Optical waveguides – Optical transmission cable
Reexamination Certificate
2000-04-04
2001-08-21
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Optical transmission cable
C385S112000, C385S109000
Reexamination Certificate
active
06278825
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical fibre cable having high tracking resistance, particularly suitable for installation along high-voltage overhead lines.
BACKGROUND OF THE INVENTION
An insulating material subjected to the combined action of an electrical gradient and external agents (moisture, solar radiation, pollutants, etc.) is subject to degradation as a result of the phenomenon known as “tracking”. This is the situation affecting the material forming the outer sheath of an optical fibre cable suspended near a high-voltage overhead line. The gradient applied is the difference between the potential of the electric field around the cable and the earth, to which the cable is connected by the clamps fixing it to the supporting pylons. In the proximity of these clamps, the insulating material is subject to the maximum variation of the electric field.
When the cable is dry, the high surface resistance of the sheath prevents the flow of current, and consequently there is no evidence of degradation. Exposure to atmospheric agents, and to solar radiation in particular, causes a surface oxidation of the material which increases its affinity for water. When the sheath starts to retain water, and with it the atmospheric pollutants, the surface resistance decreases drastically and current therefore flows through the film of water coating the sheath. This flow of current produces, by the Joule effect, a local heating of the material, with evaporation of the water. The situation becomes critical when the sheath is only partially wet, so that moist areas with low electrical resistance alternate with dry areas with much higher resistance. The high conductivity gradient at the points of separation between dry and moist areas leads to the formation of electrical discharges which intensely overheat the underlying sheath, with consequent degradation of the polymer material and formation of paths (tracking) and true areas of erosion which extend rapidly until the rupture of the material is caused. In the case of polyolefin materials, the degradation is manifested by an initial melting followed by oxidation, with a consequent increase in local wettability and therefore of the number of electrical discharges, such that the polymer is caused to ignite.
The installation of self-supporting optical cables made of dielectric material in the vicinity of high-voltage (usually from 150 to 380 kV) overhead lines therefore requires the use of sheaths with high resistance to the said tracking phenomena, which at the same time have good mechanical properties and low density, so that light cables resistant to mechanical stresses can be produced.
RELATED ART
U.S. Pat. No. 4,673,247 describes optical fibre cables which are particularly suitable for installation along high-voltage overhead lines, and which have an outer sheath consisting of a polymer material (for example an ethylene-vinyl acetate copolymer) in a mixture with a hydroxide, for exale zinc hydroxide, magnesium hydroxide or, preferably, aluminum hydroxide, in quantities corresponding to between 30% and 60% by weight, and preferably approximately 50% by weight. The high quantity of hydroxide present not only has a flame proofing effect but also makes it possible to increase the resistance of the polymer to the tracking phenomena, but inevitably leads to a deterioration of the mechanical properties in terms of modulus and of elongation at break, and to a considerable increase of the density and consequently of the total weight of the cable per unit of length.
To obtain tracking resistance combined with satisfactory mechanical properties, International Patent Application WO 93/05424 describes an optical fibre cable wherein the outer sheath is formed by a composition consisting of linear polyethylene, preferably medium-density linear polyethylene (MDPE), mixed if necessary with low-density branched polyethylene (LIDPE), containing 15% to 30% by weight of magnesium hydroxide or aluminum hydroxide. However, the Applicant has found that compositions of this type, containing low quantities of hydroxide, are capable of passing the ordinary tests for measuring tracking resistance and erosion, such as IEC Standard 587 (1984), only with low potential differences (up to 3 kV), while the results are unsatisfactory in more severe conditions (potential differences up to 4.5 kV).
SUMMARY OF THE INVENTION
EP-490 803 relates to an optical fiber cable, particularly suitable to resist local impacts (e.g. shots from hunters). In that cable, a bundle of tubes containing the optical fibers arranged around a central element resistant to traction and compression and wound with a synthetic strip. That assembly is enclosed by a polyurethane covering which should absorb the impact energy. Around the covering, a layer of aramide fibers is arranged which are in turn hold by two layers consisting of aramide strips wound in opposite directions, which should further improve resistance to impacts. The layers of aramide strips are bold in place by a nylon tape. An external sheath is provided around the tape made from an antitracking extruded material.
The Applicant has now found that it is possible to produce an optical fibre cable provided with both optimal mechanical properties and high tracking resistance, by applying an extruded double-layer outer sheath, wherein the first inner polymer layer is substantially without tracking resistance, while the second outer polymer layer is designed in such a way as to mainly impart high tracking resistance. In this way it is possible to obtain a light optical cable with good mechanical resistance, particularly suitable for installation along high-voltage overhead lines.
In a first aspect, the invention therefore relates to an optical fibre cable having high tracking resistance, comprising at least one optical fibre and a polymer-based outer sheath, characterized in the said sheath is an extruded double-layer sheath, comprising a first inner polymer layer substantially devoid of tracking resistance, and a second outer polymer layer having high tracking resistance and containing an inorganic oxide or hydroxide in an amount of at least 40% by weight with respect to the total weight of a polymeric mixture.
The term “polymer layer substantially devoid of tracking resistance” is used to indicate a polymer-based material which, when subjected to the test for measuring the resistance to tracking and erosion according to IEC Standard 587 (1984), Criterion A, Method 1, is classed in Class 1A 0 (none of the five test specimens pass the test if subjected to a potential difference of 2.5 kV for less than 6 hours).
The term “polymer layer having high tracking resistance” is used to indicate a polymer-based material which, when subjected to the test for measuring the resistance to tracking and erosion according to IEC Standard 587 (1984), Criterion A, Method 1, is classed in Class 1A 3.5 (all five test specimens pass the test if subjected to a potential difference of 3.5 kV for at least 6 hours, while none of them passes the test if a potential difference of 4.5 kV is applied for less than 6 hours), or preferably in Class 1A 4.5 (all five test specimens pass the test if subjected to a potential difference of 4.5 kV for at least 6 hours).
REFERENCES:
patent: 4673247 (1987-06-01), Oestreich
patent: 4863983 (1989-09-01), Johnson et al.
patent: 4983677 (1991-01-01), Johnson et al.
patent: 5015693 (1991-05-01), Duchesne et al.
patent: 40 09 101 (1991-09-01), None
patent: 0 490 803 (1992-06-01), None
patent: WO 93/05424 (1993-03-01), None
patent: WO 93/07627 (1993-04-01), None
patent: WO 97/10294 (1997-03-01), None
Derwent Abstract of JP 09 133847, “Non-Metallic Self-Supporting Optical Cable has Outer Face of Water Repellent Materials”, Derwent Publications Ltd., London, GB, (1997).
International Electrotechnical Commission IEC Standard 587, “Test Methods for Evaluating Resistance to Tracking and Erosion of Electrical Insulating Materials used Under Severe Ambient Conditions”, pp. 3, 5, 7, 9, 11, 13, 15-18, (1984).
Casiraghi Flavio
Consonni Enrico
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Palmer Phan T. H.
Pirelli Cavi E Sistemi S.p.A.
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