GB2302184A - Composite cable for electric power and communication signals - Google Patents

Abstract

A composite cable 1 for transmitting both electric power and communication signals comprises an inner electrically conductive power transmitting core 3, 4, 5 provided with insulation 6, 7, 8 and an electrically conductive stranded layer 9, 11 overlying the insulation. The stranded layer 9, 11 comprises a plurality of electrically conductive material strands 9, e.g copper or aluminium wires, interspersed with strands 11 of substantially the same diameter as the wire strands which provide signal communication paths along the cable. The latter strands 11 may for example comprise optical fibres in a metal tube or coaxial transmission lines. Extruded polymeric sheath 10 is shown.

Description

Composite Cable This invention relates to a composite cable for transmitting both electric power and communication signals.

It has for many years frequently been a requirement that a single cable should be used to transmit both electric power and communication signals, such as speech signals or information data. This requirement has been achieved in a number of ways. One way is to use the electric power conductors of a cable to transmit also communication signals. Where the cable is used for transmitting alternating current power the communication signals may, for example, be transmitted as modulations of a carrier signal at a frequency which is different from the frequency of the alternating current transmitted by the cable. This has presented some difficulties in the past since combining the electric power and communication signals and separating them at the receiving end is not easy.Furthermore, the electric power transmitted may, if harmonics are present, corrupt the information which is transmitted on the communication signal.

To overcome this it has been proposed that communication signal conductors be wound in a cable separate from the power conductors, for example, around a central core power conductor and insulated therefrom. Again, however, there has been the problem with this proposal in respect of the interference between the power and communication signals.

To overcome these difficulties it has further been proposed to provide in a single cable electric power conductors and an optical path for communication signals. The optical path is not likely to be affected by electric or magnetic disturbances which may occur if a sudden pulse appears on a power conductor. One way of providing such a cable is shown in the United Kingdom patent specification No. 2213960 in which a form of underwater cable has a central core of bundled optical fibres which are placed in a tube and are surrounded by conductive metal wires. The whole is then sheathed and then supported with reinforcing strands. A variation of this is known from United Kingdom patent specification No. 2157018 where a central power conductor core is surrounded by an annular insulating material member having grooves in its perimeter in which optical fibres are located.The whole is then sheathed and enclosed in a tube which is provided with an outer protective layer.

A futher form of cable for carrying both electric power and communication signals is known from European patent specification No. 285917 where a central power conductor core is surrounded by insulation which has an outer protective layer in which a tube containing optical fibres is embedded.

A frequent problem which arises with electric cables is to how to provide a branch or junction along the length of the cable, more especially since it is useful for such connections to be made while the cable is live so that it is necessary to be able to break into the cable without interrupting the supply of power while a connection is teed off. It is also useful, when the cable allows the transmission of communication signals, to be able to make such connections without interrupting the communication flow while the jointing procedure is taking place.

It is an object of the present invention to provide a novel form of composite cable for transmitting both electric power and communication signals whereby one or more of the above described problems may be alleviated.

According to the present invention, a composite cable for transmitting both electric power and communication signals comprises an inner electrically conductive power transmitting core and an overlying electrically conductive stranded layer separated from the core by electrically insulating material, the stranded layer being itself covered by a sheath of insulating material, and the strands of the stranded layer comprising a plurality of strands of electrically conductive material and at least one strand which provides a path for communication signals.

Preferably the or each strand which provides a path for communication signals comprises a communication path housed in a tubular electrically conductive outer sheath.

In one particular embodiment of the invention the or each communication path comprises an optical fibre.

Preferably the strands are layed in the cable in such a manner that substantially no torsion is imparted to them during or after laying, for example, so as to extend along the cable in a wave like or meandering form.

One composite cable in accordance with the invention will now be described, by way of example, with reference to the accompanying drawing which shows a dressed end of the cable.

Referring to the drawing, the cable 1 comprises a central core 2 made up of three sub-core electric conductors 3, 4, 5 for transmitting three phase electric power. The conductors 3, 4, 5 are each insulated by a polymeric insulator 6 and are held together by having a wrap of tape 7 around all three conductors. The taped assembly 7 is sheathed with an elastomeric insulating material 8 into which are laid electrically conductive strands 9 to form a one strand thick stranded layer. The layer of strands 9 is surrounded by an extruded polymeric sheath 10.

The majority of the strands 9 are made of copper or aluminium wire and provide the cable with a combined neutral and earth along its length. It will be appreciated that the stranded layer thus serves in known manner as a protective layer which electrically earths any metal implement, such as a spade or pick, which accidentally penetrates the cable. Certain of the strands 9 however, shown here as strands 11, comprise welded stainless steel tubes which loosely encase optical fibres 12 to provide paths along the cable 1 for communication signals.

The strands 9 and 11 extend along the cable 1 in parallel spaced relationship and are partially embedded in the insulating material 8 so as to be held in position thereby.

In addition, the strands 9 and 11 are laid so as to follow a meandering path along the length of the cable 1. Suitably the meandering path is of a regular wave, e.g. sine wave, form.

The diameter of the strands 11 is the same as the diameter of the wire strands 9 so that there are no irregularities on the outer surface of the cable 1. Since the strands 11 comprise material of lower conductivity than the strands 9, it is necessary for design purposes to make the strands 9 of slightly larger diameter than they would be if all the strands 9, 11 were made of wire so that the resistance of the stranded layer along the length of the cable 1 is not increased by introduction of the strands 11. The laying of the strands 9, 11 along a path of regular wave form has the advantage that no torsion is applied to the strands 9, 11 during or after laying.

The meandering or waveform or wave form of the strands 9 and 11 also has the advantage that if a branch connection is to be made to the cable 1 it is possible for the strands 9, 11 to be parted to insert ajointing connector through the insulation 6, 7, 8 to connect with the three sub-cores 3, 4, 5 of the cable 1. This can be done while the cable 1 is live using well known techniques. Since there is sufficient slack due to their wave form the strands 9, 11 are not themselves broken and power feed continues while the jointing process takes place. If it is required to make a joint to one of the optical fibres 12 then there is sufficient excess length in the wave form for the tube 11 containing the fibre to be taken outside the insulating sheath 10 and for a connection to be made to the fibre 12 by a suitable coupler.

It will be appreciated that the invention provides a cable which enables both power and communication signals to be transmitted and which is able to be jointed satisfactorily and effectively without interrupting either the power or the communication signals.

Although the example described above uses optical fibres to provide the communication signal paths, it will be appreciated that in other cables in accordance with the invention the communication signal paths may be provided by other means, e.g. by coaxial or parallel-pair electrical transmission lines. It will be understood that where such electrical communication paths are used the presence of the adjacent-electrically conductive material strands of the stranded layer of which the communication paths form part assists in reducing electrical interference with the communication signals.

In addition, although the example described above is a three phase cable, it will be appreciated that a cable with more or fewer than three separate conductors, for example a three phase cable with a separately insulated neutral core can be used or a single power core cable can be used.

Furthermore it should be appreciated that the communication paths need not necessarily be laid in tubes but may be inter-layered with the other strands 9. The communication signal paths may be colour coded so that when the composite cable is broken into for jointing purposes the paths carrying different signals are readily identified.

The laying of the stranded layer in such a manner as to avoid torsion being applied to them can also have the advantage that premature ageing effects are reduced.

Claims (11)

1. A composite cable for transmitting both electrical power and communications signals comprising an inner electrically conductive power transmitting core and an overlying electrically conductive stranded layer separated from the core by electrically insulating material, the stranded layer being itself covered by a sheath of insulating material, and the strands of the stranded layer comprising a plurality of strands of electrically conductive material and at least one strand which provides a path for communication signals.

2. A cable as claimed in Claim 1 wherein the or each strand which provides a path for communication signals comprises a communication path housed in a tubular electrically conductive outer sheath.

3. A cable as claimed in Claim 1 or Claim 2 wherein the or each communication path comprises an optical fibre.

4. A cable as claimed in Claim 1, Claim 2 or Claim 3 wherein the strands are all of substantially the same cross-sectional dimensions.

5. A cable as claimed in any preceding claim in which the strands are layed in the cable in such a manner that substantially no torsion is imparted to them during or after laying.

6. A cable as claimed in Claim 5 wherein the strands are layed so as to extend along the cable in a wave like or meandering form.

7. A cable as claimed in any preceding claim in which said core comprises a plurality of sub-cores insulated from one another.

8. A cable according to any preceding claim wherein the strands are layed in parallel spaced relationship.

9. A cable according to Claim 8 wherein the strands are embedded in electrically insulating material.

10. A cable according to any preceding layer wherein the stranded layer is one strand thick.

11. A composite cable for transmitting electrical power and communication signals substantially as hereinbefore described with reference to the accompanying drawing.