GB2280455A

GB2280455A - Non-metallic rope

Info

Publication number: GB2280455A
Application number: GB9415268A
Authority: GB
Inventors: Christopher Simon Pegge
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1993-07-30
Filing date: 1994-07-27
Publication date: 1995-02-01
Also published as: GB9415268D0; GB9315783D0; EP0636744A1

Description

Ropes and the Use Thereof 2280455 This invention relates to non-metallic

ropes and their use in relation to overhead electric lines.

There is currently much activity in the addition of optical telecommunications circuits to electric highvoltage overhead lines, in most cases either by the replacement of the existing earth wire with one incorporating optical fibers or by adding an optical cable either as a self-supporting element of the line for example an "all dielectric cable, or as a wrapped-on addition to the earth wire or phase conductor. If clearances are adequate, it is possible to install optical cables onto earth wires or to install self supporting cables while the phase conductors are on load, but this requires the use of electrically insulating pulling ropes. Similarly, in systems having two independent three phase circuits, cable can be installed on phase conductors of one circuit while the other circuit is on load.

However, when the electrical power lines are on -2load, electrical currents may be capacitively induced on the pulling rope due to the distributed capacitance between the rope and the power lines. The voltages on the rope will tend to reach a maximum at mid-way between the points at which it is earthed, while the current flowing along the rope will be greatest at the earthing points. The rope will normally be earthed at the pulling tower and at a minimum of one other place on the earthed conductor. Under dry conditions the induced currents will be relatively small due to the relatively high longitudinal resistance of the rope, but under wet conditions when the resistance of the rope is much lower, much higher currents will be induced. Joule heating of the rope by the induced currents can cause a short length of the rope to become dry. usually in the region of a tower where the current is highest. When this happens the major part of the induced voltage is dropped across the short dry band due to its high longitudinal resistance. and so-called "dry-band arcing" may occur. Such dry band arcing is the same phenomenon as may occur with self supporting all dielectric optical cables, but because ambient water can penetrate ropes. the arcing may occur within the body of the rope rather than on the surface as happens with optical cables. Since the heat generated by arcing within the body is not so readily removed from the arc source, catastrophic failure of the rope may occur by burning.

Greasing of the rope is only partly effective and p 1 1 -3risks contamination that presents another risk to the electrical properties of the rope.

In accordance with one aspect of the present invention, a non-metallic rope has the interstices both within and between its strands filled with a waterblocking material and is enclosed in a close-fitting water-tight sheath of electrically insulating material.

The rope may be made of polyester, aramid or other polyamide, or other conventional fibres (but synthetic ones are preferred because they have less affinity for moisture than most natural fibres) and may be of any conventional strand structure.

The water-blocking material may be of any of the three classes conventionally used for this purposes in electric and/or optical cables, namely a) greasy materials, such as silicone greases, petroleum jellies; b) soft polymeric material, such as silicone rubbers; and C) materials that swell to form local water blocks on exposure to moisture, such as polyacrylate powders.

Materials of the last group are preferred as they are more convenient to handle, have less effect on the flexibility of the rope and do not tend to facilitate relative movement between the sheath and the body of the rope, as greasy materials may.

The sheath should ideally be hydrophobic, arcresistant, infusible and incombustible, but substantial -4departure from the ideal can be tolerated provided the risk of catching fire or of gross melting is negligible since the sheath will not be relied on to contribute to the strength of the rope. Conventional cable- sheathing materials may be used. Preferably the sheath is applied by vacuum extrusion to achieve maximum grip on the underlying part of the rope. For the same reason, rope structures with smooth surfaces are best avoided. In addition, ropes will reduce in diameter significantly under tension are not preferred since this can cause the sheath to separate from the stranding and cause failure of the rope when pulled. Example 1 A first rope in accordance with the invention comprises a core about 10mm in diameter comprising a plait of 16 strands formed around a core of three strands. All 19 strands are made of aramid fibres coated in waterswellable, material, the coated fibres being sold by Akzo Chemie under the trademark 7WARON as water-blocked TWARON fibres. This core is sheathed with 1.5mm radial thickness of the. thermoplastic rubber sold by Monsanto Ltd under the trademark SANTOPRENE by vacuum extrusion at a pressure differential of 350kPa. Example 2 This is substantially the same as Example 1 except that instead of incorporating TWARON tape the fibrous part of the core is fully impregnated with a standard cablefilling grade of petroleum jelly.

i 1 1

Claims

1 A non-metallic rope having the interstices both within and between its strands filled with a water blocking material and enclosed in a close-fitting water tight sheath of electrically insulating material.

2 A rope as claimed in claim 1 made of polyester, polyamide or other synthetic fibres.

3 A rope as claimed in claim 1 or claim 2 in which the water-blocking material is selected from a) greasy materials, such as silicone greases.

petroleum jellies; b) soft polymeric material, such as silicone rubbers; and C) materials that swell to form local water blocks on exposure to moisture, such as polyacrylate powders.

4 The use of the rope claimed in any one of the preceding claims in work for modification or refurbishment of live power line installations.

The use of the rope claimed in any one of claims 1 to 4 in work for installation of communication circuits in live power line installations.