GB2096819A - Electrical conductor for telecommunications cable - Google Patents

Abstract

Insulated electrical conductor for telecommunications cable in which two layers of insulation are provided. The inner 16 of the two layers is a solid non-cellular construction and the outer layer 17 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 e.g. polyethylene or polypropylene. The cellular material may be a closed or open foam. <IMAGE>

Description

SPECIFICATION Electrical conductor for telecommunications cable This invention relates to an insulated electrical conductor for telecommunications cable.

Telecommunications cables conventionally comprise a 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 conductors. If perforations are present or are otherwise formed in the sheath e.g. due to lightning or mechanical 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 to 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 petrolatum jelly.

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 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 strength of the insulation.

Originally, the problem of increase in capacitance with grease filled cable was overcome by an increase in the thickness of the individual solid insolution 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 of 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 commercially 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 reduced, 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 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 realised 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 commercially 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 dielectric strength and capacitance properties which are within commercially acceptable levels while having a smaller outside diameter than comparable insulated conductors for air-core cable and which have a conventional solid insulation.

Accordingly, the present invention provides 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, 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 manufacturing limits, say between 79 and 87 nanofarads/mile.

From the above defined invention, it is clear that with the 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 that desirable values are achievable with suitable combinations of two parameters, i.e.

the thickness of the inner layer and the percentage blow of the foam. For instance, where the thickness of the inner layer is 54 creased, this has an undesirable effect on the capacitance and, to counteract this, a higher percentage of air space needs to be provided in the outer cellular layer. The situation is, however, that the inner layer is located at the position of greatest field intensity and is sufficient to provide the required dielectric properties in an air-core cable while 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 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 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 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 significant savings in materials may be obtained over materials required for conventionally insulated conductors in air-core cable.

One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a cross-sectional view through a telecommunications 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 of 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 embodiment, a telecommunications cable 10 comprises a core having a plurality of pairs of insulated conductors 11. The core is wrapped in a composite wrap comprising an inner layer 1 2 of plastic 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 of 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.

"Mylar" is a Registered Trade Mark.

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 1 5 covered by an inner layer 1 6 of solid non-cellular insulating material which in line with this invention has a maximum thickness of 4 mils. This may 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 structure.

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 1 3 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 effect 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 inner 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 field 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.

Further, 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 dielectric strength 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 AWG conductor. These appear in "Category A" of the following Table For comparison, the test also includes measurements of dielectric strengths of insulated conductors made for grease filled cable in which the insulation has an inner cellular layer of polypropylene and an outer non-cellular layer of medium density polyethylene and as described in the above Canadian Patent No. 952,991.

In addition, and also for comparison, the test also includes measurements of dielectric strengths of insulated conductors in which the insulation is conventional and is non-cellular low density polyethylene throughout. These measurements appear as "Category C".

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 should be stressed that these conditions should not normally be expected for air-core cable but are ones which could lead to premature dielectric breakdown.

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 substantially uniform rate until dielectric 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 length 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 procedure was then performed for 30 foot sample lengths of two twisted together insulated conductors, in water in which conductor "1" was twisted with conductor "2". Results are given under column 3.

The whole of the above procedure 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 the breakdown values given under columns 7 and 8.

No test was performed under Category 'C' for the insulated conductors twisted together.

Table I Category Category Category A B C 1 2 3 4 5 6 7 8 D.C. Voltage Average 15.2 15.1 29.4 10.5 15.3 17.5 36 48 Dielectric Minimum 11.5 11.0 22.0 8.5 4.0 8.0 12 27 Breakdown Maximum 17.0 16.5 32.5 13.0 22.0 19.0 46 60 (Kv) Outside 43.3 42.7 - 48.0 48.0 - 45.5 44.E Diameter of Insulation (mils) Thickness of 6.7 6.4 - 8.7 8.3 - - - Cellular Layer (mils) Thickness of 2.3 2.3 - 2.6 3.0 - 10.1 9.7 Non-cellular Layer (mils) % Blow 26 27 - 35 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 Categories 'A' and 'C' have a nominal mutual capacitance of 83 nanofarads/mile for air-core cable.

As may be seen from the above Table I, the dielectric 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 than 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 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 requirement of 8Kv DC by at least some authorities.

Coiumn 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 iliustrate 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 'B' construction. It should be remembered that Category 'B' insulated 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 established by the conductors in Category 'A'.

The results 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 diameters 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 conventional 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. Clearly, 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 possible to the conductor to obtain the required capacitance level.

In constructions according to the invention, the amount of air space in the total volume of the outer layer is a parameter in deciding the capacitance whereas the amount of polymeric material is a parameter for the dielectric strength. While 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 diameter of the insulation below that for insulated conductor in Category 'C', the air space may need 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 obtainable.

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, acceptable 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 !I shows the savings obtained in insulation material in both Categories 'A' and 'B' over Category 'C'. While the savings in this table for Categories 'A' and 'B' 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 according to Category 'B'.

Table II Outside Thickness diameter of Air Wt saving non-cellular insulation Wt/ft space over % Diameter SWG Construction layer (mills) (mils) (MG) % Category 'C' reduction 19 Category A 3 56.7 220 25 38 6.9 4 57.3 234 25 34 5.9 3 55.4 186 35 47 9.0 Category B 3 57.0 263 20.2 25 6.4 3 56.0 242 26.6 31 8.0 Category C - 60.9 337 - - 22 Category A 3 41.3 125 25 33 6.1 4 41.8 134 25 29 5.0 3 41.8 134 20 29 5.0 Category B 3 42.0 151 14.5 20 4.5 3 40.5 125 28.2 33 8.0 Category C - 44.0 188 - - 24 Category A 3 33.2 84 25 30 5.1 4 33.7 92 25 23 3.7 3 33.5 89 20 26 4.3 Category B 3 33.2 89 19.5 26 5.1 Category C - 35.0 120 - -

Claims (6)

Claims

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.