Optical waveguides – Optical transmission cable
Patent
1994-10-25
1996-04-30
Lee, John D.
Optical waveguides
Optical transmission cable
385101, G02B 648
Patent
active
055132928
DESCRIPTION:
BRIEF SUMMARY
This invention relates to optical cables for use in optical transmission systems, and especially to cables for use in overhead optical transmission systems in which the cable is supported along the route of the system by means of towers, masts or other upstanding supports that are also employed to support electrical power cables.
In systems of this kind it is the general practice to earth the optical cable or cables at the towers, masts or other supports (hereinafter simply referred to as towers). When the electrical power lines are on load, electrical currents may be capacitively induced on the optical cable due to the distributed capacitance between the cable and the power lines. The voltage induced on the optical cable will reach a maximum at mid span between the towers, while the current flowing along the cable will be greatest in the region of the towers. Under dry conditions the induced currents will be relatively small due to the relatively high longitudinal resistance of the cable, e.g. in the region of 10.sup.12 Ohm m.sup.-1 but under wet conditions when the surface resistance of the cable is much lower, e.g. in the region of 10M Ohm m.sup.-1, much higher currents will be induced. Joule heating of the cable surface by the induced currents can cause a short length of the cable surface 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 on the cable is dropped across the short dry band due to its high longitudinal resistance, and so called "dry-band arcing" may occur which can cause severe damage to the cable.
It is possible to overcome the problem of dry-band arcing by providing the optical cable with a longitudinally extruding electrically conductive path. However, an optical cable having such a longitudinally continuous electrically conductive path has the disadvantage that it cannot be installed safely between towers of an overhead electric power transmissions line that is on load in view of the danger of it touching one of the transmission lines; moreover, it is not always possible or desirable to interrupt the electrical power transmitted by the overhead electric power transmission line for a time sufficient to enable such an optical cable to be installed.
It is an object of the present invention to provide an improved method of installing in an overhead power transmission line which is on load an optical cable which has an overall protective sheath of electrically insulating material and which has, extending throughout substantially the whole of its length, at least one longitudinally continuous path sufficiently electrically conductive to carry along the cable any currents that may be capacitively induced.
According to the invention, the improved method comprises supporting an optical cable, which has an overall protective sheath of electrically insulating material, in long lengths between towers, employed to support at least one electric power transmission line which is on load and, after the optical cable has been so installed and whilst the overhead electric power transmission line is on load, providing on or in the supported optical cable at least one longitudinally continuous path which extends throughout substantially the whole length of the supported optical cable and which is sufficiently electrically conductive to carry along the cable any currents that may be capacitively induced.
During installation of the optical cable, the optical cable may also be suspended at least one position intermediate of its ends by a tower support employed to support the overhead electric power transmission line.
Preferably, but not necessarily, at each end of the supported optical cable and, if desired, at one or more than one of said intermediate positions at which the optical cable is suspended from a tower the or each longitudinally continuous electrically conductive path on or in the optical cable is directly or indirectly electrically (e.g. capacitively) connected to the tower, mast, or other upstanding support to wh
REFERENCES:
patent: 4776665 (1988-10-01), Oestreich
patent: 5291573 (1994-03-01), Yoshida et al.
patent: 5317665 (1994-05-01), Herrebrugh
patent: 5325457 (1994-06-01), Bottoms, Jr. et al.
"The development of Wrap-on Optical cables for use on the earthwires of high voltage power lines" Carter et al. Central Electric Research Lab. pp. 1-6 Aug. 1988.
Knight Ian G.
Rowland Simon M.
BICC Public Ltd. Co.
Lee John D.
Miller Anthony D.
Nath Gary M.
Palmer Phan T. H.
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