GB2236197A

GB2236197A - Optical aerial cable

Info

Publication number: GB2236197A
Application number: GB9013440A
Authority: GB
Inventors: John Derek Smithies Hinchliffe; Alan James Peacock; Brian Anthony Perrett
Original assignee: Telephone Cables Ltd
Current assignee: Telephone Cables Ltd
Priority date: 1989-09-20
Filing date: 1990-06-15
Publication date: 1991-03-27
Also published as: GB8921270D0; GB9013440D0

Abstract

An optical aerial cable having, within an outermost plastics sheath, a plurality of longitudinal strength members and at least one coextensive optical package comprising at least one optical fibre embedded in a coating of semi-conductive material such that the electrical resistivity of the optical package is in the range 1 to 10 megohms per metre. <IMAGE>

Description

Optical Aerial Cable This invention relates to optical aerial cable, which is typically designed to be freely suspended from spaced supports which may be up to 1000 metres apart.

When non-metallic optical aerial cables, such as the cable described in our U.K. Patent Application No. 2193583A, are hung adjacent power cables, electric voltages induced across regions of the surface of the optical cable sometimes cause local breakdown and consequential tracking, leading to deterioration of the cable. The purpose of the invention is to provide a simple, inexpensive anti-tracking measure in optical aerial cable.

In its broadest aspect the invention comprises an optical aerial cable having, within an outermost plastics sheath, a plurality of longitudinal strength members and a coextensive optical package comprising a plurality of optical fibres embedded in a coating of semi-conductive material such that the electrical resistivity of the optical package is in the range 1 to 10 megohms per metre.

It is believed that the semi-conductive material in the cable is capable of reducing its susceptibility to tracking, because it prevents the maintenance of very high potential differences across parts of the cable. It provides a longitudinal current path, but its conductivity is not so great as to give rise to significant induced currents.

The term "semi-conductive" as used in this specification means partially conductive, i.e. with a conductivity intermediate that of an insulator and a conductor.

For use as an aerial cable the strength members must, of course, provide an adequate resistance to stretching of cable to prevent an undue stress being applied to the fibres when the cable is suspended between two supports spaced apart the maximum distance for which the cable is designed to be used.

The optical package may be in the form of an optical fibre ribbon wound helically around the strength members within the plastics sheath.

Preferably, however, the optical fibre package comprises a single optical fibre coated with the semi-conductive material, loosely contained in an interstitial space between the strength members or between the strength members and the plastics sheath.

Preferably, the optical package is contained loosely within a tube, and the tube is contained loosely in the interstitial space.

Advantageously, the tube contains a plurality of such optical packages, and the or each optical package is immediately surrounded by a water blocking substance.

Two alternative forms of aerial cable embodying the invention will now be described by way of example with reference to Figures 1 and 2 of the accompanying drawing, which are cross-sections through the two forms of cable.

Referring first to Figure 1, this illustrates a non-metallic aerial cable comprising four circular sectioned strength members 1 of glass-reinforced plastics material, each having a diameter of approximately 5mm stranded together around a central filler 2 of polyethylene having a diameter of approximately 2mum, and surrounded by a close fitting polyethylene or other polymer sheath 3 having a wall thickness of approximately 1.5mm.

Within the four channels 4 formed between adjacent pairs of strength members 1 and the outer sheath 3 there are located four optical fibre elements 5, each of which comprises a bundle of four coated single mode optical fibres 6 within a 2mm diameter tube 7 of a suitable polymer having a filling of grease or other water blocking compound which allows a degree of transverse movement of the fibres within the tube. Each fibre is coated with a semi-conductive plastics material, such as cross-linked polythene impregnated with carbon black particles. The composition and thickness of the coating are such that the overall resistivity of the coated fibre is between 1 and 10 megohms per metre.

The depth of the channel 4 is such that the optical fibre elements 5 are spaced from the inner surface of the surrounding sheath 3, and thereby protected against external pressure.

The interstitial spaces between the strength members 1, optical fibre elements 5 and the sheath 3 may also be filled with a suitable water-blocking compound to prevent water penetration along the cable.

The sheath itself may contain a suitable filler to provide an adequate level of resistivity.

The second cable, illustrated in Figure 2, comprises five stranded-together strength members 1, similar to those of the cable shown in Figure 1, and similarly surrounded by a polyethylene cuter sheath 3.

However, immediately beneath the outer sheath 3 there is an optical fibre ribbon 10 wound helically around the strength members 1; the ribbon material is semi-conductive, and has the same anti-tracking function as the coating of the fibres 6 of Figure 1.

The ribbon 10 replaces some or all of the units 5 of Figure 1.

In this case the dimensions of the channels formed by the interstices between adjacent strength members 1 and the inner surface of the sheath 3 are inadequate to accommodate optical fibre elements of the diameter of those of the previous embodiment without being contacted by the sheath, but optionally sixteen optical fibres 6.1 are accommodated in a single polymer tube 8, having a diameter of approximately 3.25mm, located in the central channel 9 bounded by the stranded strength members 1. The tube 8 is filled with grease or other suitable water blocking compound which permits movement of the fibres within the tube, as in the previous embodiment, and the interstices between strength members 1, the tube 8 and the sheath 3 may also be filled with a water blocking compound. The fibres 6.1 may be stranded together or disposed in a stacked 4 x 4 array, or arranged in any other convenient manner.

As with the previous embodiment the fibres 6.1 in the tube 8 are protected by the strength members against external radial pressure. Those in the ribbon 10 are protected from radial compression by their ribbon coating and the sheath 3, and from longitudinal tensile stress by virtue of their helical lay.

The strength members may, in some cases, be coated with a plastics material to give added mechanical and/or environmental protection.

Claims

1. An optical aerial cable having, within an outermost plastics sheath, a plurality of longitudinal strength members and at least one coextensive optical package comprising at least one optical fibre embedded in a coating of semi-conductive material such that the electrical resistivity of the optical package is in the range 1 to 10 megohms per metre.

2. A cable according to Claim 1, in which the optical package is an optical fibre ribbon wound helically around the strength members within the plastics sheath.

3. A cable according to Claim 1, in which the optical package comprises a single optical fibre coated with the semi-conductive material, loosely contained in an interstitial space between the strength members or between the strength members and the plastics sheath.

4. A cable according to Claim 3, in which the optical package is contained loosely within a tube, and the tube is contained loosely in the interstitial space.

5. A cable according to Claim 4, in which the tube contains a plurality of such optical packages.

6. An optical aerial cable as claimed in Claim 4 or 5 wherein the or each of said tubes is a grease filled polymer tube.

7. An optical aerial cable as claimed in Claim 6 wherein the or each tube contains a semi-conductive material so as to constitute said layer of semi-conductive material.

8. An optical aerial cable as claimed in Claim 1 wherein the or each of the optical packages are coated with a semi-conductive material thereby to form said layer of semi-conductive material.

9. An optical aerial cable as claimed in Claim 1 wherein said semi-conductive material layer comprises a binding tape or a extruded layer applied over the or each of said optical fibre units and under said plastics sheath.

10. An optical aerial cable substantially as hereinbefore described with reference to either Figure 1 or Figure 2 of the accompanying drawings.

10. An optical aerial cable as claimed in any one of the preceding claims wherein said strength members are cylindrical and the or each optical fibre unit is loosely contained in an interstitial space between the strength members or between the strength members and said plastics sheath.

11. An optical aerial cable substantially as hereinbefore described with reference to either Figure 1 or Figure 2 of the accompanying drawings.

Amendments to the claims have been filed as follows 1. An optical aerial cable having, within an outermost plastics sheath, a plurality of longitudinal strength members, at least one optical fibre contained loosely adjacent the strength members, and a semi-conductive layer whose properties are such that the electrical resistance of the cable is in the range 1 to 10 ,negohms per metre.

2. A cable according to Claim 1, in which the optical fibre is an optical fibre ribbon wound helically around the strength members within a plastics sheath.

3. A cable according to Claim 1, in which a single optical fibre coated with the semi-conductive material is loosely contained in an interstitial space between the strength members or between the strength members and the plastics sheath.

4. A cable according to Claim 3, in which the or each optical fibre is contained loosely within a tube, and the tube is contained loosely in the interstitial space.

5.. An optical aerial cable as claimed in Claim 4 wherein there are a plurality of said tubes each containing at least one optical fibre, the or each of said tubes being a grease filled polymer tube.

6. An optical aerial cable as claimed in Claim 5 wherein the or each tube contains a semi-conductive material so as to constitute said layer of semi-conductive material.

7. An optical aerial cable as claimed in Claim 1 wherein the or each of the optical fibres is or are coated with a semi-conductive material thereby to form said layer of semi-conductive material.

8. An optical aerial cable as claimed in Claim 1 wherein said semi-conductive material layer comprises a binding tape or an extruded layer applied over the or each of said optical fibre units and under said plastics sheath.

9. An optical aerial cable as claimed in any one of the preceding claims wherein said strength members are cylindrical and the or each optical fibre is loosely contained in an interstitial space between the strength members or between the strength members and a plastics sheath.