GB2230109A

GB2230109A - Core for an optical fibre cable or wire

Info

Publication number: GB2230109A
Application number: GB9006707A
Authority: GB
Inventors: Franco Galimberti; Ubaldo Vallauri
Original assignee: Pirelli Cavi SpA; Cavi Pirelli SpA
Current assignee: Pirelli and C SpA
Priority date: 1989-03-24
Filing date: 1990-03-26
Publication date: 1990-10-10
Anticipated expiration: 2010-03-26
Also published as: GB2230109B; IT1228878B; GB9006707D0; ES2022008A6; BR9001647A; AU5204290A; NZ233009A; AU614462B2; IT8919900A0

Abstract

The core comprises a central reinforcing member 1 formed of reinforcing elements such as glass fibres embedded in a polyphenylene sulphide plastics material and a layer 2 of a polyether-ether-ketone extruded over the member 1 and provided with passages 3 for accommodating optical fibres 5. The plastics material of the central member 1 has a lower melting point than that of the layer 2 and the core may include (Figure 2, not shown) a metal sheath (11) to carry electric signals. <IMAGE>

Description

CORE FOR AN OPTICAL FIBRE CABLE OR OVERHEAD GROUND WIRE The present invention relates to a core for an optical fibre cable or overhead ground wire.

Overhead ground wires of aerial systems for distributing electrical energy have in recent times also been used to accommodate optical fibres for the transmission of optical signals. However, transient situations at high temperature are possible when said overhead ground wires are struck by lightning carry a short-circuit current.

Under this situation it is known that in the -ddliy innermost region of the overheat ground nlle it is possible to reach temperatures even in the order of 3000C.

In the optical fibre telecommunication cables as well, the occurrence of a transient situation at high temperature cannot be excluded, due to accidents taking place in the environment in which the cables are positioned or as a result of possible short circuits in the conductors feeding optoelectronic repeaters which may be incorporated therein.

Support structures, or cores, for known optical fibre cables comprise a reinforcing member in the form of a rod made of plastic material and including mechanically tensile resistant fibres and/or threads occupying the radially innermost position in the structure, surrounded by a plastics layer or matrix provided with passages or hollows extending along the longitudinal axis of the matrix and in which the optical fibres are loosely accommodated.

Also known are overhead ground wires and optical fibre cables incorporating the above described structure.

Known overhead ground wires which carry optical fibres have the following structure.

At the longitudinal axis of the overhead ground wires there is a reinforcing member consisting of a mad made t-t plastic material and incorpcrat .!g mechanically tensile resistant and electrically nonconductive threads or fibres, for example fibre glass.

A matrix or tubular layer, disposed around the rod, is made of plastic material and is provided with passages or hollows extending along the overhead ground wire axis, for example forming an open or closed helix with respect to said axis. Loosely housed in these passages or hollows are the optical fibres.

In particular the passages provided in said matrix or tubular layer may consist of grooves formed in the radially outermost surface of the same.

The matrix is surrounded by a metal covering or envelope comprising a sheath made for example of aluminium and around said sheath there are layers formed by windings of mechanically tensile resistant metal elements such as wires and/or metal rods and/or metal quoins and/or metal straps which together form the mechanically tensile resistant structure of the overhead ground wire adapted to bear the mechanical stresses including those arising when a lightning strikes the overhead ground wire itself.

Known optical fibre cables to which the present invention refers have a structure which can differ from the one described above in that the mechanically tensile resistant treads or fibres incorporated ir the plastics rod can also be made of metal and in that the metal envelope need not be provided with the layers of wire and the like wrappings around the sheath.

In the above described known optical fibre overhead ground wires said mechanically resistant structure, besides performing the function of exerting a mechanical resistance, also accomplishes the task of dissipating heat generated therein due to the shortcircuit current produced by a lightning striking, in order to prevent the temperature from reaching such high-values that the plastics support structure of the optical fibres incorporated therein may be damaged.

The thicker the mechanically resistant structure of the overhead ground wire is, the higher the heat dissipation.

Present optical fibre overhead ground wires have a mechanically resistant covering, or armour, much greater in thickness than it would be necessary for resisting the mechanical stresses applied thereto, so as to allow heat generated by the short-circuit current originating from a lightning, to be dissipated. This dissipation must be sufficient to avoid the plastics support structure of the optical fibres housed therein being damaged.

As a result known overhead ground wires, due to the oversizing of their mechanically resistant armour, have a very high weight per unit length and low flexibility. Their stringing in aerial systems for the distribution of electric energy is therefore difficult. In addition, due to the high weight of the known optical fibre overhead ground wires, the support structures for the overhead ground wires must be oversized as well.

A further drawback present in known optical fibre overhead ground wires and known optical fibre cables having a core as described above is the possible presence of unevennesses on the surface of the passages, or hollows, in the layer around the reinforcing member which may cause damage to the optical fibres due to microbending. In fact on the occurrence of this damage overhead ground wires become unserviceable as regards their capability of transmitting optical signals and optical fibre cables become unserviceable as well.

An object of the present invention is to provide an improved core comprising a central reinforcing member comprising a plastics material and having a layer of polymeric material extruded thereover and provided with passages extending along the length of the core, which will allow the above mentioned drawbacks to be avoided or at least reduced. That is to enable overhead ground wires incorporating the core to have a lower weight per unit length and greater flexibility and to reduce the possibilities of the surfaces of the passages being formed unevenly.

The invention provides a core for an optical fibre cable or overhead ground wire comprising a central reinforcing member having a layer of a polymeric material having a melting temperature not lower than about 3400C extruded thereover, said layer being provided with passages extending along the core for accommodating optical fibres, and said reinforcing member comprising a plastics material having a melting temperature lower than the melting temperature of the polymeric material of the layer and a gas emission temperature not lower than said temperature.

The invention also includes a core for an optical fibre cable or overhead ground wire comprising a central reinforcing member having a layer of a polyether-ether-ketone extruded thereover, said layer being provided with passages extending along the core of accommodating optical fibres, and said reinforcing member comprising a plastics material having a melting temperature lower than the melting temperature of the polyether-ether-ketone and a gas emission temperature not lower than said temperature. In this case preferably the - plastics material has a gas emission temperature not lower than about 3400C.

The plastics material preferably has a melting temperature of at least about 2800C.

In order that the invention may be well understood, two embodiments thereof, which are given by way of example only, will now be described with reference to the accompany drawing, in which: Fig. 1 is a sectional view of an optical fibre overhead ground wire; and Fig. 2 is a sectional view of an optical fibre cable.

As shown in figure 1 the structure of the optical fibre overhead ground wire is as set forth hereinafter starting from the inner part thereof and going towards the outside.

The radially innermost element in the overhead ground wire is a rod or reinforcing member 1 made by impregnating mechanically resistant and electrically non-conductive threads or fibres, for example fibre glass, threads and/or fibres of an aromatic polyamide and the like, with plastic material.

Disposed around the rod 1 and in contact therewith, is a matrix or tubular layer 2 formed by plastics extrusion and generally provided with hollows, passages, or extending in a longitudinal airection to the matrix or tubular layer 2 and, in Ine particular embodiment shown in Fig. 1, with grooves 3 (a particular embodiment of hollows) extending in a longitudinal direction and recessed in the radially outermost surface of the matrix or tubular layer 2.

Preferably grooves 3 extend in a helical form with respect to the longitudinal axis of the matrix or tubular layer 2 and said helix can be either of the closed. type or of the open type that is formed with alternate S-shaped and Z-shaped lengths.

Loosely housed in each groove 3 is at least one optical fibre 5. At least one tape wrapping (not shown in the figures) made of a polyamide for example, closes grooves 3. A metal envelope covers the radially outermost surface of said tape wrapping.

This metal envelope comprises a sheath 4, preferably made of metal, for example aluminium, and a number of metal wires 6 disposed around the sheath so as to form coaxial and superposed layers forming together the mechanical resistant armour of the overhead ground wire. In particular the number of said metal wires 6 and their section is selected so as to realise a structure offering only mechanical resistance to the mechanical stresses to which the overhead ground wire can be submitted in the conditions of use it is intended for.

Alternatively, instead of the metal wires 6 it is possible to use metal rods, metal quoins or metal straps in order to form the mechanically resistant armour of the overhead ground wire.

An essential feature of this embodiment is that the plastic material of which the matrix or tubular layer 2 provided with hollows for loosely housing the optical fibres is made, is a polyether-ether ketone of extrudable type and that the plastic material forming the rod 1 constituting the reinforcing element for said matrix has a decomposition temperature with gas emission not lower than 3400C.

A polyphenylene sulphide is one example of the plastic material adapted for rod 1.

One example of polyether-ether ketone of extrudable type of which the matrix or tubular layer 2 is made is the one having the following features: - Apparent viscosity determined with a capillary rheometer at 3800C and at a cutting speed of 3000 sec'l 200 to 300 pascal x sec - density 1.25 to 1.35 gr/cm3 - melting temperature 340 - 3700C - minimum temperature of the melt material coming out of the extruder 345"C One example of the polyphenylene sulphide of which the rod 1 is made is Lhe one having the following features:: - Apparent viscosity determined with a capillary rheometer at 2900C and at a cutting speed of 3000 sec-l 100 to 200 pascal x sec - density 1.30 to 1.40 gr/cm3 - melting temperature 280 - 3100C - decomposition temperature producing gas formation 3400C.

The cable shown in Fig. 2, starting from the inside and going outwardly has a rod 7 the characteristics of which are the same as previously set fbrth with reference to the rod 1 of the optical fibre overhead ground wire shown in Fig. 1, except that the mechanically resistant threads or fibres included in the rod can also be electrically conductive, for example metal wires.

Extending around the rod 7 is a matrix or tubular layer 8 formed by extrusion and having the same features as the matrix or tubular layer 2 of the optical fibre overhead ground wire 1.

The matrix or tubular layer 8 is generally provided with passages, or hollows, extending longitudinally to the cable and in the particular embodiment shown in Fig. 2 said hollows consist of grooves 9 extending longitudinally to the cable for example in the form of a helix and are recessed in the radially outermost surface of the matrix or tubular layer 8.

Loosely housed within each groove 9 is at least one optical fibre 10 and said grooves are closed outwardly by at least one tape wrapping made of a polymeric material (not shown in Fig. 2) and the whole is covered with a metal covering or envelope, 11 in the form of a metal sheath, for example of copper or aluminium, which can also perform the function of an electric conductor for powering optoelectronic signal repeaters for signals transmitted along the cable.

While not shown in Fig. 2, in the case of submarine cables, the metal envelope 11 comprises further reinforcement, for example a number of metal wires, or the like, disposed around the sheath so as to form coaxial and superposed layers which together create the structure offering mechanical resistance to the stresses applied during the cable laying or recovery.

A layer of polymeric material 12 having electrical insulation properties is disposed around the metal envelope 11.

From the above described particular embodiments of an optical fibre overhead ground wire and optical fibre cable and from the following considerations it will be apparent that they achieve the intended purposes as stated above.

The matrix or tubular layer 2 or 8 provided with the passages or hollows loosely accommodating the optical fibres is made of a polyether-ether ketone, that is a polymeric material having a melting temperature not lower than 3400C, i.e. much higher than the temperature of 3000C which can normally be reached in the radially innermost region of any overhead ground wire (i.e. even one not incorporating optical fibres) when submitted to a short circuit caused by a lightning.Therefore an overhead ground wire in accordance with the embodiment, unlike the known overhead ground wires which incorporate optical fibres, does not need any oversizing of its mechanically resistant armour, since the melting temperature of the material of layer 2 is higher than the temperature which can be reached by effect of short-circuit currents passing through the overhead ground wire when struck by a lightning and consequently deformations of said layer due to temperature are not to be feared.

However, polyether-ether ketone, in spite of its high melting temperature, cannot provide by itself the core accommodating the optical fibres due to insufficient mechanical characteristics as it is a polymeric material.

Provision of the core with sufficient mechanical strength requires the presence of a mechanically resistant member in the form of the rod made of plastic material reinforced with mechanically resistant fibres or threads, which are electrically non-conductive in the case of optical fibre overhead ground wire. The presence of such a rod must not however cause any defect in the tubular layer of the core provided with the passages for loosely accommodating the optical fibres and in particular on the surface of said passages with which said optical fibres may be brought into contact.

For the above reason, the plastic material of which the reinforcing rod is made, besides having a melting temperature in the order of 2800C so as to allow the extruded plastic material of the layer bond thereon, should have a decomposition temperature with gas emission much higher than the melting temperature of the material of which said layer is formed and in particular a temperature not lower than 3400C.

In this manner during the manufacture by extrusion of the core no decomposition with gas formation can take place as regards the plastic material of the rod.

Therefore there is no formation of gas bubbles in the layer and consequently no unevennesses occur on the surfaces of the passages formed in the layer for accommodating the optical fibres.

It is therefore clear why the two essential elements pointed out avoid the need for any oversizing of the mechanically resisting structure for overhead ground wires and why both in the optical fibre overhead ground wires and optical fibre cables there is less risk that the optical fibres loosely housed in the passages provided in the layer of the core should be damaged, since the cause of formation of unevennesses in the surfaces of said passages is eliminated.

While some embodiments of the invention have been described and illustrated, it is understood that all modifications accessible to a person skilled in the art are intended to fall within the scope of the invention idea.

Claims

CLAIMS:

1. A core for an optical fibre cable or overhead ground wire comprising a central reinforcing member having a layer of a polymeric material having a melting temperature not lower than about 340"C extruded thereover, said layer being provided with passages extending along the core for accommodating optical fibres, and said reinforcing member comprising a plastics material having a melting temperature lower than the melting temperature of the polymeric material of the layer and a gas emission temperature not lower than said temperature.

2. A cue Zor an optical fibre cable or overhead ground wire comprising a central reinforcing member having a layer of a polyether-ether-ketone extruded thereover, said layer being provided with passages extending along the core of accommodating optical fibres, and said reinforcing member comprising a plastics material having a melting temperature lower than the melting temperature of the polyether-etherketone- and a gas emission temperature not lower than said temperature.

3. A core as claimed in claim 2, wherein said plastics material has a gas emission temperature not lower than about 3400C.

4. A core as claimed in claim 1, 2 or 3, wherein said plastics material has a melting temperature of at least about 2800C.

5. A core as claimed in any one of the preceding claims, wherein said reinforcing member comprises tensile resistant elements embedded in said plastics material.

6. A core as claimed in claim 5, wherein said plastics material comprises a polyphenelene sulphide.

7. A core as claimed in claim 5 or 6, wherein said tensile resistant elements are electrically nonconductive.

8. A core as claimed in claim 7, wherein said tensile resistant elements comprise glass fibres, or fibres of an aromatic polyamide.

9. A core as claimed in any one of the preceding claims, wherein said passages comprise grooves provided in the outer surface of said layer.

10. An optical fibre cable or overhead ground wire having a core as claimed in any one of the preceding claims, optical fibres loosely accommodated in the passages in said layer, and a metal covering around said core.

11. An optical fibre cable or overhead ground wire as claimed in claim 10, wherein said metal covering comprises a tubular sheath.

12. An optical fibre cable or overhead ground wire as claimed in claim 10 or 11, wherein said metal covering comprises stranded metal elements.

13. A core for an optical fibre cable or overhead ground wire substantially as hereinbefore described with reference to the accompanying drawings.