GB2084757A - Overhead cables or earth conductors containing telecommunication elements - Google Patents

Abstract

Overhead power cables and earth conductors (10) comprise optical fibre telecommunication elements (11) in a conductive tube (12) surrounded by reinforcing elements (14) and conductive elements (16), at least the outermost surfaces of the tube (12) and elements (14, 16) being formed of the same metal or of metals which are electro-chemically compatible to reduce or prevent corrosion thereof. <IMAGE>

Description

SPECIFICATION Overhead cables or earth conductors containing telecommunication elements The present invention relates to overhead power cables and earth conductors, which contain telecommunication elements.

Overhead power cables are suspended, by means of insulators, from supports that are spaced apart one from the other. Uninsulated cables that are known in the art as "earth conductors" can, in their turn, be suspended to run above the overhead power cables. The earth conductors are directiy hooked onto the supports in a non-insulated way.

The purposes of the earth conductors is to protect the power cables from lightning strikes (which, in the event pass to earth through the earth conductors and the supports).

For some time, it has been known to include in overhead power cables and earth conductors, telecommunication elements, in particular co-axial pairs for transmitting signals through the pulse code modulation system.

Such a co-axial pair may have an outer diameter of only a few millimeters, and it is because of this very small size that such elements can be disposed, singly or in a plurality, inside overhead power cables or earth conductors in which there is a crown of resistant and/or conductive elongate elements. The resistant (mechanically reinforcing) elongate elements are generally steel wires, whereas the conductive elongate elements are, for example, aluminium. With the ever more widespread use of optical fibres in the field of telecommunications, and because of the similarity between what occurs therein and what occurs in co-axial pairs, and further because of their small diameters, it has recently been suggested to include optical fibres as the telecommunications elements inside overhead power cables and earth conductors. One such construction is described, for example, in U.K.Patent Specification No. 2,029,043A. In this construction, optical fibres are loosely disposed inside an aluminium hermetic tube, surrounded by resistant cylindrical elements of steel, or in an aluminium alloy. In fact, in the art generally, the resistant elements and the conductive elements are always formed out of different metals or metal alloys.

This use of juxtaposed different metals and alloys gives rise to a problem in that, because of electrochemical effects, corrosive processes tend to occur.

When corrosion occurs, perforations form which permit water to penetrate in between the fibres, and to spread along the entire lengths of the fibres.

Moreover, such water is likely to hasten the propagation of any micro-cracks in the optical fibres, especially when the latter are under stress and this reduces the efficiency of the optical transmission.

Optical fibres cannot be stretched without risk of damage, and stretching can occur in the course of instailing the fibres themselves in the cables or earth conductors from overhead supports. As appreciated, such stretching will increase the stress in the optical fibres and promote micro-cracks.

We have now devised a construction by which this problem can be reduced or overcome.

According to the invention, there is provided an overhead power cable or earth conductor, which comprises one or more optical fibres enclosed in the longitudinal cavity of a fluid-tight conductive tubularm element, surrounded by elongate resistant (reinforcing) and conductive elements, wherein said tubular conductive element, the resistant and the conductive elements are composed, at least at their peripheries, of metals that are electro-chemically compatible with each other.

In the cables and earth conductors of the present invention, the optical fibres may be bare fibres, or covered by at least a primary coating. The resistant (mechanically reinforcing) elements and the conductive elements may be tubular or wire-shaped, and they are constituted (at least at their outer surfaces) of materials that are electro-chemically compatible with each other. The cables and earth conductors are preferably so constructed as to be highly inextensible, to prevent any excessive elongation during their installation, whilst still preserving adequate flexibility.

In one preferred form of the present invention, cylindrical resistant elements are used in the form of rods having a steel core clad with aluminium, and both the tubular conductive element and the elongate conductive elements are made of aluminium.

Preferably, the elongate resistant elements and the elongate conductive elements constitute together an anti-torsional structure.

In order that the invention may be more fully understood, one embodiment thereof wili now be described, by way of example only, with reference to the accompanying drawing which is a perspective cut-away view of one end of a cable or earth conductor.

Referring to the drawing, the cable or earth conductor 10, comprises one or more fibres 11, enclosed in the longitudinal cavity of a fluid-tight tubular conductive element 12. The latter can be formed in any suitable manner, for example, by extrusion or by longitudinally winding a tape of conductive material and welding its edges. The tubular conductive element 12 is surrounded by a first crown 13 of elongate resistant elements 14, in this particular case of a cylindrical shape.

The first crown 13 is surrounded by a second crown 15 of elongate conductive elements 16, which in this particular case are also cylindrical. The elements 12, resistant cylindrical elements 14 and conductive cylindrical elements 16 are, at least with respect to their outer surfaces, made of electrochemically compatible metals.

The elongate resistant elements and the elongate conductive elements could be disposed with respect to each other, in a way that is different from that indicated in the drawing. For example, they could be disposed in an alternate manner on the same crown.

In one preferred arrangement, the conductive cylindrical elements 16 and the conductive tubular element 12 are made entireiy of aluminium, while the resistant cylindrical elements 14 are steel rods 17, covered with an outermost layer 18 of alumi nium. Wires, having a steel core and an aluminium covering, and which are commercially available underthe name "Allumoweld", have proved to be suitable for the above purpose.

All the elements 14 can also be constituted, for their entire thickness, out of a single material which is the same as, or electro-chemically compatible with, that of the elements 12 and 16. For example, the elements 12, 14, 16, could entirely be made of aluminium alloy; or else they could be made only on their outermost layer, of aluminium alloy.

The first crown 13 and the second crown 15 are, as indicated in the drawing, helicoids 13 and 15, respectively. The resistant cylindrical elements 14 and the conductive cylindrical elements 16 are wires wound helicoidally. The outer helicoid 15 is wound on the inner one- with more or less the same inclination, with respect to the X-X axis of the cable, but in the contrary sense. The system constituted by the inner helicoid 13 and the outer helicoid 15 is an anti-torsional system.

This anti-torsionality is achieved, as those skilled in the art already know, by opportunely equilibrating the following parameters: S = (section of each single wire of the helicoids) = = (helix pitch) = = (average diameter of the helices) E = (Young's modulus) G = (Modulus of tangential elasticity).

In this way, the cables and earth conductors of the invention can be provided with the desired characteristics of inextensibility and good flexibility, and also the ability to withstand some twisting without incurring any substantial damage.

It will be appreciated that, with the aforesaid metals being electro-chemically compatible, in the presence of ambient water as electrolyte, no or very little e.m.f. is generated and as a result no or negligible corrosion of any of the metals occurs.

Claims (5)

1. An overhead power cable or earth conductor, which comprises one or more optical fibres enclosed in the longitudinal cavity of a fluid-tight conductive tubular element, surrounded by elongate resistant (reinforcing) and conductive elements, wherein said tubular conductive element, the resistant and the conductive elements are composed, at least at their peripheries, of metals that are electro-chemically compatible with each other.

2. A cable or earth conductor according to claim 1, wherein said elongate resistant elements are rods having a steel core covered with aluminium, and wherein both the conductive tubular element and the elongate conductive elements are of aluminium.

3. A cable or earth conductor according to claim 1, wherein the resistant elongate elements are rods having a steel core covered with aluminium alloy, and both the conductive tubular element and the elongate conductive elements are of aluminium alloy.

4. A cable or earth conductor according to any of claims 1 to 3, wherein the elongate resistant elements and elongate conductive elements together constitute an anti-torsional structure.

5. An overhead power cable or earth conductor substantially as herein described with reference to the accompanying drawing.