WO2008003970A2

WO2008003970A2 - Electrical cable

Info

Publication number: WO2008003970A2
Application number: PCT/GB2007/002511
Authority: WO
Inventors: Mark Billows
Original assignee: Linkranch Limited
Priority date: 2006-07-05
Filing date: 2007-07-05
Publication date: 2008-01-10
Also published as: WO2008003970A3

Abstract

An electrical cable has a set of conductor wires extending along the length of the cable, each having a conductor core and a respective insulator sheath about the conductor core. The conductor wires are spaced and held in a respective tube-form space provided in a cable jacket. The cable jacket is tubed onto the conductor wire set such that a respective discrete tube-form boundary wall is present for a respective conductor wire and an air gap exists between the tube-form boundary wall and the respective insulator sheath of the conductor wire. The cable provides advantages in diminishing cross talk effects for data cable and also provides benefits for high energy cable.

Description

Electrical Cable

The present invention relates to electrical cables.

The present invention is applicable to cables for various uses for example energy cables, data cables, audio video power cables and other cables intended for other general or specialist applications.

WO2005/027148 and WO2003/075287 disclose electrical cables designed to have technical features addressing perceived problems with cables .

Developments are being made in electrical cable technology to minimise undesired effects such as 'near end crosstalk' (NEXT) PSNEXT PSELFEXT return loss, insertion loss and other performance reducing effects. WO2005/027148 and WO2003/075287 disclose electrical cables designed to have technical features addressing these problems. Other examples of electrical cables designed to address crosstalk and other performance issues are disclosed in US 6297454 and WO2001/54139. Improvements are particularly desired in relation to the ability to increase bandwidth capacity for cables .

An improved electrical cable and manufacturing technique and apparatus has now been devised.

According to a first aspect, the present invention provides an electrical cable comprising a set of conductor wires extending along the length of the cable, the respective conductor wires comprising a conductor core and a respective insulator sheath about the conductor core, the conductor wires being spaced and held in a respective tube-form space provided in a cable jacket, the cable jacket being tubed onto the conductor wire set such that a respective discrete tube-form boundary wall is present for a respective conductor wire and an air gap exists between the tube-form boundary wall and the respective insulator sheath of the conductor wire.

The air gap that exists may be no more than a very small separation gap at the interface between the insulator sheaths of the wires and the tube form boundary wall of the tube form space in the jacket. The important feature is that there is some air gap and no fusion or bond at that interface, thus enabling the surface of the insulator sheath of the wire to move/slide/slip with respect to the tube form boundary wall of the jacket. This prevents kinking, fracture or breakage of wire that could otherwise occur. The air gap also provides other technical benefits in respect of minimising cross talk and with respect to capacitance.

During tubing on of the jacket, the diameter of the void spaces contracts from a relatively large size at formation to be a snug, cling fit about the respective sheathed wires. Some air gap will be present.

The cable is preferably flexible.

Such a cable has, for certain embodiments, reduced capacitance interaction between wires, resulting in improved energy transmission characteristics. In other realisations the cable of the invention provides a data or audio/visual cable having improved cross-talk and/or bandwidth capacity. In this context, the spacing between the wires is frequently important, and in certain embodiments it is therefore preferred that the void spaces are spaced by a distance equal to or greater than the diameter of the respective void spaces at formation. Beneficially, the cable jacket comprises an insulator material and may be a PVC based Thermoplastic material.

The insulator sheaths of the respective conductor wires are also preferably of plastics material as used conventionally in the art for insulator sheathing conductor cores .

The conductor cores of the respective wires may be solid cores. Alternatively, in certain embodiments (particularly energy cable applications) the cores may include a plurality of conductor strands wound to form a core. Typically for such wound cores, 7 or more conductor strands may be wound to form a core. In a further alternative embodiment particularly suited to data (audio/video etc) transmission, the conductor wires are provided as twisted pairs of wires, each member of a respective pair having a conductor core and respective insulator sheath.

For energy cables typically the conductor set will comprise between 2 and 5 conductor cores (or wires) . British Standards BS 6500 and BS 7919 give an indication of various cables for ordinary and light duty energy supply uses.

In one embodiment, it is preferred that the respective tube form spaces for holding the conductor wires are provided in respective projecting zones (such as limbs or lobes) which extend outwardly from the axis of the cable jacket. Beneficially, the zones (lobes or limbs) are formed (for example having waisted portions or other zones of relative weakness) enabling the respective zones (lobes or limbs) to be stripped apart from one another. This provides beneficial characteristics for electrical connection to appliances, sockets, plugs and so on. In certain embodiments, the respective tube-form spaces for holding the twisted conductor pairs are provided in head portions which project transversely outwardly with respect to the lobes or limbs. This facility conveniently enables the correct and sufficient spacing of the wires in the jacket.

The cable jacket provides a central or core portion of the cable. In certain embodiments the cable jacket is provided with an internal bore, void-space or cavity. More preferably the cable is provided with an axial bore extending longitudinally of the cable. The provision of such a technical feature reduces the capacitance of the cable significantly, resulting in a reduction in the power requirement for a given cable to transmit a given amount of energy.

Such an arrangement is believed to be novel and inventive per se .

Accordingly, a second aspect of the invention provides an electrical cable comprising a set of conductor wires extending along the length of the cable, the respective conductor wires being spaced and held in a respective tube-form space of a cable jacket, the cable jacket being tubed onto the conductor wire set; wherein

the respective tube form spaces for holding the respective wires are provided in discrete lobes of the jacket, the discrete lobes being strippable to come apart from one another; and/or,

the cable jacket is provided with a vacant internal bore, void-space or cavity.

It is preferred that the tube-form boundary wall of the cable jacket is a continuous boundary around the respective insulator sheath of a respective conductor wire. More preferably, the tube-form boundary wall is without steps, apexes or projections. As such the tube form boundary wall is preferably smoothly curved.

A lubricant or separation layer or material is preferably provided between the tube-form boundary wall of the cable jacket and the respective insulator sheath of the respective conductor wire. The lubricant or separation material may be provided in powder form. In a preferred embodiment, the lubricant or separation material comprises chalk or talc. This aids in separating the jacket and insulator sheaths of the wires and ensuring that the air gap forms .

It is preferred that the cable, including the cable jacket, is helically twisted along its length. Beneficially, for data cable, the lay length of the twisted cable jacket is in the range 100mm to 140mm.

In one embodiment there are three conductor wires (cores) spaced about the axis of the cable. In this embodiment the jacket may appear "clover" shaped in section. In an alternative embodiment there are four sets of conductor wires (or twisted conductor pairs) spaced about the axis of the cable.

According to a second aspect of the invention, there is provided a method of manufacturing an electrical cable comprising a set of conductor wires extending along the length of the cable, the respective conductor wires comprising a conductor core and a respective insulator sheath about the conductor core, the conductor wires being spaced and held in a respective tube-form space provided in a cable jacket, wherein the cable jacket is tubed onto the conductor set such that a discrete tube-form boundary wall is present and an air gap exists between the tube-form boundary wall of the jacket and the respective insulator sheath of the respective conductor wire .

Beneficially, the tube-form boundary is already formed before tubing onto the respective insulator sheath of a respective conductor wire.

It is preferred that the solid jacket is formed upstream of a point at which the cable jacket is tubed onto the respective insulator sheath of a respective conductor wire.

Typically the cable jacket will be formed by extrusion, typically of Thermoplastics material.

In one realisation, the conductor wires, already with their respective insulator sheaths, pass through extrusion apparatus and the cable jacket is extruded and tubed on around the wires.

Where the conductor wires are provided as a twisted conductor pair (e.g. for use in data applications) it is preferred that the tube-form boundary is already formed before tubing onto the respective twisted conductor pair preferably upstream of a point at which the cable jacket is tubed onto the conductor pair .

In one realisation, the twisted conductor pairs pass through extrusion apparatus and the cable jacket is extruded and tubed on around the insulated sheaths of the twisted conductor pairs.

Beneficially, a lubricant or separation layer or material (such as chalk or talc) is introduced between the tube-form boundary wall and the respective insulator sheath of a respective conductor wire. Following extrusion, the cable, including the cable jacket, is preferably helically twisted along its length. The cable may then be wound for storage in a conventional manner.

According to a further aspect, the invention provides a method of manufacturing an electrical cable comprising a set of conductor wires extending along the length of the cable, the respective conductor wires being spaced and held in a respective projection zone (limb or lobe) of a cable jacket, wherein the cable jacket is formed by extrusion and tubed onto the conductor wire set, the extrusion of the jacket providing a vacant bore, void-space or cavity internally of the jacket.

According to a further aspect, the invention provides extrusion apparatus for forming a cable, the apparatus including:

a die having a die aperture;

an extruder body having:

a plurality of guide channels for guiding the passage of respective conductor wires and

a surface for guiding flowable material which solidifies to form a jacket about the respective conductor wires;

wherein the die aperture is provided substantially level with the end of the extruder body, such that a solid cable jacket is formed by the extruder body upstream of a point at which the cable jacket is tubed onto the conductor wires.

In one embodiment the extruder body has a plurality of projecting pipes defining the ends of the plurality of guide channels for guiding the passage of respective wires, the extruder body and die being so spaced to ensure formation of tube-form spaces in the jacket, with the tube-form boundary walls being formed on the projecting pipes.

The present invention will now be further described by way of example only, and with reference to the accompanying drawings, in which;

Figure 1 is a schematic sectional view of an electric cable in accordance with the invention/

Figure 2 is a schematic sectional view of extrusion die apparatus for forming the cable of the invention;

Figure 3 is facing view of the extrusion die point body of the apparatus of figure 2;

Figure 4 is a side view of the die point body of figure 3;

Figure 5 is schematic perspective view of a part of the cable of figure 1;

Figure 6 is a is a schematic sectional view of extrusion die apparatus for forming a cable outside the scope of the invention;

Figure 7 is a sectional view of a prior art cable formed with the apparatus of figure β.

Figure 8 is a schematic sectional view of an alternative embodiment electric cable in accordance with the invention;

Figure 8a is a detailed view of a part of the cable of figure Figure 9 is a schematic sectional view of extrusion die apparatus for forming the cable of figure 8;

Figure 10 is facing view of the extrusion die point body of the apparatus of figure 9; and,

Figure 11 is a side view of the die point body of figure 10.

Referring to the drawings and initially to figures 1 and 5 there is shown an electrical cable 1 comprising a cable jacket 2 formed about twisted pairs of conductors 3, 4, 5, 6. Each of the twisted pairs of conductors comprises two respective conductors 3a, 3b 4a, 4c etc, each sheathed in a respective Thermoplastic insulator sheath 7. The cable jacket 2 is beneficially of flexible material enabling the tube to be twisted into a lay along its length and also give flexibility to the cable as will be described in detail. A suitable material for the jacket 2 is a PVC based Thermoplastic elastomer .

The embodiment primarily described has four sets of twisted pairs of conductors 3, 4, 5, 6 however it is important to note that the invention has applicability where other numbers of twisted pairs or other conductor sets are present. The invention in this aspect is directed to minimise crosstalk and other effects (such as other interference effects) between pairs of conductors in the same cable and also between conductors in adjacent but discrete cables, such as so called 'alien crosstalk'. These effects and proposed solutions are described, for example in WO2005/027148 and WO2003/075287. The invention, in this aspect, is applicable to power cables, data, audio and video cables and specialist cables to provide improved performance over Cat-5 type, Cat-β type and Cat-7 type cables. In this aspect the function of the jacket in spacing the conductors is important. The spacing may vary dependent upon specific application but will typically be in the range of 3mm to 10mm.

The twisted pairs of conductors, 3, 4, 5, 6 are twisted to be intertwined along their length as is common in the prior art and also as described in WO2005/027148 and WO2003/075287. An important feature of the invention is that in forming the jacket 2, a receiving tube 9 for the relevant pair of twisted conductors is formed, in such a fashion that the twisted conductor pair (e.g. pair 3a, 3b) is not embedded within the jacket 2. Rather that a discrete non fused tube boundary 9 and air gap 10 exists between the cable jacket 2 and the respective conductor sheaths 7 of each conductor in the pair. In the drawing of figure 1, the tube 9 boundary is shown as circular, this is exemplary and shows the condition of the tube 9 boundary immediately following formation. In tubing of the jacket 2 onto the wires, the tube 9 boundary contracts and collapses out of circular form to cling about the sheaths 7 of the wires. The air gap that exists may be no more than a very small separation gap at the interface between the insulator sheaths of the wires and the tube form boundary wall of the tube form space in the jacket. The important feature is that there is some air gap and no fusion or bond at that interface, thus enabling the surface of the insulator sheath of the wire to move/slide/slip with respect to the tube form boundary wall of the jacket. This prevents kinking, fracture or breakage of wire that could otherwise occur. The air gap also provides other technical benefits in respect of minimising cross talk and with respect to capacitance.

This technical configuration and arrangement is different to the arrangements described for example in WO2005/027148 and WO2003/075287 in which the twisted conductor pairs are embedded in the jacket by pressure type plastics injection moulding to embed the conductor pairs in the jacket. In these prior art arrangements described, there is no presence of a receiving tube for the relevant pair of twisted conductors formed, in such a fashion that the twisted conductor pair is not embedded within the jacket 2. These prior art disclosures do not disclose a discrete non fused tube boundary and gap existant between the cable jacket and the respective conductors.

In accordance with the present invention the desired result (the air gap 10 and separation boundary 9) may be achieved by a plastics moulding process in which the jacket is tubed on to the respective conductor pairs. This will be described in greater detail. In addition a separator or lubricant substance or layer may be introduced at manufacturing to enhance the boundary and separation effect between the non fused tube boundary 9 and air gap 10 existing between the cable jacket 2 and the respective conductor sheaths 7 of each conductor in the pair. In a technically realisable procedure chalk powder or dust may be utilised as the separator or lubricant. The tube boundary 9 of the tube formed in the jacket is without steps, apexes or projections interstitially of the conductors in a respective twisted pair.

The technical benefits realised by a cable construction according to the present invention, in which the twisted conductor pair (e.g. pair 3a, 3b) is not embedded within the jacket 2, but rather a discrete non fused tube boundary 9 and gap 10 exists between the cable jacket 2 and the respective conductor sheaths 7 of each conductor in the pair, include enhanced performance in terms of crosstalk reduction. This is believed to be due to the insulation properties of the air gap 10. Additionally enhanced performance has been found using twisting of the cable (including the cable jacket) along its longitudinal axis into a relatively tight helical lay-form. Cable having the separation air gap 10 (enhanced by lubrication) enables the twisting into lay-form to be consistently achieved with reduced damage to the respective twisted pairs such as kinking, buckling or stretching, all of which can adversely affect performance. This is because the twisted pairs are not fully embedded within the jacket 2.

Twisting cable having fully embedded twisted pairs such as described in WO2005/027148 and WO2003/075287 will be prone to damaging effects such as kinking, buckling or stretching. The air gap 10 shown in figures 1 and 5 is pronounced, primarily for the purposes of explanation. In reality the air gap produced will more likely be of the order of lmm or less. The tube boundary 9 is similarly shown as circular but in reality will become collapsed or flattened to an irregular ovoid form about the twisted conductors. The air gap, however small, exists because of the fact that there is a separate tube boundary 9 formed in the cable jacket 2.

Referring to figures 2 to 4, there is shown extrusion apparatus for producing cable according to the invention having the air gap 10 and separation interface at the tube on boundary 9. The apparatus comprises an extrusion die 14 having a die aperture conforming to the external profile configuration of the cable jacket 2. In the case of the cable of figure 1, the external profile 12 of the cable jacket 2 is in the form of a cross shape having limbs provided with transverse, 'hammer head' terminations 15. In this case the die 14 aperture is correspondingly shaped in the form of a cross shape having 'hammer head' terminations.

An extruder point body 18 is positioned upstream of the extrusion die 14 and has respective bores 23, 24 25 26 extending through the extruder point body 18 through which are fed respective twisted conductor pairs. In figure 2 only upper and lower bores 23 25 for twisted pairs 3 5 are shown, the twisted pairs being fed from the left hand side of the apparatus . The pairs of conductors pass out from the extruder point body 18 via respective cylindrical pipes 33,34,35,36 which have their respective downstream ends approximately co- terminal with the die aperture of the die body 14.

The molten Thermoplastic material to form the cable jacket 2 flows along a generally conical outer surface 43 of the extruder point body 18 an over and around the pipes 33, 34, 35, 36 and out through the die aperture. The flow of material has a set rate through the extruder point body 18 and die 14 and is not restricted, or choked. Because the flow rate is uniform through the die, the radial thickness of the jacket 2 can be maintained accurately via adjustments to the line feed speed. The die aperture of the die 14 is large enough for the extruder point body 18 to be positioned within its periphery.

The solid jacket is formed on the pipes 33, 34, 35, 36 upstream of the point of insertion of the conductor pairs into the jacket (via the ends of the pipes) . In this way, the already formed jacket is tubed onto the twisted pairs of conductors ensuring that a discrete non fused tube boundary 9 and gap 10 exists between the cable jacket 2 and the respective conductor sheaths 7 of each conductor in the pair. The die 14 aperture is provided substantially level with the of the end of the extruder body 18, such that a solid cable jacket is formed on the extruder body upstream of a point at which the cable jacket is tubed onto the conductor pairs.

This is contrasted with the prior art techniques such as certain embodiments described in WO2005/027148 and WO2003/075287 in which the extrusion point is behind the die aperture and the flow of material is restricted causing pressure build up and the extruded material to form about the twisted pairs themselves filling the interstices between the members of a twisted pair and fully embedding the pairs in the jacket .

Figures 6 and 7 show such a prior art technique falling outside the scope of the present invention, in which the downstream end of the extruder point body 318 is spaced upstream of the die aperture of the die 314 such that the molten Thermoplastic material of the jacket forms directly on the twisted conductor pairs 103 105 fully embedding the twisted pairs which then form an integral part of the Thermoplastic cable jacket. See figure 7 in which there is no gap equivalent to the air gap 10 shown about the twisted pairs of conductors. This latter extrusion technique is often referred to as pressure extrusion. The flow of material is restricted causing pressure to build up at the die 314. This ensures that the interstices between the conductor pair are filled with extruded material.

Particularly beneficial results have been achieved with cable in accordance with the present invention that has a longitudinal twist laid during manufacture. It has been found to be particularly technically beneficial to provide a cable having the combined technical features of a longitudinal cable lay twist and pairs of twisted conductors extending in a tube pre formed within a cable jacket so as to have discrete non- fused tube boundary 9 and gap 10 existing between the cable jacket 2 and the respective conductor sheaths 7 of each conductor in the pair. For data cables lay lengths of the order of 100mm to 140mm have been found to produce beneficial effects. Other preferred lay lengths for cables for other uses and applications are envisaged.

The lay twist is applied following forming of the jacket 2 and positioning of the twisted conductor pairs in the respective tubes formed in the cable jacket 2, at a forming station provided downstream of the extrusion apparatus. In tests the cable of the present invention having the separately formed tubed on cable jacket defining the air gap between the cable jacket and the twisted conductor pairs, has been found to be of improved performance in terms of reduced crosstalk effects and also bandwidth. This performance is enhanced by helically twisting the cable including the cable jacket once extruded. Additionally the use of a separator or lubricant layer or substance (such as chalk) at the interface of the tub boundary 9 and conductor pairs has been found to improve manufacturing and subsequent performance.

Referring to figure 8 there is shown an alternative embodiment of an electrical cable 101 particularly suitable to high energy transmission applications, the cable 101 comprises a cable jacket 102 formed about a set of three conductor- wires 103, 104, 105. Each of the conductor wires 103, 104, 105 comprises a respective conductor core 103a, 104a, 105a, sheathed in a respective Thermoplastic insulator sheath 107. The cable jacket 102 is beneficially of flexible material enabling the tube to be twisted into a lay along its length and also give flexibility to the cable as will be described in detail. A suitable material for the jacket 102 is a PVC based Thermoplastic material.

The jacket 102 includes respective lobes 106a, 106b, 106c within which the wires are held, and neck portions 131a, 131b, 131c extending toward a central zone. The central zone is provided with an axial air-space bore 185.

The embodiment primarily described has a set of 3 conductor wires. However it is important to note that the invention has applicability where other numbers of conductor wires are present. Typically between 2 and 5 conductor wires will be present. The conductor cores 103 of the respective wires may be solid cores (for example of copper) or alternatively may include a plurality of conductor strands (as shown in figure 8a) wound to form a core. Typically for such wound cores 7 ox more conductor strands may be wound to form a core.

The invention, in this aspect, is directed to minimise capacitance effects between the wires 103, 104, 105, and thereby provide enhanced energy delivery capability and characteristics. the jacket 102 acts to space the wires 103, 104, 105 from one another. The optimum spacing of the wires will vary depending upon particular technical applications. Generally the greater the spacing between cables, the greater the reduction in capacitance, however this needs to be counterbalanced by a desire not to make the overall width of the cable un-manageably large. Typically, the spacing will be greater than the diameter of the void space 109 at its formation (typically in the range 3mm to 10mm) .

An important feature of the invention is that in forming the jacket 102, a receiving tube for each of the respective conductor wires is formed, in such a fashion that the wires 103, 104, 105 are not embedded within the jacket 102. Rather that a discrete non fused tube boundary 109 and air gap 110 exists between the cable jacket 102 and the respective insulator sheaths 107 of each conductor wire. This is shown most clearly in figure 8a.

This technical configuration and arrangement is different to the arrangements described for example in WO2005/027148 and WO2003/075287 in which the conductors are embedded in the jacket by pressure type plastics injection moulding to embed the conductor in the jacket. In these prior art arrangements described, there is no presence of a receiving tube for the conductors. These prior art disclosures do not disclose a discrete non fused tube boundary and gap existing between the cable jacket and the respective conductors.

In accordance with the present invention the desired result (the air gap 110 and separation boundary 109) may be achieved by a plastics moulding process in which the jacket is tubed on to the respective conductor wire, about the insulator sheath 107 of the respective wire. This will be described in greater detail. In addition a separator or lubricant substance or layer may be introduced at manufacturing to enhance the boundary and separation effect at the non fused tube boundary 109 and air gap 110 existing between the cable jacket 102 and the respective conductor sheaths 107 of each conductor wire. In a technically realisable procedure chalk or talc powder or dust may be utilised as the separator or lubricant. The tube boundary 109 of the tube formed in the jacket is without steps, apexes or projections.

The technical benefits realised by a cable construction according to the present invention, in which the conductor wires are not embedded within the jacket 102, but rather a discrete non fused tube boundary 109 and gap 110 exists between the cable jacket 102 and the respective conductor sheaths 107 of each conductor wire, include enhanced performance in terms of reduced capacitance and magnetic interaction between the separate conductor wires 103, 104, 105. This is believed to be due to the presence of the discrete separation boundary 109 at the interface between the cable jacket 102 and the insulator sheath 107.

The material of the insulator sheath preferably has a dielectric constant less than 50. This aids in reducing capacitance. The conductor wires are preferably spaced equidistant from the two most closely adjacent conductor wires. This would result in the wires being positioned at the apexes of an equilateral triangle for a 3 wire cable (such as the ' cloverleaf ' design shown in figure 1). Similarly, the wires would be positioned at the corners of a square for a four wire cable, and at the apexes of a pentangle for a five wire cable.

Additionally enhanced performance has been found using twisting of the cable (including the cable jacket) along its longitudinal axis into a relatively tight helical lay-form. Cable having the separation air gap 110 (enhanced by lubrication) enables the twisting into lay-form to be consistently achieved with reduced damage to the respective such as kinking, buckling or stretching, all of which can adversely affect performance. This is because the wires are not fully embedded within the jacket 2, but able to rotate within the jacket to a degree.

Twisting cable having fully embedded wires such as described in WO2005/027148 and WO2003/075287 will be prone to damaging effects such as kinking, buckling or stretching. The air gap 110 shown in figure Ia is pronounced, primarily for the purposes of explanation. In reality the air gap produced will more likely be 0.5mm or less and in fact may be no more than the inherent space between adjacent and abutting surfaces of the tube form bore of the sheath 102 and the external surface of the wire insulation sheath 107. The air gap, however small, exists because of the fact that there is a separate tube boundary 109 formed in the cable jacket 102.

The construction of the jacket having lobes 106 a, 106b, 106c enables the wires to be stripped apart at the ends of the cable, whilst remaining in the jacket 102. In effect the respective lobes peel apart along their longitudinal connection to one another. This provides benefits in terms of connecting the cable to appliances, sockets, plugs etc. The presence of the neck portions enhances the strip-ability of the cable, as does the presence of the axial bore 185.

The axial air space bore 185 in the jacket 102 has also been shown to yet further improve the performance of the cable in terms of reduced capacitance and other electromagnetic effects making the cable yet further efficient. It should however be understood that in its broadest sense the invention does not require the presence of this axial bore 185.

Referring to figures 9 to 11, there is shown extrusion apparatus for producing cable according to the invention having the air gap 110 and separation interface at the tube on boundary 109. The apparatus comprises an extrusion die 114 having a die aperture conforming to the external profile configuration of the cable jacket 102. In the case of the cable of figure 1, the external profile of the cable jacket 102 is in the form of a three lobed 'cloverleaf shape. In this case the die 114 aperture is correspondingly shaped in the form of a cloverleaf shape.

An extruder point body 118 is positioned upstream of the extrusion die 114 and has respective bores 123, 124, 125 extending through the extruder point body 118 through which are fed respective conductor wires, already having their insulator sheaths 107 in place. In figure 2 only upper and lower bores 123 and 125 (for wires 103, 105) are shown, the conductor wires 103 105 being fed from the left hand side of the apparatus. The insulated conductor wires pass out from the extruder point body 118 via respective cylindrical pipes 133,134,135 which have their respective downstream ends approximately co-terminal with the die aperture of the die body 114. The molten Thermoplastic material to form the cable jacket 102 flows along a generally conical outer surface 143 of the extruder point body 118 an over and around the pipes 133, 134, 135 and out through the die aperture. The flow of material has a set rate through the extruder point body 118 and die 114 and is not restricted, or choked. Because the flow rate is uniform through the die, the thickness of the jacket 112 can be maintained accurately via adjustments to the line feed speed. The die aperture of the die 114 is large enough for the extruder point body 118 to be positioned within its periphery, the extruder point body 118 includes a solid pin projection 285 for forming the axial air space bore 185 in the jacket. If the axial air space bore is not to be present in the cable, the pin projection 285 does not need to be present.

The solid jacket is formed on the pipes 133, 134, 135, an the pin projection 285 upstream of the point of insertion of the conductor into the jacket (via the ends of the pipes) . In this way, the already formed jacket is tubed onto the insulation sheaths 107 of the wire conductors 103, 104, 105 of conductors ensuring that a discrete non fused tube boundary 109 and separation gap 110 exists between the cable jacket 102 and the respective conductor sheaths 107 of each conductor wire. The die 114 aperture is provided substantially level with the end of the extruder body 118, such that a solid cable jacket is formed on the extruder body upstream of a point at which the cable jacket is tubed onto the conductor wires.

This is contrasted with the prior art techniques such as certain embodiments described in WO2005/027148 and WO2003/075287 in which the extrusion point is behind the die aperture and the flow of material is restricted causing pressure build up and the extruded material to form about the conductors and fully embedding integrally in the jacket. Particularly beneficial results have been achieved with cable in accordance with the present invention that has a longitudinal twist laid during manufacture.

The lay twist is applied following forming of the jacket 102 and positioning of the conductor in the respective tubes formed in the cable jacket 102, at a forming station provided downstream of the extrusion apparatus.

The capacitance reduction that can be achieved using cable of the present invention is significant. Tests have shown that a prior art 3 core cable of conductor area 1.5mm2 can have a mutual capacitance of 110pf/m. The 'clover leaf design of figure 1 has been shown to have a mutual capacitance of in the region of 50pf/m for the same core area wires.

Claims

Claims :

1. An electrical cable comprising a set of conductor wires extending along the length of the cable, the respective conductor wires comprising a conductor core and a respective insulator sheath about the conductor core, the conductor wires being spaced and held in a respective tube-form space provided in a cable jacket, the cable jacket being tubed onto the conductor wire set such that a respective discrete tube-form boundary wall is present for a respective conductor wire and an air gap exists between the tube-form boundary wall and the respective insulator sheath of the conductor wire.

2. A cable according to claim 1, wherein the cable jacket comprises an insulator material.

3. A cable according to claim 1 or claim 2, wherein the cable jacket comprises a Thermoplastics material.

4. A cable according to any preceding claim, wherein the respective tube form spaces for holding the respective wires are provided in discrete projections (such as limbs or lobes) of the jacket, which extend outwardly from the axis of the cable jacket.

5. A cable according to claim 4, wherein the discrete lobes are strippable to come apart from one another.

6. A cable according to any preceding claim, wherein: i) the cable jacket provides a central or core portion of the cable; or ii) the cable jacket is provided with an internal bore, void-space or cavity.

7. A cable according to claim β, wherein the cable is provided with an axial bore extending longitudinally of the cable.

8. A cable according to any preceding claim, wherein the tube-form boundary wall is a continuous boundary around the respective conductor wire.

9. A cable according to any preceding claim, wherein a lubricant or separation layer or material is provided between the tube-form boundary wall and the respective insulator sheath of the conductor wire.

10. A cable according to claim 9 wherein the lubricant or separation material is provided in powder form.

11. A cable according to claim 10, wherein the lubricant or separation material comprises chalk or talc.

12. A cable according to any preceding claim, wherein the cable, including the cable jacket, is helically twisted along its length.

13. A cable according to claim 12 wherein the lay length of the twisted cable jacket is in the range: i) 75mm to 200mm; or ii) 100mm to 140mm

14. A cable according to any preceding claim wherein: i) the conductor set comprises three or more conductor wires spaced about the axis of the cable; and/or ii) the conductor set comprises three or more sets of twisted conductor pairs spaced about the axis of the cable.

15. A cable according to any preceding claim wherein a plurality of spaced sets of twisted pairs of conductor wires are provided, the members of each twisted pair each having a respective insulating sheath, and respective twisted pairs being held in a respective tube-form space.

16. A cable according to any preceding claim, wherein the surface of the insulator sheath about the wires and the boundary wall of the tube-form space of the jacket can move with respect to one another as the cable is flexed.

17. A cable according to any preceding claim, wherein the void spaces are spaced by a distance substantially equal to or greater than the diameter of the respective void spaces at formation.

18. A cable according to any preceding claim wherein the diameter of the void spaces at formation is relatively large and contracts to a relatively smaller dimension about the wires.

19. An electrical cable comprising a set of conductor wires extending along the length of the cable, the respective conductor wires being spaced and held in a respective tube-form space of a cable jacket, the cable jacket being tubed onto the conductor wire set; wherein

the respective tube form spaces for holding the respective wires are provided in discrete limbs or lobes of the jacket, the discrete limbs or lobes being strippable to come apart from one another; and/or,

the cable jacket is provided with a vacant internal bore, void-space or cavity.

20. A method of manufacturing an electrical cable comprising a set of conductor wires extending along the length of the cable, the respective conductor wires comprising a conductor core and a respective insulator sheath about the conductor core, the conductor wires being spaced and held in a respective tube-form space provided in a cable jacket, wherein the cable jacket is tubed onto the conductor set such that a discrete tube-form boundary wall is present and an air gap exists between the tube- form boundary wall of the jacket and the respective insulator sheath of the respective conductor wire.

21. A method according to claim 20, wherein the tube-form boundary is already formed before tubing onto the respective wire.

22. A method according to claim 20 or claim 21, wherein the solid cable jacket is formed upstream of a point at which the cable jacket is tubed onto the conductor wire.

23. A method according to any of claims 20 to 22, in which the cable jacket is extruded on to the conductor wire set.

24. A method according to claim 23 in which the insulator sheathed conductor wires pass through extrusion apparatus and the cable jacket is extruded and tubed on around the insulator sheathed conductor wires.

25. A method according to any of claims 20 to 24, wherein a lubricant or separation layer or material is introduced between the tube-form boundary wall and the respective insulator sheathed conductor wire.

26. A method according to claim 25, wherein the lubricant or separation material is chalk or talc.

27. A method according to any of claims 20 to 26, wherein the cable, including the cable jacket, is helically twisted along its length, during or subsequent to the jacket extrusion process.

28. A method according to any of claims 20 to 27, wherein the diameter of the jacket void spaces at formation is relatively large and contracts to a relatively smaller size about the sheathed wires .

29. A method according to any of claims 20 to 28, wherein the cable jacket is extruded to have a vacant internal bore, void-space or cavity.

30. A method according to any of claims 20 to 29, wherein the jacket is tubed onto twisted pairs of conductors, the individual members of respective twisted pairs having insulating (plastics) sheaths.

31. A method of manufacturing an electrical cable comprising, a set of conductor wires extending along the length of the cable, the respective conductor wires being spaced and held in a respective lobe or limb of a cable jacket, wherein the cable jacket is preferably formed by extrusion and tubed onto the conductor wire set, the extrusion of the jacket providing a vacant bore, void- space or cavity internally of the jacket.

32. Extrusion apparatus for forming a cable, the apparatus including:

a die having a die aperture; an extruder body having:

wherein the die aperture is provided substantially level with the of the end of the extruder body, such that a solid cable jacket is formed on the extruder body upstream of a point at which the cable jacket is tubed onto the conductor wires.

27. Extrusion apparatus according to claim 26, wherein the- extruder body has a plurality of projecting pipes defining the ends of the plurality of guide channels for guiding the passage of respective of conductor wires, the extruder body and die being so positioned to ensure formation of tube-form spaces in the jacket with the tube-form boundary walls being formed on the projecting pipes.

28. Extrusion apparatus according to claim 26 or claim 27, wherein the extruder body has a formation for defining a vacant internal bore, void-space or cavity in the extruded cable jacket.