GB1565385A - Telephone cable elements - Google Patents

Description

(54) TELEPHONE CABLE ELEMENTS (71) We, S. A. DESCABLERIES ET TREFILERIES DE COSSONAY, of 1305 Cossonay Gare, Switzerland, a Swiss Com- pany, SOCIETY D'EXPLOITATION DES CABLES ELECTRIQUES SYSTEME BER THOUD BOREL ET CIE., of 2016 Cortaillod, Switzerland, a Swiss Company, and CABLERIES DE BRUGG S.A., of 5200 Brugg, Switzerland, a Swiss Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described, in and by the following statement: This invention relates to telephone cable elements.

Telephone cables are formed from elements consisting of insulated wires grouped in twos or fours, known respectively as pairs and quads.

The pairs may consist of either two coaxial conductors or helically twisted wires. In this latter case they are known as symmetrical pairs. The quads are either formed from four twisted wires (star quad), or from twisted wire pairs which are themselves twisted, and known as DM (Dieselhorst-Martin) quads or symmetrical pair quads. These pairs or quads form the elementary circuits of a cable. The present invention has particular applications to star quads, DM quads and symmetrical pairs.

The proximity of the conductors grouped in a cable means that the telephone circuits are not totally independent of each other.

Interactions are produced, and parasite signals may be detected in a particular circuit, gener ated by the passage of signals over other circuits. This phenomenon is known as crosstalk and is manifested in practice by the awareness of a telephone conversation transmitted over a neighbouring circuit Crosswalk is influenced by the electrical resistance of the elementary circuits, by the capacitance of these circuits, and in particular by their capacitance dissymmetry. These same parameters also contribute to line attenuation, which results in a decrease in the sound level of the transmitted conversation and should of course be as low as possible.

The electrical resistance of the circuit is defined by parameters which can be fairly easily controlled, namely the resistivity of the metal used for the conductor, the constancy of this resistivity along the line, and the conductor dimensions.

The capacitance and capacitance dissyme metries, the influence of which is preponderant in cross-talk questions, depend on the dielectric constant of the insulant utilised, which is a measurable and reproducible parameter, but they also depend on other parameters much more difficult to control and which are related to the geometry of the quad.

This geometry results from the helical assembly of the quad wires, and it is obvious that it is very difficult to control such geometry with rigorous precision, and in particular to ensure that no displacement of the wires in the quad occurs during the further cable manufacturing operations.

For a long time, cables were insulated with spirally wound paper strip, but this insulating material, which is relatively fragile, has a low rate of production, and leads to complications in splicing, has now been replaced by plastics insulation applied by extruding machines.

This plastics insulation has the disadvantage of a higher dielectric constant, which requires the thickness of the insulation to be increased to obtain the same line attenuation.

In addition to this defect deriving from the nature of the insulation, other defects may be indicated which derive indirectly from the use of plastics insulation.

First, it is difficult to ensure accurate centering of the conductor in the sheath produced by an extrusion machine. Eccentricity of the conductor has repercussions over the entire length of the wire, and creates capacitance dissymmetries. Furthermore, even when this defect is practically non-existant, the stability of the geometry of quads formed from wires insulated by plastics sheaths is poor because of the low coefficient of friction between the sheaths, so that the wires may become displaced and create capacitance dissymmeties, in particular during the cable manufacturing operations which follow the manufacture of the quads themselves.

Finally, and in contrast to paper insulation, plastics insulation offers no protection to the cable against water infiltration if the cable envelope becomes defective.

Among the numerous solutions which have been advocated, it has been proposed in British Patent No. 1,408,068 which discloses and cIaims an insulated electrical wire, suitable for manufacturing cables, the insulation of which consists of a sheath made of a plastics material, the sheath being surrounded by a hydrophilic zone constituted by a plurality of cellulose fibres of substantially equal size anchored to the surface of the sheath so as ro prevent or hinder the progression of water along the cable if the latter ruptures.

The object of the present invention is to improve the capacitance symmetry of telephone circuits, using wires the insulation sheath of which is surrounded by a plurality of fibres.

The present invention provides a telephone cable element formed of at least one pair of helically twisted conductors, each of these conductors being enveloped in an insulating sheath made of a synthetic plastics material in which are embedded a plurality of fibres projecting all around the sheath, which fibres are about 25 denier and have a mean length, constant over the entire length of said conductor, about a millimetre, the weight of these fibres embedded in the sheath which envelops each conductor being about 400 grams per kilometre of conductor.

The lengths of the fibres and their density are chosen so as to maintain between the conductors a determined spacing which is a function of the desired capacitance between the conductors, and the average length of said fibres and the density at which they are embedded in said sheaths are kept constant so that said spacing remains uniform over the entire length of the cable element. The pro jecting portions of said fibres fasten the sheath of one conductor to the sheath of the other conductor by the mutual interpenetration of the fibres of the adjacent sheaths whereby, the pitch of the helix is kept constant and uniform over the entire length of the cable element.

Telephone cable elements constructed in accordance with the invention have shown that, regardless of the nature of the fibres utilised, the presence of these fibres of given average length embedded in the sheath at a constant density enables a spacing to be maintained between the conductors which is a function of the desired capacitance, and to give said capacitance a symmetry which is absolutely surprising, and very difficult to obtain with extruded insulation.Moreover, when bundles and cables are manufactured using pairs of quads according to the inven tion, it is found that these elements have not undergone any deformation, because of the interpenetration of the fibres in the adjacent regions of the conductors, and in contrast to that which happens with all other known forms of insulation, which allow the twisted conductors to become displaced relative to each other during the further cable manufacturing operations.

The importance of these advantages and of other accompanying advantages will be more evident from the description given hereinafter.

The accompanying drawing represents a diagrammatic illustration by way of example of cable elements according to the present invention. In the drawing: Figure 1 is a perspective view of a pair; Figure 2 is a cross-section through a quad of symmetrical pairs; Figure 3 is a cross-section of a star quad; Figure 4 is a diagram which is useful in defining the parameters k which are a measure of capacitance dissymmetries; and Figure 5 is a diagram showing the various factors which can modify the value of the aforementioned parameters.

Figure 1 shows a pair 25 formed from two wires 24 twisted into a helix of constant pitch.

Each of the wires comprises a conductor 19 enveloped by a plastics sheath 2 in which a plurality of fibres 21 are anchored. The pair 25 is designed to constitute a telephone cable element forming a telephone circuit or line.

As can be seen in Figure 1, the fibres 21 which cover the respective insulating sheaths 2 mutually interpenetrate in the adjacent parts of the twisted wires. Consequently, these fibres act as distance pieces. between the insulating sheaths formed about the conductors 19, the diameter of which in this example is 0.6 mm, the sheaths being preferably of expanded polyethylene about 0.2 rnm thick. The average length of the fibres is about 1 rum and their diameter 25 denier. The polyethylene layer ensures good mechanical strength and sufficient electrical strength. The fibres, which are preferably cellulose fibres, have a good insulation resistance when dry, and a low dielectric constant which is less than that of the plastics material alone.

If the surrounding cable envelope (not shown in Figure 1) is ruptured and water penetrates into its interior, the cellulose fibres have a dual function of impeding, by their swelling the progression of the water along the cable and, through a lowering of their electric resistance, sharply increasing the leakage current and thus signalling the existence of a defect. The blocking of the flow by the swollen fibres is highly effective and limits the damage to a short length of cable. The location of the leak can be readily pinpointed by measurements of the current flow and the voltage drop along the line.

Even if the effective length of the individual projecting fibres varies somewhat about its mean value but that their density remains constant all over the length of the plastics sheath 2, the spacing of the conductor cores remains substantially unchanged during handling so that no significant variations in the shunt capacitance between the conductors occur. Stabilization of this shunt capacitance at a predetermined magnitude is essential for the suppression of cross-talk between circuits of a quad including the conductor pair of Figure 1, as more fully discussed hereinafter with reference to Figure 4.

Fibres of the dimensions given above have a weight of about 400 grams per kilometre of conductor.

The sheath is not extruded but is produced by the melting of polyethylene powder, as described and claimed in the abovementioned British Patent No. 1,408,068, which also contributes to the maintenance of the desired symmetry inasmuch as any irregularities in the coating process tend to be distributed at random around the conductor axis; on the other hand, any irregularity of an extrusion nozzle leads to a distinct eccentricity of the sheath. The interpenetrating fibres resist any relative axial shifting of the conductors which in the absence of the fibres could occur during handling, thereby unbalancing the circuit.

Figure 2 shows two symmetrical pairs 25 of the type illustrated in Figure 1 within a common, flexible envelope illustrated diagrammatically at 26.

Figure 3 shows a star quad 25' disposed within an envelope 26; the star quad 25' consists of four insulated conductors 24 twisted about a common axis; in cross-section the conductor axes lying at the corners of a square. In this instance, the fibres 21 interpenetrate not only in the spaces between adjoining conductors but also in a central channel 27- which would be completely empty in a quad composed of conventionally sheathed conductors including those with paper wrappings. While such wrappings could form a barrier between adjoining conductors, they would not swell sufficiently to block the flow of water in the vicinity of the cable axis.

Envelope 26 may, of course, embrace a multiplicity of symmetrical pairs 25 and/or star quads 25.

We shall now refer to Figure 4 for a discussion of the part played by the various shunt capacitances in a quad forming three signalling circuits. In Figure 4 the four conductors are designated a, b, c and d; the interconductor capacitances are Cae, Cube) Cad and Cbd; and the shunt capacitances with reference to ground are C,, Cbo, C and Cdo. The first circuit consists of wires a (outgoing) and b (incoming), the second circuit consists of wires c (outgoing) and d (incoming), and the third or phantom circuit consists of wires a, b (outgoing) and c, d (incoming).We can then define the cross-talk among those circuits in terms of three parameters, namely a factor kl relating to the first and second circuits, a factor k2 relating to the first and third circuits and a factor k, relating to the second and third circuits. These parameters are given by the following equations: kl=Cae+CbdCbeCad CbaCao kZ=Cbc+Cbd-Cac-Cad+ --- 2 Ci.oCco k3 = Cad + Cbd C^e Cbe + 2 In the ideal case, k1=k2=k3=O.

In practice, deviations from this ideal case are determined by the following parameters indicated in Figure 5: dielectric constants B, eb, e,, d wire radii ra', rb', rc', rd' outer radii of insulation ra", rb", rc", rd" eccentricites Ra, Rb, Rc, Rd of wire axes angular spacing rua, ab, ac, ad of axial planes.

The conductor insulation according to -che present invention ensures the essential constancy of the foregoing parameters over the entire length of the cable.

A cable according to the invention can be manufactured in a relatively simple manner and is lighter as well as more flexible than those of the paper-insulated type while being also considerably easier to splice with the aid of automatic equipment. The risk of unravelling, as can happen with paper wrappings, is eliminated.

WHAT WE CLAIM ZS:- 1. A telephone cable element formed of at least one pair of helically twisted conductors, each of these conductors being enveloped in an insulating sheath made of a synthetic plastics material in which are embedded a plurality of fibres projecting all around the sheath, which fibres are about 25 denier and have a mean length, constant over the entire length of said conductor, about a millimetre, the weight of these fibres embedded in the sheath which envelops each conductor being about 400 grams per kilometre of conductor.

2. An element as claimed in claim 1 in which the thickness of said sheath is about 0.2 mm.

3. An element as claimed in claim 1 or 2, in which the fibres are hydrophilic.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   constant all over the length of the plastics sheath 2, the spacing of the conductor cores remains substantially unchanged during handling so that no significant variations in the shunt capacitance between the conductors occur. Stabilization of this shunt capacitance at a predetermined magnitude is essential for the suppression of cross-talk between circuits of a quad including the conductor pair of Figure 1, as more fully discussed hereinafter with reference to Figure 4.

   Fibres of the dimensions given above have a weight of about 400 grams per kilometre of conductor.

   The sheath is not extruded but is produced by the melting of polyethylene powder, as described and claimed in the abovementioned British Patent No. 1,408,068, which also contributes to the maintenance of the desired symmetry inasmuch as any irregularities in the coating process tend to be distributed at random around the conductor axis; on the other hand, any irregularity of an extrusion nozzle leads to a distinct eccentricity of the sheath. The interpenetrating fibres resist any relative axial shifting of the conductors which in the absence of the fibres could occur during handling, thereby unbalancing the circuit.

   Figure 2 shows two symmetrical pairs 25 of the type illustrated in Figure 1 within a common, flexible envelope illustrated diagrammatically at 26.

   Figure 3 shows a star quad 25' disposed within an envelope 26; the star quad 25' consists of four insulated conductors 24 twisted about a common axis; in cross-section the conductor axes lying at the corners of a square. In this instance, the fibres 21 interpenetrate not only in the spaces between adjoining conductors but also in a central channel 27- which would be completely empty in a quad composed of conventionally sheathed conductors including those with paper wrappings. While such wrappings could form a barrier between adjoining conductors, they would not swell sufficiently to block the flow of water in the vicinity of the cable axis.

   Envelope 26 may, of course, embrace a multiplicity of symmetrical pairs 25 and/or star quads 25.

   We shall now refer to Figure 4 for a discussion of the part played by the various shunt capacitances in a quad forming three signalling circuits. In Figure 4 the four conductors are designated a, b, c and d; the interconductor capacitances are Cae, Cube) Cad and Cbd; and the shunt capacitances with reference to ground are C,, Cbo, C and Cdo. The first circuit consists of wires a (outgoing) and b (incoming), the second circuit consists of wires c (outgoing) and d (incoming), and the third or phantom circuit consists of wires a, b (outgoing) and c, d (incoming).We can then define the cross-talk among those circuits in terms of three parameters, namely a factor kl relating to the first and second circuits, a factor k2 relating to the first and third circuits and a factor k, relating to the second and third circuits. These parameters are given by the following equations: kl=Cae+CbdCbeCad CbaCao kZ=Cbc+Cbd-Cac-Cad+ ---

   2 Ci.oCco k3 = Cad + Cbd C^e Cbe + 2 In the ideal case, k1=k2=k3=O.

   In practice, deviations from this ideal case are determined by the following parameters indicated in Figure 5: dielectric constants B, eb, e,, d wire radii ra', rb', rc', rd' outer radii of insulation ra", rb", rc", rd" eccentricites Ra, Rb, Rc, Rd of wire axes angular spacing rua, ab, ac, ad of axial planes.

   The conductor insulation according to -che present invention ensures the essential constancy of the foregoing parameters over the entire length of the cable.

   A cable according to the invention can be manufactured in a relatively simple manner and is lighter as well as more flexible than those of the paper-insulated type while being also considerably easier to splice with the aid of automatic equipment. The risk of unravelling, as can happen with paper wrappings, is eliminated.

   WHAT WE CLAIM ZS:- 1. A telephone cable element formed of at least one pair of helically twisted conductors, each of these conductors being enveloped in an insulating sheath made of a synthetic plastics material in which are embedded a plurality of fibres projecting all around the sheath, which fibres are about 25 denier and have a mean length, constant over the entire length of said conductor, about a millimetre, the weight of these fibres embedded in the sheath which envelops each conductor being about 400 grams per kilometre of conductor.
2. 2. An element as claimed in claim 1 in which the thickness of said sheath is about 0.2 mm.
3. 3. An element as claimed in claim 1 or 2, in which the fibres are hydrophilic.
4. 4. An element as claimed in claim 3 in which the fibres are cellulosic.
5. 5. An element as claimed in claim 1, 2, 3 or 4 in which the sheaths consist of expanded plastics.