, expansion of the outer layer is outwardly from the inner layer and has no effec~ upon the inner layer which has just been extruded. Thus the inner layer is not stressed by its contact with the expanding outer layer and there is no likelihood of pinholes being formed in the lnner layer because of stress build-up.
The thickness of each of the layers 13, 14 is predetermined primarily to give a desired nominal mutual capacitance value of 83 nanofarads/mile in the completed cable. Also to give the required dielectric properties, the inner layer is located at the position of greatest ~ield intensity and its thickness is calculated to give satisfactory dielectric strength and thus to enable the outer cellular layer to lie as close as possible to the conductor so as not ; to detract from the required mutual capacitance.
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~ ~5~9~10 Further9 the material of the outer layer may be pigmented without detracting from the mutual capacitance properties unduly.
While it is known that pigmentation may deleteriously affect the dielec~ric s~rength properties of an insulating layer, the inner layer is not pigmented and thus its dielectric strength is not so affected.
In the following, measurements were taken of the dielectric strengths of insulated conductor according to the above described embodiment for 22 A~G conductor. These appear in "Category Ali of the following Table I~
For comparison, the test also includes measurements of dielectric strengths of insulated conductors made for grease filled cable in which the insulatiun has an inner cellular layer of polypropylene and an outer non-cellular layer of medium density polyethylene and as described in ~he above Canadian Paten~ No.
952,991.
In addition, and also for comparison, the test also includes measurements of dielectric strenciths of insulated conductors in which the insulation is conventional and is non-cellular low density polyethylene throughout. These measurements appear as "Category C".
;~ 20 The test was conducted while submerging the insulated conductors concerned under water. This was done to simulate the worst possible conditions which insulated conductors in an air core cable could experience, i.e. conditions in which the core is completely waterlogged. It shou1d be stressed that these conditions should nnt normally be expected for air-core cable but are ones which could lead to premature dielectric breakdown.
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A 1000 foot length of insulated conductor in Category 'A' and insulated on one production run ("1" in Table I) was tested in 30 foot sample lengths. Each sample length was immersed in water and a DC
; potential passed through it. The voltage was increased at a substan~ially uniform rate un~il dielec~ric breakdown occurred. The maximum and minimum dielectric breakdown values (Kv), recorded for all of the 30 foot sample lengths, are recorded in Table I together with the average breakdown figure. The above test procedure was then repeated for another 1000 foot leng$h of conductor in Category 'A' which had been insulated on a different production run ("2" in Table I) and the results similarly recorded.
The test precedure was then performed for 30 foot sample leng~hs of two twisted toge~her insulated conductors, in water in which conductor "1" was twisted with conductor "2". Results are given under column 3.
The whole of the above proceclure was then repeated for two 1000 foot lengths of insulated conductor made under Category 'B' and dielectric breakdown values given under columns 4, 5 and 6.
Under Category 'C', tests were made and breakdawn values given under columns 7 and 8. No test was performed under Category 'C' for the insulated conductors twisted together.
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TABLE I
CATEGORY CATEGORY CATEGORY
A B . _ C .
1 2 3 4 5 ~ 7 8 D.C. Voltage Average15.215.1 29.4 10.5 15.3 17.5 36 48 Dielectric Minimum11.511.0 22.0 8.5 4.0 8.0 12 27 Breakdown Maximum17.016.5 32.5 13.0 22.0 19.0 46 60 (Kv) ~ .
Outslde : Diameter of Insulation 43.3 42.7 - 48.0 48.0 -- 45.5 44.8 (Inils) ~ ' Thickness o~
~ Cellular 6.7 6.4 -- 8.7 8.3 : 10 Layer (mils) Thickness o~
: Non-Cellular 2.3 2.3 -- 2.6 3.0 -- 10.1 9.7 Layer (mils) . ~. j i . . .... .
;~ % Blow . ~ . 26_ ~7 -~ 35 - ,-- ~ --.~ :It should be made clear at this stage that the insulated conductors in Category 'B' were designed for grease filled cable. The desired mutual capacitance of 83 nanofarads/mile would not be achieved between conductors of this construction for air-core cable. However, in contrast, conductors in both of Gategories 'A' and 'C' have a nominal mutual capaci~ance of 83 nanofarads/mile for air-core cable.
As may been seen from the above Table I, the dielectric : 20 breakdown values for conventionally insulated conductor (Category 'C') ~:
` were consistently very high with very high average breakdown values of .~
36Kv and 48Kv. While the breakdown values for insulated conductors:
according to the embodiment described above were much lower $han those of Category C, these values for the embodiment are extremely satisfactory (Category A) and are significantly above one requirement for commercially acceptable air-core cable. This requirement is for , .
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a length of insulated conductor to withstand a voltage of 8Kv DC
between conductors for a period of 1 to 3 seconds without dielectric breakdown. Column 3 shows these breakdown values between conductors for Category 'A'. The minimum is 22Kv DC which is well above the reqirement of 8Kv DC by at least some authorities. Column 3 results are interesting in that they indicate values approximately twice those `~ obtained for the single wires in columns 1 and 2. This doubling in values between conductors illustrate not only that current needed to pass through two layers of insulation on both conductors (as distinct from two layers on one conductor in columns 1 and 2), but also that the inner insulation layers of solid material were adding their dielectric strength characteristics without these being degraded by flaws in the layers. This illustrates that there were no physical stresses causing flaws in the inner layers and no impurities, e.g. colour pigments in the layers, both of which would tend to deleteriously affect the results obtained. As a means of comparison with Category 'B', it may be seen that the dielectric breakdown values in column 6 are certainly not of the order of double those obtained for single conductors in columns 3 and 4. In fact, they are not significantly different from columns 3 and 4. It is believed, that the lack of the doubling value effect in column 6 can be blamed upon physical stresses imposed by the inner cellular layers, during extrusion upon the outer solid layers oF
Category 'B' construction, whereby flaws and pinholes are formed, and ; upon the use of colour pigmentation in these outer layers.
Hence, the dielectric strength between conductors for the Category 'A' construction is significantly higher than for the Category ~e~ construction. It should be remembered that Category 'B' insulated .
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conductor was made for grease filled cable and would have a dielectric strength suitable for this purpose. However, if insulated conductor under Category 'B' were designed for air-core cable while providing the desired nominal 83 nanofarads/mile mutual capacitance and having a diameter less than that of Category 'C', then this would lead to a dielectric strength below that es~ablished by the conductors in Category 'A'.
The resul~s obtained for the construction of the invention were, as already stressed, well above the acceptable levels specified, and because of the use of an outer layer of cellular material with a blow of 35% or less (i.e. 35% of air space in the total volume of the outer layer), there was a significant saving in material compared to the construction of Category 'C', with attendant cost ; saving. In addition) these commercially acceptable results were obtained with outside diameters of insulation in the Category 'A' construction which were at least 1.5 mils less than the outside d~ameters of the Category 'C' construction. Hence, it follows that a resultant air-core cable made with insulated conductors according to the invention will have an outside diameter less than one made using conductors of~ conventlonal Category 'C' while being more economic and providing well above the commercially acceptable levels of dielectric breakdown between conductors.
The recorded values in Table I indicate that constructions according to the invention are a desirable replacement for constructions using a single layer of solid material. C7early~ in most practical constructions, the inner layer should have a maximum thickness of 4 mils to enable the cellular layer to lie as close as .
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~1~5~0 possible to the conductor to obtain the required capacitance level.
In constructions according to $he invention, the amount of air space in the total volume of the outer layer is a parameter in deriding the capacitance whereas the amount of polymeric material is a parameter for the dielectric strength. Whîle the air space may be as much as 50% or more of this volume, to obtain a desirable balance between desired capacitance and desired dielectric strength while ~ enabling a reduction in outside dia~eter of the insulation below that ; for insulated conductor in Category 'C', the air space may n~ed to be at a maximum of 40% and a minimum of 10% for the use of an inner non-cellular layer of maximum thickness of 4 mils.
In addition, conductors according to this invention and as described in the embodiment, may be used for cores filled with particulate material, as acceptable dielectric strengths are obtainableO
The invention is applicable to all conductor gauges which are useful for telecommunications cable and, for all these gauges, that is 19, 22, 24, 26, and 28 at least, accept:able dielectric strengths are obtainable with maximum thicknesses of 4 mils for the non-cellular inner layer. The following Table II compares the constructions of Categories A, B and C in conductor gauges 19~ 22 and 24. Table II
shows the savings obtained in insulation material in both Categories 'A' and 'B' over Category 'C'. While the savings in ~his table for Categories 'A' and IB' are comparable, it should be remembered From the above discussion that the insulated conductors of the invention (Category 'A') provide dielectric strengths for air-core cable which are more acceptable than conductors acording to Category 'B'.
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This invention relates to an insulated electrical conductor for telecommunications cable.
; Telecommunications cables conventionally comprise d plurality of individually insulated conductors, usually twisted together in pairs, the conductors forming a core encased in a cable sheath. In "air core" polyolefin insulated cables, i.e. those not filled, the usual practice in some countries is to use an insulation consisting of solid non-cellular polymeric material.
Interstices exist between the insulated conduc~ors. If perforations are present or are otherwise formed in the sheath e.g. due to lightning or ~echanical damage, it is possible in certain applications for moisture entering into the cable to reach these ; interstices and to Fill them for long distances along the cable by migration. The presence of this moisture degrades the electrical performance of the cable and may even cause short circuits between conductors when pinholes or other defects are present in the individual insulation of the conductors. The moisture acts as an electrolyte ~o lead to corrosion of exposed metal surfaces directly or by facilitating galvanic action.
In view of all these problems, for instance for buried cable, the interstices between conductors in cable cores have been filled with a water repellent and water impermeable medium such as grease or petrola~:um iel 1YD
Known filling materials all have a permittivity greater than 1 which is the permittivity of air. Hence, displacement of the air from between the insulated conductors by these filling materials affects the electrical characteristics and thus telecommunication .. ~ . . . . - .
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characteristics compared to air-core cable. For instance, where grease is used as filling material, these changes are in some respects deleterious in that the filling materials increase the capacitance between adjacent conductors, but it is also found that the grease advantageously increases the dielectric strPngth of the insulation.
Qriginally9 the problem of increase in capacitance with grease filled cable was overcome by an increase in the thickness of the individual solid insulation on the conductors, but this resulted in an increase in the amount of insulation material required over that for air-core cable and hence an increase in cable diameter which, is undesirable for cost and installation reasons.
The above further problem o~ increase in the amount of insulation material and cable diameter has been overcome by an invention described in Canadian Patent No. 952,991. In this patent, there is described a communication cable having a filled core of a plurality of insulated conductors, the insulation on each conductor comprising an inner layer of cellular polymeric material and a relatively thin outer layer of solid polymeric material. The cellular polymeric material has the advantage that it has a lower permittivity than solid non-cellular materials and is adjacent to the conductor to retain the capacitance down to commerc;ally acceptable levels. This also results in a saving in materials in replacing solid material with cellular material and the overall diameter of each insulated conductor is reduced9 thereby advantageously reducing the outside cable diameter for filled cable. In an example given in the copending applications, the inner layer of cellular insulation, on 22AWG aluminum conductor, has a thickness of 9 mils with 40% of its volume being air, and the . ,;, .
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outer solid layer has a thickness of 2.5 mils, the overall diameter of the insulated conductor being approximately 48 mils. The dielectric strength between conductors is held at acceptable levels mainly by the combined dielectric properties of the outer solid layer and the surrounding filling material in the core.
Unfortunately, in this described construction, the dielectric strength between conductors would be lower and possibly may not be acceptable if this cable was air-core cable. In addition, it should be real k ed that these results would be obtained with an outside diameter of 48 mils for 22 AWG which is greater than a conventionally insulated conductor of less than 45 mils and which provides cornmercially acceptable levels of nominal capacitance and dielectric strength. However, it is extremely important that cable diameters should be as small as possible as the spaces for accepting cable are very restricted. Of course, cable diameter is governed by outside diameter of insulated conductor.
The present invention is concerned with the provision of an electrically insulated conductor for telecommunications cable which is useful for air-core and filled cable particularly when filled with particulate material and when included in air-core cable exhibits d~electric strength and capacitance properties which are within commercially acceptable levels while having a smaller outside diameter ~han comparable insulated conduc~ors for air-core cable and which have a conventional solid insula~ion.
Accordingly, the present invention provides an insulated electrical conductor for telecommunications cable comprising a telecornmunications conductor and an insulation comprising an inner ."',d~
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layer and an outer layer of electrically insulating material, the inner layer of solid non~cellular construction and the outer layer of cellular polymeric material, and wherein the nomiral mutual capacitance between the conductor and an identically insulated conductor in a pair is at a desired value and a dielectric breakdown value above an acceptable minimum is obtained between the conductor and an identically insulated conduc~or while having an outside diameter across the insulation which is less than an insulated electrical conductor of the same gauge which provides the same mutual capacitance and which is insulated with ~olid non-cellular material only, that material being the same material as said inner layer. The inner layer may be pigmented but there should normally be no reason for this. In any case, avoidance of pigmentation provides the better dielectric strength.
The desired mutual capacitance is dependent upon requirements laid down by any particular authority. For instance, in some cases, a nominal mutual capacitance value of 83 nanofarads/mile is the requirement. This of course may vary between acceptable ~anufacturing limits, say between 79 and 87 nanofarads/mile.
From the above defined invention, it is clear that w;th ~he materials arranged in the layers as specified, the desired values of nominal mutual capacitance and of dielectric strength are achievable with an outside diameter across the insulation which is less than with a conductor having a single layer of insulation.
It is found tha~ desirable values are acnievable with suitable combinations of two parameters, i.e. the thickness of the inner layer and the percentage blow of the foam. For instance, where ., ~I ~ '.
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the thickness of ~he inner layer is 54creased, this has an undesirable effect on the capacitance and, to counteract this, a higher percentage of air space needs to be provided in the ou~er cellular layer. The situation is, however, that the inner layer is located at the position of greatest field intensity and is sufticient to provide the required dielectric properties in an air-core cable whil still being sufficiently thin to enable the cellular outer layer to be disposed as close to the conductor as possible and provide the required capacitance value.
The invention also includes a telecommunlcations cable having an air-core or a core filled with particulate material in which a plurality of insulated electrical conductors are provided, each of which comprises a conductor having insulation comprising an inner layer of solid non-cellular material and an outer layer of cellular polymeric material and wherein the nominal mutual capacitance between conductors is at a desired value and a dielectric breakdown value above an acceptable minimum is obtained between conductors wi~h each conductor having an outside diameter across the insulation which is less than an electrical insulated conductor of the same conductor gauge which ~`
provides the same mutual capacitance and breakdown values and which is insulated with solid material only, the material being the same material of said inner layer.
In preferred constructions, the air space volume in the ; total volume of the~cellular layer in the construction according to the invention is at least 20% whereby signi~icant savings in materials may ; be obtained over materials required for conventionally insulated conductors in air-core cable.
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One embodiment of the invention will now be described, by way of example, wikh reference to the accornpanying drawings in which:-Figure 1 is a cross-sectional view through a telecommuni-cations cable; and Figure 2 is a cross-sectional view of an insulated conductor incorporated in the cable.
In the embodiment now to be described~ specific dimensions of conductor and insulation layers will not be referred to.
Dimensions will be discussed at the end o~ the description for different gauges of conductor to enable comparisons to be made between the dielectric strength and capacity values of constructions of the embodiment and other insulated conductors not within the scope of this ~, invention.
In the embodimen~, a telecommunications cable 10 comprises a core having a plurality of pairs of insulated conduc~ors 11. The core is wrapped in a composite wrap comprising an inner layer 12 of plas~ic tape, e.g. 3 mils thick, such as "Mylar" tape. The inner layer may comprise other materials such as paper or polyethylene or combinations of these materials. Around this is another layer 13 o~
aluminum tape, e.g. 8 mils thick which has been coated on both sides with polyethylene, followed by a medium density block polyethylene outer layer 14 of about 80 mils thickness.
The core, commonly referred to as an air-core, has each insulated conductor 11 of each pair constructed in the manner shown in ; Figure 2. Each insulated conductor comprises a conductor 15 covered by an inner layer 16 of solid non-cellular insulating material which in line with this invention has a maximum thickness of 4 mils. This may `
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be made from any suitable electrically insulating plastics material such as polypropylene or medium density polyethylene. An outer layer 17 enclosing the inner layer is cellular polypropylene which is preferably closed cell but may be of open cell struc~ure.
Alternatively, the inner layer and outer layer are both formed from high density polyethylene with the outer layer, of course, being cellular.
The insulated conductor is manufactured by passing conductor through a two stage extruder (not shown), the first stage providing the inner non-cellular insulating layer 13 and the second stage extruding the cellular layer. The cellular layer is formed by `
normal foam extrusion techniques.
It is found that while the cells expand directly after extrusion, expansion of the outer layer is outwardly from the inner layer and has no effec~ upon the inner layer which has just been extruded. Thus the inner layer is not stressed by its contact with the expanding outer layer and there is no likelihood of pinholes being formed in the lnner layer because of stress build-up.
The thickness of each of the layers 13, 14 is predetermined primarily to give a desired nominal mutual capacitance value of 83 nanofarads/mile in the completed cable. Also to give the required dielectric properties, the inner layer is located at the position of greatest ~ield intensity and its thickness is calculated to give satisfactory dielectric strength and thus to enable the outer cellular layer to lie as close as possible to the conductor so as not ; to detract from the required mutual capacitance.
, , .
~ ~5~9~10 Further9 the material of the outer layer may be pigmented without detracting from the mutual capacitance properties unduly.
While it is known that pigmentation may deleteriously affect the dielec~ric s~rength properties of an insulating layer, the inner layer is not pigmented and thus its dielectric strength is not so affected.
In the following, measurements were taken of the dielectric strengths of insulated conductor according to the above described embodiment for 22 A~G conductor. These appear in "Category Ali of the following Table I~
For comparison, the test also includes measurements of dielectric strengths of insulated conductors made for grease filled cable in which the insulatiun has an inner cellular layer of polypropylene and an outer non-cellular layer of medium density polyethylene and as described in ~he above Canadian Paten~ No.
952,991.
In addition, and also for comparison, the test also includes measurements of dielectric strenciths of insulated conductors in which the insulation is conventional and is non-cellular low density polyethylene throughout. These measurements appear as "Category C".
;~ 20 The test was conducted while submerging the insulated conductors concerned under water. This was done to simulate the worst possible conditions which insulated conductors in an air core cable could experience, i.e. conditions in which the core is completely waterlogged. It shou1d be stressed that these conditions should nnt normally be expected for air-core cable but are ones which could lead to premature dielectric breakdown.
, i , . , .
,. . , , ' ' : ' ' ~
~ ~55~
A 1000 foot length of insulated conductor in Category 'A' and insulated on one production run ("1" in Table I) was tested in 30 foot sample lengths. Each sample length was immersed in water and a DC
; potential passed through it. The voltage was increased at a substan~ially uniform rate un~il dielec~ric breakdown occurred. The maximum and minimum dielectric breakdown values (Kv), recorded for all of the 30 foot sample lengths, are recorded in Table I together with the average breakdown figure. The above test procedure was then repeated for another 1000 foot leng$h of conductor in Category 'A' which had been insulated on a different production run ("2" in Table I) and the results similarly recorded.
The test precedure was then performed for 30 foot sample leng~hs of two twisted toge~her insulated conductors, in water in which conductor "1" was twisted with conductor "2". Results are given under column 3.
The whole of the above proceclure was then repeated for two 1000 foot lengths of insulated conductor made under Category 'B' and dielectric breakdown values given under columns 4, 5 and 6.
Under Category 'C', tests were made and breakdawn values given under columns 7 and 8. No test was performed under Category 'C' for the insulated conductors twisted together.
`~, , . .
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TABLE I
CATEGORY CATEGORY CATEGORY
A B . _ C .
1 2 3 4 5 ~ 7 8 D.C. Voltage Average15.215.1 29.4 10.5 15.3 17.5 36 48 Dielectric Minimum11.511.0 22.0 8.5 4.0 8.0 12 27 Breakdown Maximum17.016.5 32.5 13.0 22.0 19.0 46 60 (Kv) ~ .
Outslde : Diameter of Insulation 43.3 42.7 - 48.0 48.0 -- 45.5 44.8 (Inils) ~ ' Thickness o~
~ Cellular 6.7 6.4 -- 8.7 8.3 : 10 Layer (mils) Thickness o~
: Non-Cellular 2.3 2.3 -- 2.6 3.0 -- 10.1 9.7 Layer (mils) . ~. j i . . .... .
;~ % Blow . ~ . 26_ ~7 -~ 35 - ,-- ~ --.~ :It should be made clear at this stage that the insulated conductors in Category 'B' were designed for grease filled cable. The desired mutual capacitance of 83 nanofarads/mile would not be achieved between conductors of this construction for air-core cable. However, in contrast, conductors in both of Gategories 'A' and 'C' have a nominal mutual capaci~ance of 83 nanofarads/mile for air-core cable.
As may been seen from the above Table I, the dielectric : 20 breakdown values for conventionally insulated conductor (Category 'C') ~:
` were consistently very high with very high average breakdown values of .~
36Kv and 48Kv. While the breakdown values for insulated conductors:
according to the embodiment described above were much lower $han those of Category C, these values for the embodiment are extremely satisfactory (Category A) and are significantly above one requirement for commercially acceptable air-core cable. This requirement is for , .
, :
9~
a length of insulated conductor to withstand a voltage of 8Kv DC
between conductors for a period of 1 to 3 seconds without dielectric breakdown. Column 3 shows these breakdown values between conductors for Category 'A'. The minimum is 22Kv DC which is well above the reqirement of 8Kv DC by at least some authorities. Column 3 results are interesting in that they indicate values approximately twice those `~ obtained for the single wires in columns 1 and 2. This doubling in values between conductors illustrate not only that current needed to pass through two layers of insulation on both conductors (as distinct from two layers on one conductor in columns 1 and 2), but also that the inner insulation layers of solid material were adding their dielectric strength characteristics without these being degraded by flaws in the layers. This illustrates that there were no physical stresses causing flaws in the inner layers and no impurities, e.g. colour pigments in the layers, both of which would tend to deleteriously affect the results obtained. As a means of comparison with Category 'B', it may be seen that the dielectric breakdown values in column 6 are certainly not of the order of double those obtained for single conductors in columns 3 and 4. In fact, they are not significantly different from columns 3 and 4. It is believed, that the lack of the doubling value effect in column 6 can be blamed upon physical stresses imposed by the inner cellular layers, during extrusion upon the outer solid layers oF
Category 'B' construction, whereby flaws and pinholes are formed, and ; upon the use of colour pigmentation in these outer layers.
Hence, the dielectric strength between conductors for the Category 'A' construction is significantly higher than for the Category ~e~ construction. It should be remembered that Category 'B' insulated .
~1 , "' 1 1~594~
conductor was made for grease filled cable and would have a dielectric strength suitable for this purpose. However, if insulated conductor under Category 'B' were designed for air-core cable while providing the desired nominal 83 nanofarads/mile mutual capacitance and having a diameter less than that of Category 'C', then this would lead to a dielectric strength below that es~ablished by the conductors in Category 'A'.
The resul~s obtained for the construction of the invention were, as already stressed, well above the acceptable levels specified, and because of the use of an outer layer of cellular material with a blow of 35% or less (i.e. 35% of air space in the total volume of the outer layer), there was a significant saving in material compared to the construction of Category 'C', with attendant cost ; saving. In addition) these commercially acceptable results were obtained with outside diameters of insulation in the Category 'A' construction which were at least 1.5 mils less than the outside d~ameters of the Category 'C' construction. Hence, it follows that a resultant air-core cable made with insulated conductors according to the invention will have an outside diameter less than one made using conductors of~ conventlonal Category 'C' while being more economic and providing well above the commercially acceptable levels of dielectric breakdown between conductors.
The recorded values in Table I indicate that constructions according to the invention are a desirable replacement for constructions using a single layer of solid material. C7early~ in most practical constructions, the inner layer should have a maximum thickness of 4 mils to enable the cellular layer to lie as close as .
' ' ". ': ' .. . .
:
~1~5~0 possible to the conductor to obtain the required capacitance level.
In constructions according to $he invention, the amount of air space in the total volume of the outer layer is a parameter in deriding the capacitance whereas the amount of polymeric material is a parameter for the dielectric strength. Whîle the air space may be as much as 50% or more of this volume, to obtain a desirable balance between desired capacitance and desired dielectric strength while ~ enabling a reduction in outside dia~eter of the insulation below that ; for insulated conductor in Category 'C', the air space may n~ed to be at a maximum of 40% and a minimum of 10% for the use of an inner non-cellular layer of maximum thickness of 4 mils.
In addition, conductors according to this invention and as described in the embodiment, may be used for cores filled with particulate material, as acceptable dielectric strengths are obtainableO
The invention is applicable to all conductor gauges which are useful for telecommunications cable and, for all these gauges, that is 19, 22, 24, 26, and 28 at least, accept:able dielectric strengths are obtainable with maximum thicknesses of 4 mils for the non-cellular inner layer. The following Table II compares the constructions of Categories A, B and C in conductor gauges 19~ 22 and 24. Table II
shows the savings obtained in insulation material in both Categories 'A' and 'B' over Category 'C'. While the savings in ~his table for Categories 'A' and IB' are comparable, it should be remembered From the above discussion that the insulated conductors of the invention (Category 'A') provide dielectric strengths for air-core cable which are more acceptable than conductors acording to Category 'B'.
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Claims (6)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An insulated electrical conductor for telecommunications cable comprising a telecommunications conductor and an insulation comprising an inner layer and an outer layer of electrically insulating material, the inner layer of solid non cellular construction and the outer layer of cellular polymeric material, and wherein the nominal mutual capacitance between the conductor and an identically insulated conductor in a pair is at a desired value and a dielectric breakdown value above an acceptable minimum is obtained between the conductor and an identically insulated conductor while having an outside diameter across the insulation which is less than an insulated electrical conductor of the same gauge which provides the same mutual capacitance and which is insulated with solid non-cellular material only, the material being the same material as said inner layer.
2. A conductor according to claim 1 wherein the inner layer has a maximum thickness of 4 mils.
3. A conductor according to claim 1 wherein the outer layer has closed cells.
4. A conductor according to claim 1 wherein the cells provide an air space which is at least 10% of the total volume of the outer layer.
5. An electrically insulated conductor according to claim 4 wherein the air space in the outer layer is between 10% and 40%.
6. A telecommunications cable having an air-core or a core filled with particulate material in which a plurality of insulated electrical conductors are provided, each of which comprises a conductor having insulation comprising an inner layer of solid non-cellular material and an outer layer of cellular polymeric material and wherein the nominal mutual capacitance between conductors is at a desired value and a dielectric breakdown value of above an acceptable minimum is obtained between conductors with each conductor having an outside diameter across the insulation which is less than an electrical insulated conductor of the same conductor gauge which provides the same mutual capacitance and which is insulated with solid material only, the material being the same material of said inner layer.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000375110A CA1155940A (en) | 1981-04-09 | 1981-04-09 | Electrical conductor for telecommunications cable |
| GB8208961A GB2096819A (en) | 1981-04-09 | 1982-03-26 | Electrical conductor for telecommunications cable |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000375110A CA1155940A (en) | 1981-04-09 | 1981-04-09 | Electrical conductor for telecommunications cable |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1155940A true CA1155940A (en) | 1983-10-25 |
Family
ID=4119698
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000375110A Expired CA1155940A (en) | 1981-04-09 | 1981-04-09 | Electrical conductor for telecommunications cable |
Country Status (2)
| Country | Link |
|---|---|
| CA (1) | CA1155940A (en) |
| GB (1) | GB2096819A (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1495474B1 (en) * | 2002-04-16 | 2008-11-12 | Prysmian Cavi e Sistemi Energia S.r.l. | Electric cable and manufacturing process thereof |
-
1981
- 1981-04-09 CA CA000375110A patent/CA1155940A/en not_active Expired
-
1982
- 1982-03-26 GB GB8208961A patent/GB2096819A/en not_active Withdrawn
Also Published As
| Publication number | Publication date |
|---|---|
| GB2096819A (en) | 1982-10-20 |
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