NZ216819A - Optical fibre cable: electric stress relief where fibre leaves h.v. field - Google Patents

Description

.•■•••• V^'Vr-sl.« C-. 2168 1 9 /7T V No.: Date: NEW ZEALAND Under the provisions of Regti- latlon 23 ;i; t;ic- .Cov^^-4^ Spcc!.1cr:c- „.:cr. cmc-datcd to 13- Dec. 19 22.

PATENTS ACT, 1953 initials Divided from no. 206,555 12 December 1983 COMPLETE SPECIFICATION PROTECTIVE BOOT FOR OVERHEAD FIBRE OPTIC CABLES I 11j**Lt986 J i We. RAYCHEM LIMITED, a company organised according to the laws of the United Kingdom of Rolls House, 7 Rolls Buildings, Fetter Lane, London EC4A 1NL, England hereby declare the invention for which-f* / we pray that a patent may be granted to at€/ue, and the method by which it is to be performed, to be particularly described in and by the following statement: - (followed by page la) 2 168 • ntn6or<23 O FIBRE OPTIC CABLE ARRANGEMENTS This invention relates to arrangements including fibre optic cables, and in particular to arrangements including fibre optic cables suitable for installation closely adjacent to, or in physical contact with high voltage equipment such as a high voltage conductor.

By high voltage equipment is meant equipment, for examplefa conductor rated at ikV or above, and may be for example a 33•<V or 400kV power conductor.

The use of a fibre optic cable in association with a high voltage conductor is known from UK Patent Application Publication Nos. 2083647A and 2101351A. These publications disclose arrangements in which a fibre optic cable extends inside a high voltage conductor and thus is not subject to its electric field gradient. The arrangements provide for the cable to leave the conductor in a metal housing and to extend through a porcelain chamber filled with insulating oil to a location at earth potential. An article in Distribution Developments (September, 1902) discloses the mounting of a fibre optic cable having an outer sheath manufactured from a non-tracking low erosion compound on an overhead conductor.

It is one object of the present invention to prevent, or at least to reduce, damage to a fibre optic cable that is to be located in the vicinity of a high voltage conductor, including the region where the cable leaves the conductor, and that accordingly is subject to flow of leakage current therealong.

It is another object of the invention to minimise other detrimental effects of leakage current or induced 4 * - f^ N •!* V* w JL J ' rr-. \ -2- RK180 NZ3 current, such as radiated interference and spurious optical signals.

The present invention provides a fibre optic cable arrangement whereby the cable may be used in close proximity to, and subject to the electric field of, high voltage equipment, for example high voltage conductors, switchgear, transformers, motors and generators, and whereby the cable may be routed through or around equipment at different potentials, including earth potential, without adversely affecting the operation of the cable.

•—\ The fibre optic cable may transmit optically encoded data, such data may be in either digital or analogue form, and such data may be used for the monitoring and control of an electrical network of which the conductor, for example, forms part. However, the cable may be employed for the transmission of data not relevant to the operation of the network, such data including telecommunications and similar signals normally carried by fibre optic links. Furthermore, it is envisaged that the fibre optic cable could itself serve as a sensor to monitor the operating parameters of an electrical system, including those of the conductor.

In accordance with one aspect of the present invention, there is provided an arrangement comprising high voltage equip-r-s ment, a fibre optic cable that is located in a region such ^ that it is exposed to the electric field of the equipment, and electrical stress control means that is located so as to receive the fibre optic cable from said region and to cooperate with the cable such that the effect of said electric field is substantially insufficient to cause damage to the fibre optic cable.

Advantageously, the jacket of the fibre optic cable is of a substantially electrically non-tracking material,and may. electrically insulating, substantially non-wetting and// weather resistant. Thus, if 1 ' tJUNw; 1 r 2168 1 9 - 3 - Mc-tsuim such a jacket, or coating, is applied to the fibre optic cable throughout its length, the cable may be applied to the surface of a conductor that is energised. For this latter application to be successful, that is to say for the arrangement to be such that integrity of the cable, and thus the transmitted optical signal, remains intact, the coating must be sufficiently robust to resist electrical activity resulting from the electrical field surrounding the conductor. Three formidable problems must be overcome before the arrangement can be expected to provide acceptable long term performance. These problems include: stress control at the point where the fibre optic cable leaves the high voltage conductor and where it approaches ground, or earth, potential; control of surface leakage currents which will flow along the surface of the fibre optic cable sheath as a result of contamination; and the maintenance of the integrity of the fibre optic cable after prolonged outdoor exposure to high electric stress. Problems of these general kinds are Known within the electricity distribution industry, and in many instances typical devices including insulators, cable terminations and the like, which must of necessity encompass phase and ground potential, have been designed with these problems in mind.

However, solutions appropriate to insulators or high voltage power cables are not appropriate to fibre optic cables.

Stress control in high voltage cable terminations can be effected either resistively or capacitively by coupling with the energised centre conductor of the cable. A fibre optic cable, however, has no such conductor and in some respects can be considered to be more similar to a high voltage insulator. A high voltage insulator is 2168 1 9 - 4 - RTCTBO NZ3 designed to control surface leakage currents by the provision of sheds on its surface. The sheds have several functions: increasing the creepage distance, often to more than three times the actual phase-to-ground distance; provision of numerous "dry" areas of relatively high resistance, which can maintain the leakage currents at acceptably low levels; and the provision of protected areas which limit the deposition of pollutants.

By comparison, the diameter of a fibre optic cable is typically one tenth to one hundredth that of a high voltage insulator and the fibre optic cable has negligible strength. The provision of shedded areas to produce the same result as in typical high voltage insulators is clearly impractical and other solutions must therefore be sought to both problems.

Insulators are of solid core or occasionally oil filled construction to eliminate, or at least to reduce, internal discharge activity which might result in failure. By comparison optical fibres are frequently of "loose tube" construction, or laid up in such a way as to form extended internal voids. Since air is a good dielectric material it might be supposed that such voids in so small a structure would have negligible affect. However, experience and experiment have shown that such voids are detrimental to longer term service life and must either be eliminated or their potentially detrimental effects curtailed in some other way.

The high voltage equipment may be an elongate conductor, or may be other equipment such as switchgear or a transformer. The cable may extend along or alongside, and preferably be helically wrapped around, the conductor.

The invention thus allows a fibre optic cable to be mounted externally of a high voltage conductor, for example, and to be removed therefrom, or to provide for splicing thereto, at any position along the length of the conductor, and to be brought safely to a location of appreciably different electrical potential, without having to terminate, splice, or otherwise interfere with the conductor itself.

The invention also allows a fibre optic cable to extend safely from a sensor associated with high voltage electrical equipment to a much lower, for example earth, potent ial.

The stress control means may be arranged to enclose the fibre optic cable in the region where the cable leaves the immediate vicinity of the field of the conductor.

Preferably, the electrical stress control means comprises an electrically-conductive housing, of conductive polymeric material, or metal, for example, that is mounted on the equipment, and the cable is arranged to leave the equipment and pass towards a site of significantly different electrical potential, eg earth potential, through the appreciably-reduced field environment of the housing.

The housing may thus provide an enclosure that is substantially free from the electric field associated with the high voltage conductor, so that the fibre optic cable, which is initially at the voltage of the conductor along which it may, in use, extend, can leave the conductor in the vicinity of the housing and can safely be lead away from the high voltage conductor to, or at least towards, a position at a much lower potential.

Thus, the housing need extend away from the conductor only so far as to ensure that when the fibre optic cable 2 1681 2 1 68 1 RK100 NZ3 exits therefrom into any electric field of the conductor, the field strength is too small to effect any substantial damage of the cable or have such adverse effects as discussed above. To this end, the configuration of the housing may be such as to reduce the effect of the electric stress from the field of the conductor. Advantageously the cable exit of the housing may open outwards away from the conductor, being conical or bell-shape for example. It will be appreciated that the configuration, including the shaping and size, of the housing may be different for high voltage conductors of different voltage ratings, 30 as to optimise the electrical protection of the fibre optic cable. It will be understood that the function of the housing is to act in a manner similar to that of a Faraday Cage, and thus it need not completely physically enclose the fibre optic cable. Furthermore, the housing will not, in general, be required to carry any appreciable electric current. To these ends, the stress control effect may be achieved by a partially open configuration, for example a lattice work wrapping of electrically conductive material around the region where the fibre optic cable leaves the immediate vicinity of the electric field of the high voltage conductor.

It is further envisaged that an electrically-conductive housing may be arranged to protect the cable at the location where the cable is brought to earth potential.

Preferably the housing has a passage therethrough and an inlet and an outlet spaced from but in communication with the passage, whereby the high voltage conductor extends through the passage,and supports the housing, and the fibre optic cable is arranged to leave the conductor just outside the housing, to enter through the inlet and to exit from the housing through said luminal mm ■- j 2168 - 9 outlet. It will be appreciated that the fibre optic cable should leave the conductor, and thus enter the housing, at a position close enough to the housing such that no significant potential drop exists along the fibre optic cable. Preferably, the fibre optic cable should not extend laterally of the high voltage conductor for more than about three times the diameter of the conductor before entering the conductive housing.

Alternatively, the fibre optic cable may enter the housing into the passage together with the conductor, and thus leave the conductor within the housing itself.

The housing may comprise two half shells that are secured together, by screws for example, so as to allow the conductor to pass therethrough and so as to extend transversely thereof, preferably flaring outwardly, to guide the fibre optic cable away from the conductor.

The housing may be split at one end, for example by being bifurcated, so that it can be pushed on to the conductor, and may be provided with a closure cap, as a screw fit, push fit or snap-on for example.

Alternatively, or in addition, the housing may be a two-part housing with a suitable closure mechanism, or be of a one-part wraparound configuration. The housing may be substantially elongate, or may have some other shape, for example a T-shape whereby the arms of the T extend along the conductor.

In one configuration, the housing is generally elongate, having bifurcations at one end leading to a transverse passage receiving the conductor, with an outlet at the other end of the housing leading the fibre optic cable away from the conductor.

Preferably the housing is environmentally sealed on to the high voltage conductor, advantageously by means of electrically conducting mastic or sealant. * 2 1 68 RJHS3—JIZ3 Envlronmental protection, for example from water or salt or other pollutants, of the fibre optic cable after leaving the housing may be provided by an electrically insulating, non-tracking, weather resistant and preferably tubular, member that interengages with the housing and leads the cable away therefrom, advantageously all the way to the earthed location. This member may be convoluted and/or shedded on its outer surface, for extending the creepage path length. Its inner surface may be convoluted, and it may be filled to prevent water deposition therealong. Alternatively, the fibre optic cable may be loosely disposed within the tubular member.

U.S. Patent No. 3746424 discloses an isolating device for guiding an optical fibre bundle from a current measuring device associated with a high voltage conductor to a detection device at earth potential, in which the isolating device has a plurality of skirts along its length. However, there is no provision for, for example, the fibre optic bundle to extend externally of the conductor so as to be subject to its electric field.

Further protection of the optical fibre may be achieved by completely filling the region between the cable and the enclosing housing and/or the tubular insulating member to be described. One way of achieving this is to arrange for the insulating member to be shrinkable, for example heat-shrinkable, on to the optical cable, preferably with an internal sealant or adhesive coating, for example a mastic or a hot melt adhesive. Another way is to ensure that the region between the optical cable and the insulating member is filled with an oil, resin, foam, or other suitable insulating medium. With such provision, the sealing of the housing on to the conductor is not so important, and may be dispensed with entirely. "• -7 • o 2168 19 - 9 - RI£>8ft~*T3—• In an alternative configuration of the arrangement of the Invention, the or a further stress control means may be located at a position where the cable, initially subject to a high voltage, approaches or reaches a much lower, for example earth, potential. Such stress control means may act as an earth leakage current collector and thus ensure safe earthing of the cable without causing any appreciable electrical damage thereto or minimise other adverse effects of the leakage current.

The fibre optic cable used in the arrangement of the invention may be of any suitable configuration, and may, for example, comprise one or more optic fibres, each within its individual sheath or sheaths, and where more than one fibre is present, these may be grouped together within a single outer sheath.

Two configurations of high voltage fibre optic cable are proposed which may be used separately or in conjunction with one another. It is envisaged that the fibre optic cable may have other components, in addition to those specifically mentioned.

The first configuration of fibre optic cable may comprise a substantially non-tracking outer jacket, a non metallic strength member, one or more fibre optic elements and a compatible, protective filler.

The filler is arranged substantially to eliminate, or at least inhibit, the formation or existence of significant elongate voids, or the significant transmission of moisture within the structure in the event of puncture of said outer protective jacket. The same filler may be used to achieve each of these results, or separate fillers may be required.

The compatible filler may be incorporated into the construction during manufacture, and may comprise a ... >v «^*sgwgaiwaw*!Wii riwuiimii. i" mm wi n ■ whimx •" 2 168 1 similar material to the jacket; a gum-like material, for example baaed on silicone, butyl, or ethylene propylene elastomers; a wax or jelly-like material for example a petroleum jelly; or an oil which may be incorporated during manufacture or subsequently drawn into the interstices of the construction. The filler may be of the same material as, and integral with, the cable Jacket, such that filling of the cable, for example by complete impregration of a braided component thereof, i3 effected when the jacket is added during manufacture.

(~~- The jacket and filler may be subsequently cross-linked by high energy radiation or chemical means.

The second configuration of high voltage fibre optic cable may comprise a conductive outer jacket, which may be of metal or polymer, a strength member and one or more fibre optic elements.

The more straightforward construction, and the conductive outer sheath of the second configuration cable make it particularly, though not exclusively, suited to installation adjacent to a live conductor at high voltages, whereas the first configuration cable is particularly though not exclusively, suited to the transition from high voltage to ground potential. ^ Thus, a cable of the second configuration is particularly suited to operation at a single potential and may be connected to a cable of the first configuration to effect a transition between high -~n voltage and ground potential.

It will be appreciated that the material of the outer jacket of the fibre optic cable would be selected in dependence on the electrical environment in which it is located. For a cable subject to a high voltage field, for example by being wound on an uninsulated overhead HI IIIIIIWII I..' .;■ ■ jig* ■■«':■>*** 2 168 - 11 - —RK-180 NZ3 power conductor, a conductive outer jacket of a material having a resistance of less than about 10^ ohms per cm of Its length would be suitable. Such a material ensures in general, that the potential of the cable jacket is sufficiently close to that of the conductor so that any surface discharge activity that may take place on the cable will be insufficient to cause any damage thereto. However, if 3uch a jacket were insulating, having a resistance of more than about 10? or 10® ohms per cm length, then it should either be encased in a further jacket of a non-tracking insulating material, or made entirely from such non-tracking insulating material. Thus, even though there may be an appreciable difference in potential between the conductor and the cable, the cable jacket has a protective outer surface. Where the fibre optic cable is required to extend between phase potential and ground potential, the outer jacket should have a resistance of at least about 10? or 10® ohms per cm length and be non-tracking. If the resistance of the outer jacket of such a cable were of significantly lower potential, then the leakage current flowing therealong, and the power dissipation, could be unacceptably high. Thus, it will be understood that at higher voltages electrical tracking along the cable surface has to be minimised, whilst at lower voltages the leakage current has to be maintained within an acceptable limit. The particular resistance limit3 will, accordingly, be dependant on the operating voltage to which the cable is subject.

Although usually it will be desirable to provide filling for the fibre optic cable only when an insulating outer covering is provided, it may in some circumstances be advantageous to fill the cable when it has a conductive outer covering. For example, the fibre optic cable may be provided with a non-tracking jacket and be disposed 2168 1 9 w^^so-iiza withln a metal tube. Such a tube would enhance the mechanical protection and moisture proofing of the cable. However, on splicing or terminating such a cable, the metal tube would be removed at the cable end and thus subject the cable to axial ingress of moisture. In such circumstances, water blocking of the cable would be advantageous. Filling or blocking the cable is particularly important where the cable extends between locations at appreciably different voltages, for example from a conductor at phase voltage to ground potential. However, ingress of moisture at any part of the cable may cause long term damage to its optical properties, if water comes into contact with the glass fibre, for example.

The non-tracking outer Jacket material used in the lower potential, and preferably also in the transition region from the high potential region, may be a replacement for said conductive outer jacket material, or alternatively may be provided as a further layer thereover.

The cable may be located in a tubular member, and may but and need not be environmentally sealed. For example a protective housing for the tube may be perforated, so as to avoid the build-up of moisture for example.

The arrangement may comprise means for conducting any leakage current flowing from the high voltage equipment along the fibre optic cable to a lower potential, preferably earth, which means may be an earth leakage current collector or a means for reducing the leakage current density, and may conveniently comprise two electrically-conducting half-shells clamped directly on to the cable. When the cable is itself enclosed within an enviromentally-protecting member, such as the tubular member described above, the earth leakage current collector may further comprise means, similar to the said electrically conductive means, for connecting such ; r-c - ' RKIQO *!Z3 member to earth and thus for earthing any leakage current flowing therealong. Advantageously, the tubular or other protection member extends all the way along the fibre optic cable between the stress control means and the leakage current collecting means.

The earth leakage current collector may alternatively be provided by a hybrid arrangement comprising a pair of Insulating, for example porcelain, half shells that are mounted on the,cable and that abut a pair of metal earthing half-shells that are mounted on the cable in the direction of the lower potential from the insulating half shells. Such an arrangement is disclosed and claimed in EP-A-0125884.

It will be appreciated that depending on the operating conditions, the invention may provide an arrangement that comprises any two or all three of the above-mentioned features, namely said electrical stress control means for minimising any electrical stress to which the fibre optic cable is subjected, means for earthing any leakage current flowing along the cable, and environmental protection means for the cable as it passes from a location at one potential to a location at an appreciably different potential, for example from high voltage to earth.

One advantageous arrangement of the invention comprises a fibre optic cable associated with a high voltage conductor, which may be an overhead conductor, stress control means arranged such that substantially no electrical damage is caused to the cable where it leaves the conductor, a protective tubular member for guiding the cable towards a region of significantly lower electrical potential, for example earth, and further stress control means for electrically terminating the cable without electrical damage at said lower potential. *1 t JUNI987 *. ' "r.j n Ik. w J. O j. «l -14- RKiao .*1Z3 In accordance with another aspect of the present invention, there is provided a method of providing electrical protection for a fibre optic cable that is located In a region such that it is exposed to the electric field of high voltage equipment, wherein where the cable leaves the immediate vicinity of the field of the equipment, the electrical stress on the cable is controlled such that the effect of said electric field is substantially insufficient to cause damage to the fibre optic cable.

Electrically conductive means may be mounted on the cable in a region of lower, for example earth, potential, so as to conduct to a point at said lower potential any leakage current flowing along the cable jacket from the high voltage electrical equipment.

The method of the invention may produce an arrangement in accordance with said one aspect of the present invention and having features thereof as described above.

A method of providing electrical protection for a fibre optic cable that is associated with a high voltage conductor, and an arrangement comprising the cable, conductor and electrical stress control means, each in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a sectional elevation of a housing of a stress control means; Figure la is a sectional elevation of half of another embodiment of the housing; —Figure 2 is a side elevation of one embodiment of the ^ arrangement, including the housing of Figure 1; Figures 2a,3 and 4 are side elevations of other embodiments of the arrangement; 2168 1 9 Figures 5a and 5b show in partly cutaway and cross- section respectively a modification of part of the arrangements of Figures 2,2a,3 and 4; Figures 6a,6b and 6c are diagramatic representations of a portion of further arrangements that comprise other embodiments of the stress control means; Figure 7 is a vertical section through another stress control housing that is suitable for effecting ground connection of the fibre optic cable; and Figure 8 is a schematic representation of the arrangement of Figure 3 positioned adjacent the termination of a high voltage conductor at a transformer station, and including the earthing housing of Figure 7.

Referring to Figure 1, a housing 2 is made from an electrically-conductive polymeric material, and provides electrical protection for a fibre optic cable that • r>- O r^ ' V 2 1 68 1 -16- JIK4€0-!!123. extends along a high voltage conductor. The conductor and cable are shown in outline in Figure ', and will be described in more detail with reference to Figure 2. The housing 2 comprises a first elongate part 4 that is open at each end and that has an upper generally right cylindrical portion 6 that continues downwards into a generally conical portion 8. The housing 2 has a second part to, that provides a closure cap for the housing portion 6, and for this purpose the closure cap 10 has an internal thread and the cylindrical portion 6 has a mating external thread.

The cylindrical portion 6 is bifurcated st its open end so that a slit '2 therein extends froa the open end to generally circular openings in opposing side walls of the portion 6 that define a generally tubular passageway M therethrough.

Figure 2 shows the housing 2 of Figure 1 mounted on a high voltage uninsulated overhead conductor 16 that has a fibre optic cable 18 wound helically therearound. The cable 18 may have any suitable construction as hereinbefore described. The bifurcated cylindrical portion 6 of the housing 2 is spread apart so as to enlarge tr.e slit 12 to an extent that the conductor 16 can pass therealong so as to be located within the housing passageway M. The fibre optic cable 18 extends with the conductor 16 into the passageway 14, but within the housing 2 is lead off the conductor 16 and down through the open end of the conical housing portion 8. A conductive mastic material 20 is wrapped around the cable carrymq conductor 16 where it extends into, through, and out of the housing 2 to prevent ingress of moisture into the housing along the passageway 14. The slit 12 is closed, and the housing cap 10 screwed on to the cylindrical portion 6, thereby sealing the top of the housing 2.

'.NV". , Cr y. ■ . ., 2168 1 9 _! 7. -RK l80^-3- Figure la shows on« half 3 of a cast metal housing, that is secured to a corresponding half shall by scraws or bolts so as to enclose tha high voltaga conductor. Tha housing formad from tha half shalls 3 functions electrically in tha sama way as tha housing 2 of Pigura 1, but «ach half shell extends integrally above and balow the conductor, one on each sida thereof. Thus, tha conductor extends througn a channel 5, the fibre optic cable enters througn aperture 7, and internal ribs 9 provide for securing of a protective tube (not shown) for guiding the fibre opric cable transvarsaly away froa tha conductor.

Referring to Figure 2a, the assembly has a housing 21 that differs from the housing 2 in its provision for entry of the fibre optic cable 18. In this arrangement, the cable 19 passes away from the conductor 16 at a distance of about 5 0mm before the conductor enters the housing. The cable 18 then extends up to tha housing cap 23 that has an aperture 25 that allows tha cable to pass down into the housing 21. In another alternative configuration, an inlet aperture 27 for tha optical cable 18 is on tne side of tna cap, and tha path of the cable ^8 is tnen as shown by tha broken lines in Figure 2a. Since the cable 18 anters tha housing 21 through apertjre 25, or 27, rathar than entering along tha conductor 16, as shown n Pigura 2, there will be no chafing between tha fibre optic cabla 18, conductor 16 and the housing passage entranca. Furthermore, sinca the cable 18 is only away from tha conductor 16 for a short distance before entering the conductive housing, no appreciable electrical stress is applied to the cable. a The configuration of the housings 2 and 21 is such that in the region where the fibra optic cable 18 extends out from tha conical portion 8 thereof, tha alactric stress ?. 1 68 - 13- RX180 on the cable arising fro* the field associated with th« conductor 16 is insufficient to cause damage to the cable or other adverse effects. To this end# parameters such as the length of the housing 2 and the angle of the conical portion 8 are selected in accordance with the voltage rating of the conductor 16. For a 33kV conductor, for example, the length of the housing extending therebelow is typically Sen to 8cm, with the conical surface of the housing making an angle of approximately to* with the vertical (as seen in the Figures).

In the modified assembly shown in Figure 3, further, mechanical, protection of the fibre optic cable 18 is provided on leaving the housing by enclosing it within a convoluted tube 22 of electrically insulating polymeric material. Tube 22 engages internally with a threaded section 23 of the cylindrical housing portion 6, and extends downwards from the open end of the housing 2.

The further embodiment of the assembly shown in Figure 4 comprises a housing 24 that differs from the housing 2 in having an external screw-thread 26 at its lower open end, to which is externally threaded one end of a convoluted tubing 28.

The protective assembly of Figure 4 has the advantage over the protective assembly of Figure 3 in that the housing 24 does not shield the tubing 28 from water, and thus minimises the risk of dry-band formation on the outer surface of the combination of housing and tubing, which, in the assembly of Figure 3, could in some circumstances give rise to electrical damage to the tubing by way of tracking and erosion.

Figures Sa and 5b refer to alternative means to the 2168 1 9 -19- Ml80 NZ3 - convoluted tubings 22 and 28 for guiding a fibre optic cable fro* a high voltage area to a lower voltaga area, and is particularly# though not exclusively applicable in higher voltage applications, say above 132kV, where increased creepage path length is required. Thus# a fibre optic cable 50, which comprises five fibre optical fibres surrounding a strengtheninq member in a fully water-blocked construction, is embedded in a non-tracking outer jacket 52 thac has sheds 54 on its outer surface.

Alternative housings to housings 2,21 and 24 previously described are shown in Figures 6a, 6b and 6c. The housings of Pigure 6a,6b and 6c are of open helical configuration, of metal, or fairly rigid conductive polymer, provide a substantially spherical enclosure, and art mounted on the overhead high voltage conductor.

The housing 56 of Figure 6a is coiled around a generally vertical axis, and is clamped at diametrically-opposed locations 58,(0 on to the conductor 62. A fibre optic cable 64 that extends helically along the condcutor 62 is guided therefrom within the housing 56 by a coiled member 66, and passes to a cable splice enclosure, shown diagramatically at 68, that is mounted on the housing 56. After splicing within the enclosure 68, the fLbre optic cable leaves the housing 56 in a convoluted and/or shedded tub* 70 as previously described.

The housing 72 of Figure Sb differs from that of Figure 6a, in that it extends helically about an axis that is coincident with the overhead conductor. The detachment, splicing and subsequent guiding of the fibre optic cable is not shown, but may conveniently be as previously described. 2 168 « - 2 n - -RKl80N?T It will be appreciated that each of the enclosures 56 and 72 of Figures 6a and 6b usually have to be mounted on to the overhead conductor without it having to be cut; that is to say, they will have to be attached frosi the side rather than be slid on fro* an end of the conductor. The inherent rigidity required of these open configuration housings may make this rather difficult in soae instances. The housing 73 of Figure 6c overcoaes this difficulty, since Lt is formed from a metal or fairly rigid conductive polymeric tubular member that is secured at its aid point, put into a U-snape and then wound about a mandrel such that the limbs of the U extend axially in opposite directions. The housing 73 is aounted on the conductor by looping the mid point thereover and subsequently rotating the entire housing about the conductor until gripping engagement is achieved at each end of the housing.

Figure 7 shows a housing 74 that is suitable for attachment to the convoluted tubing that guides the fibre optic cable from the overhead conductor to its station at earth potential and serves as a stress control arrangement, an earthing medium and earth leakage current collector. It may also serve as an enclosure for a fibre optic splice or connector.

The housing 74, shown open in Figure 7, comprises two generally seai-cylindrical brass half-shells 76 each of which has an integrally-ridged neck portion 78 at each end for gripping the convoluted tubing 80 that guides the fibre optic cable 82. A further pair of generally semi-sylindrical brass half-shells 84 are located within the housing 74 and are claaped in contact with the half-shells 76 on closure of the housing. The non-tracking convoluted tubing 80 extends into the housing 74 and the fibre optic cable 82 exits therefroa and passes into a 2 1 68 - 21 - RlU80fi73 passageway formed by the half-shells 84. The larger upper part of the passageway encloses the fibre optic cable loosely, and is filled with a potting composition, and the narrower, lower part of the passageway grips the fibre optic cable firmly, but not so as to effect any damage thereto. Thus, the housing 74 makes direct electrical contact with the outer non-tracking tubing 90 and with the fibre optic cable 82. A conductive connecemg stud 86 is mounted on the outside of one of the half-shells 76, and a conductor (not shown) extends therefrom to a point at earth potential.

The housing 74 contains a splice enclosure, shown diagrammatical1y at 88, and the fibre optic cable passes therefrom into a further portion of the convoluted tubing 80 for onward transmission to a decoding station.

It is to be understood that in many instances, the fibre optic cable may not need to be protected, such as by a convoluted tube, and that this may be so whether or not the high voltage stress control arrangement is applied to the cable. Thus, referring to Figure 7, if there is no requirement for the protective insulating, non-traclcing convoluted tubing 80 to protect the fibre optic cable 82, then the outer pair of half shells 76 of the earth leakage current collector 74 may be dispensed with, in this case, the half shells 84 would be directly connected to earth potential.

Referring to Figure 8, a high voltage conductor 30 witn the fibre optic cable 18 spirally round thereon extends from one switchgear/transformer station (not shorn), over a ten meter high support pole 32, and ehence to a terminating switchgear/transformer station 34. The conductor 30 is secured to the pole 32 by tensioners 36 and is electrically isolated therefrom by insulators 38. Just before reaching the pole 32, the fibre optic cable 2 168 -72- RK180_H73 18 is lead away from the nam conductor 30 along a drop wire 40, that at ont and is crimpad on to the conductor 30 and that at its other and is connected to a surge divactar 42 and thence to tha pola 32 at aarth potential. Thus, tha drop wire 40 is at tha same voltage, 33kV say, as the conductor 30. The fibre optic cabla 18 is taken off the drop wire 40 in advance of the surga diverter 42 via the electrical1y-conductive housing 2 or 21, and thence throijgh the insulated convoluted tubing 22 that is secured to the pole 32. At tha bottom and of the pola the fibre optic cable 18 leaves the conductive tubing 22 and enters into switchgear (not shown), which is at earth potential. This occurs after the cabla and tubing has been earthed, for example as shown by maans of tha housing 74.

It will be appreciated that usually the conductor 30 will be accompanied by two other conductors, thus providing a three-phasa power supply, but only one of tha conductors will, in general ba required to carry a fibre optic cabla.

It is to be understood that features of ffiDre optic cables and cabla arrangements mentioned herein may oe combined together in any suitable combination to achieve particular objects.

Claims (32)

- 23 - t CfcfriMS W'HAT CLAIM

1. An arrangement comprising high voltage equipment, a fibre optic cable having a portion located in the electric field of the equipment, and electrical stress control means which co-operates with the cable where the cable leaves said electric field such that the effect of electrical stresses produced by said electric field is substantially insufficient to cause damage to the cable.

2. An arrangement according to Claim 1, wherein the stress control means is a housing which encloses a portion of the cable where the cable leaves said electric field.

3. An arrangement according to Claim 1 or 2, wherein said equipment comprises an elongate high voltage conductor.

4. An arrangement according to Claim 3 as dependent on Claim 2, wherein the housing is mounted on the conductor.

5. An arrangement according to Claim 4, wherein the housing has a passage therethrough in which the high voltage conductor is located, and an outlet through which the cable leaves the housing.

6. An arrangement according Claim 5, wherein the housing has an inlet through which the cable enters the housing, the inlet being spaced from said passage

7. An arrangement according to Claim 4, wherein the housing has a passage therethrough in which the conductor is located, and an outlet leading i L,%[ <=l - 24 - therefrom, the cable extending away from the conductor within the passage and passing out from the housing through said outlet.

8. An arrangement according to any one of Claims 4 to 7, wherein the housing comprises two half shells adapted for mounting around the conductor and for guiding the fibre optic cable transversely away therefrom.

9. An arrangement according to any one of Claims 4 to 7, wherein the housing is bifurcated so as to receive therein the conductor with the cable thereon, the bifurcations being secured together by a closure cap.

10. An arrangement according to any one of Claims 4 to 9, wherein the housing is environmentally sealed around the conductor and the cable.

11. An arrangement according to any one of Claims 4 to 7, wherein the housing has an open configuration.

12. An arrangement according to any one of claims 4 to 11, wherein the cable is guided away from the housing in a tubular member.

13. An arrangement according to Claim 12, wherein the tubular member is convoluted, and/or has a shedded outer surface.

14. An arrangement according to Claim 12 or 13, wherein the tubular member is of electrically insulating material. - 25 - (l i j 61

15. An arrangement according to any one of Claims 12 to 14, wherein the cable is loosely disposed within the tubular member.

16. An arrangement according to any one of Claims 12 to 14, wherein a filler medium occupies the space between the cable and the tubular member.

17. An arrangement according to any preceding claim, including electrically conductive means mounted on the cable so as to conduct to a point at earth potential any leakage current flowing along the cable.

18. An arrangement according to Claim 17, wherein the electrically conductive means comprises an enclosure that encloses a portion of the cable.

19. An arrangement according to Claim 18, wherein said enclosure comprises two interengaging half-shelIs.

20. An arrangement according to any one of Claims 1 to 16, including an earth leakage current collector mounted on the cable, spaced from said stress control means, for conducting to a point at earth potential any leakage current flowing along the cable, said collector comprising a pair of electrically insulating half-shells mounted on the cable, and a pair of metal half-shells mounted on the cable, in abutment with said insulating half-shells and in the direction of lower electrical potential from the insulating half-shells.

21. An arrangement according to Claim 20, wherein said insulating half shells are formed from porcelain. - 26 -

22. An arrangement according to any preceding claim, wherein the cable has an electrically insulating outer surface.

23. An arrangement according to any one of Claims 1 to 21, wherein the portion of the cable located in the electric field has an electrically conductive outer surface.

24. An arrangement according to any one of Claims 1 to 21, wherein the portion of the cable in the electric field has an electrically conductive outer surface, and the portion of the cable that extends out of the electric field has a substantially electrically non-tracking outer surface.

25. An arrangement according to Claim 22 or 24, wherein the cable comprises a substantially electrically non-tracking outer jacket, a non-metallic strengthening member, at least one fibre optic element within the jacket, and filler material surrounding the strengthening member and the fibre optic element.

26. An arrangement according to Claim 23 or 24, wherein the cable comprises an electrically conductive outer jacket, and a strengthening member and at least one fibre optic element within the jacket.

27. An arrangement comprising a high voltage conductor, a fibre optic cable, and electrical stress control means, substantially as hereinbefore described with reference to Figures 1 to 6 of the accompanying drawings. - 27 -

28. A method of providing electrical protection for a fibre optic cable having a portion located in the electric field of high voltage equipment, comprising controlling the electrical stress on the cable where the cable leaves the electric field such that the effect of the electrical stress is substantially-insufficient to cause damage to the cable.

29. A method according to Claim 28, wherein the electrical stress is controlled by enclosing a portion of the cable in an electrically-conductive housing.

30. A method according to Claim 29, wherein said housing has a passage therethrough and an inlet and an outlet in communication with the passage and spaced therefrom, wherein the equipment comprises an elongate high voltage conductor which extends through the passage and the cable is arranged to extend away from the conductor in advance of the housing, to enter through said inlet, and to exit from the housing through said outlet thereof.

31. A method according to any one of Claims 28 to 30, wherein any leakage current flowing along cable is conducted to a point at earth potential.

32. A method of providing electrical protection for a fibre optic cable, substantially as hereinbefore described with reference to Figures 1 to 6 of the accompanying drawings. " ftf W#/their authorised Agaw*. A I PARK * SON.