GB2240189A - Optical cables - Google Patents

Abstract

The invention concerns the self-supporting optical fibre cables. These are susceptible to damage by induced voltages. The invention proposes to counteract this by providing an outer sheathing layer 6 which is partially conducting. This layer is rendered conductive by the presence of conducting fibres or particles of, for example, metal or carbon therein. <IMAGE>

Description

Partially Conducting Sheaths for Self-Supporting Aerial Cables This invention relates to aerial optical fibre cables and to self supporting optical fibre cables in particular.

Self supporting, metal free, optical fibre cables have been successfully used on power lines for the last decade. However, they are susceptible to damage by induced voltages. These induced voltages seem to occur when the optical fibre cables are installed near high voltage overhead power cables. The induced voltages are thought to occur as a result of the conductance of the cable and the pollutants on it, and the capacitive impedance from the cable to the line and ground.

Electrical stress mechanisms which contribute to the aging of the outer sheath of the optical fibre cable are; corona, partial discharges in a non-uniform electric field which are often luminous and audible, and tracking, a process which produces conducting paths of localised deterioration as a result of the action of electric discharges on or close to the insulation surface.

The damage generally occurs adjacent to the cable supports.

Insulators have been incorporated into the conductor fittings to no avail.

Two approaches to solving the problem are possible; one is to use track resistant sheaths and the second is to make the sheath or the interior of the cable partially conducting and rely on capacitive effects.

The solution needs to satisfy not only the electrical requirements but also the mechanical requirements for strength and flexibility.

The track resistant sheaths appear to have worked quite well. However, a test regime which can accurately predict the lifetime performance has yet to be developed.

The present invention is concerned with the second approach.

According to the invention a cable for carrying optical fibres has an outer sheathing layer which is resistant to damage by induced voltages because it is partially conducting, the sheathing layer being rendered partially conducting by the presence of conducting fibres or particles therein.

Preferably where the conducting material is Kevlar given a surface treatment to render it conducting, steel or carbon as a filing or carbon black as particles.

The invention will now be described by way of example only with reference to the accompanying drawings, in which Figure 1 is a cross section through a typical optical fibre cable embodying the invention, and Figure 2 is side elevation of the outer sheath of the cable.

Referring to Figure 1, a core 1 is surrounded by a plurality of sheaths 2 containing optical fibres 3. The core 1 and the sheathed optical fibres 4 are held in place by an inner jacket 5 and an outer jacket 6. The inner jacket 5 and the outer jacket 6 are generally made of polymeric material. The core 1 and the sheathed optical fibres 4 are generally surrounded by a waterblocking filler 7. The purpose of the lubricant 7 is to prevent the ingress of moisture in case of cable fracture and prevent the optical fibres 2 being damaged if they come into contact with each other.

Referring now to Figure 2 also, the outer jacket 6 has been made partially conducting by including conducting particles 7 and fibres 8 of different lengths 9, 10 in the polymer 11. The long fibres 9 have a stiffening effect and the short fibres 10 have a toughening effect. The quantities and properties of long and short fibres 9, 10 and of the particles 7 can be altered until the sheath has the desired physical and mechanical properties.

The volume fraction of carbon fibres needed in the sheath to produce the desired axial electrical resistance depends on the sheath thickness and the range of fibre lengths present but is expected to be between 10 and 20% (based on experience with carbon particle loading).

The above description applies equally well to the inner jacket 5.

In making the sheath 6 partially conducting it is important that there is no degradation of the mechanical properties. The sheath material 11 should have mechanical strength of the order of 20MPa; the elongation at failure should be high enough to prevent damage on bending and the aging characteristics should be such that a service life of 25 years is possible. The sheath 6 should also be tough and resistant to abrasion e.g. by shotgun pellets. Other hazards are weathering e.g. U-V radiation acid rain and the influence of electric fields.

The loading of polymeric materials 11 with powder filler materials, to increase conductance or tracking resistance, degrades their mechanical properties. The incorporation of short fibres can improve the mechanical properties of polymers; this effect is dependant on the volume fraction of fibre, fibre length, orientation and strength of interface between fibre and matrix. To achieve high values for the stiffness and strength of a fibre composite, it is necessary to use long fibres 9, well bonded to the matrix. However, for maximum fracture toughness the opposite is true. It is desirable to have a weak interface with the matrix and the fibres to be short.

Much is to be gained by having a mix of fibre lengths or fibre types present.

In a typical example the outer jacket 6 can be made of a polymeric material such as high density polyethylene (HDPE) containing conducting particles 7 or fibres 8 of treated Kevlar or carbon. The particles 7 can be smaller than 1 u in diameter. Some particles, e.g. carbon black, tend to agglomerate making it difficult to measure the particle size.

The fibres 8 will not generally exceed 3mm in length, but should ideally be as short as possible.

Alignment of the fibres 8 is beneficial but difficult to achieve in practice.

The conducting particles 7 or fibres 8 can be made from any material which is compatible with and does not degrade the sheath material 11. Thus both the fibres and the particles can be of treated Kevlar (RTM), carbon or a metal such as steel or copper.

The incorporation of metallic particles will necessitate the addition of stabilising compounds to the sheathing polymer to prevent degradation.

Claims (8)

1. A cable for carrying optical fibres has an outer sheathing layer which is resistant to damage by induced voltages because it is partially conducting, the sheathing layer being rendered partially conducting by the presence of conducting fibres or particles therein.

2. An optical fibre cable as claimed in Claim 1 where the conducting fibres have lengths less than approximately 3mm.

3. An optical fibre cable as claimed in either of the preceding claims where the volume fraction of the conducting material in the sheathing layer is in the range of 10-20%.

4. An optical fibre cable as claimed in any of Claims 1 to 3 where the conducting material is Kevlar.

5. An optical fibre cable as claimed in any of Claims 1 to 3 where the conducting material is steel.

6. An optical fibre cable as claimed in any of Claims 1 to 3 where the conducting material is carbon.

7. An optical fibre cable as claimed in any of Claims 1 to 3 where the fibres are aligned.

8. An optical fibre cable substantially as herein described with reference to the accompanying drawings.