WO2007113522A1

WO2007113522A1 - Cable installation

Info

Publication number: WO2007113522A1
Application number: PCT/GB2007/001182
Authority: WO
Inventors: Philip Alfred Barker; Christopher Nigel Munnings; John Michael Haley
Original assignee: British Telecommunications Public Limited Company
Priority date: 2006-04-03
Filing date: 2007-04-02
Publication date: 2007-10-11

Abstract

A method of protecting a weak point along a cable (34), the cable being subject to stress, comprising the steps of (i) selecting a section of the cable between a source of the strain and up to and including the weak point, (ii) using the section of the cable to form a cable configuration comprising a pair of coils, by - putting a twist in the cable by winding the cable in a first direction to form a first coil, and - taking the twist out of the cable by winding the cable in a second direction to form a second coil, and (iii) securing under tension the cable configuration formed.

Description

CABLE INSTALLATION

The present invention relates to methods and apparatus relating to the installation of cables, particularly but not exclusively to optical fibre cables deployed by blown fibre methods in aerial installations.

In the UK, the telecommunications core network is now composed primarily of optical fibre and in the future will expand to the access network possibly using passive optical network (PON) technology. The "last mile" of the network leading to customer premises has however traditionally comprised copper wires, which has proved to be a transmission choke point because of the significantly reduced transmission capacity of copper wire compared to optical fibre. While many commercial and industrial entities have since migrated to optical fibre over the last mile, most private or residential customers still remain connected by copper twisted pairs for reasons mainly of cost. However with demand in the UK for greater telecommunications bandwidth from residential customers, the migration to an all-optical network is gradually taking place, where the benefits of a fibre-based core network can be realised by leading fibre to the home or premises (FTTH or FTTP) to deliver significantly improved data transmission rates and capacities to residential customers.

The current practice is to terminate optical fibres from the core optical network at a fibre node at a distribution point, typically sited on the street some distance away from the target residential premises, within a weather resistant enclosure such as a joint box or cabinet on a telephone pole, at street level or underground. An optical fibre node converts the optical signal from the fibre to an electrical format to be conveyed to the customer's premises over the copper wires.

Currently, about 60% of the UK's residential customers obtain their telecommunications connections via overhead copper dropwires originating from a local distribution point. Distribution points for overhead distribution are usually pole-mounted and are fed by a multi-pair cable either underground or overhead from a street cabinet or primary connection point. Most of the remaining customers are connected by underground feeds. In an overhead copper feed, lengths of dropwire or aerial cable are suspended between utility or telephone poles which lead from the distribution point to each of the premises served by the distribution point. One way of upgrading the "last mile" from copper to fibre, is to deploy optical fibre cables. Such cables can take the form of optical fibre cables where the fibre element is provided in a protective sheath for installation. Alternatively, "blown fibre" can be installed using the technique described in EP108590, wherein an empty fibre tube is first installed along the route between the two points to be connected, so that the fibre element - often a bundle of fibres making up a fibre unit - can be installed into the waiting tube at a later date, by "blowing" it through using compressed air to generate viscous drag along the surface of the fibre unit. In this description, where the context permits, references to "cables" can be used interchangeably with references to individual optical fibres and bundles of fibre units as well as cables comprising such fibres and fibre units, and tubes including the various types of conduits and ducts deployed along various sections of the network.

Aerial cables are known and currently used in fibre installations. They face high levels of environmental stress. Load factors in the form of temperature changes, wind, rain, ice build-up, birds, as well as the cable's own weight put a tensile load on the cable, particularly at its ends, resulting in expansion and contraction of the cable. Overhead copper pairs have been in long use, and are typically self supporting (incorporating a steel strength member), or are lashed to messenger wires (supporting wires strung between the poles).

Optical fibre is increasingly now deployed for aerial use as well, although special considerations are required to take into account the fragility of the fibre and other properties particular to fibre which do not exist in copper cables, such as the minimum bend radius of optical fibres and strain. Optical fibre is already susceptible to both mechanical failure (e.g. hairline cracking and breakage) and optical loss (e.g. signal attenuation from excessive bending). When used in a high-tension overhead installation, these problems are exacerbated.

As an alternative to the aerial deployment of conventional optical cables described above, the applicants have developed a blown fibre droptube (BFD) for use in conjunction with the FTTP plans to provide premises with optical connections to the telecommunications network using blown fibre. BFDs are installed along the same routes currently taken by copper overhead cables. Specifically, the BFD will typically be installed between a distribution point (in the form of a fibre node which is typically sited in an underground joint box or up a telephone pole) in suspended spans on telephone or utility poles to customer premises. After the BFD has been installed, it can be subsequently populated by having a fibre unit blown through when the connection is required. The optical fibre is terminated at each end typically in a splice, although it can be fitted e.g. with an optical connector plug for plugging into a mating socket on equipment.

One particular problem is the pulling force exerted on the optical fibre, particularly on its ends e.g. where the fibre is connected or spliced at a termination point, The aerial cable is already under strain from its own weight, and environmental loading factors can increase the pull on the cable and at the ends even further. It is known that optical fibre generally can cope with stress levels (tensile or compressive) of about 1%, but stresses beyond this can cause premature failure. The fragile fibre connection or splice in particular is a mechanically weak point at risk if this happens, and may be pulled out of its closure in the worst case. Such weak points are also vulnerable to other stresses propagated along the cable in the form of e.g. shear stress, torsion, excessive vibrations, or the like. The term "stress" herein shall include references to all such forces.

For the purposes of this description, "weak points" could include any point of fragility along the cable, such as a connectorised end of the cable, where the connector is flimsy, or other mechanically delicate point or section.

One solution is to isolate the weak point from such stresses, by reducing or completely removing the tension along the cable leading to up to it. This can be done by e.g. clamping the cable at a point between the source of load and the weak point, so that the section of the cable leading up to the weak point is at low tension compared to the other side leading to suspended section of the cable. However, simply clamping on the fibre in this way is not ideal: although an optical fibre will withstand some measure of crushing, it is very possible that the pulling strain at the point of clamping will itself create another point of stress and weakness along the cable at the clamped point.

Another solution is known in the context of conventional optical cables having an optical fibre loosely disposed within buffer tube, where the buffer tube is encased tightly in a strong outer jacket. A "coupling coil" can be formed by coiling the cable in one or more loops. The sidewall friction between the optical fibre and the inside of the buffer tube has the effect of reducing movement of the optical fibre relative to the buffer tube, thus discouraging retraction of the fibre into the buffer tube. Using this method avoids the creation of a specific point of stress as would be the case in applying a direct clamp to the cable as discussed above. This is because the stress is distributed along the length of the coil instead of being concentrated at a single point. EP1557707A1 discusses how the size of the coupling coil can be reduced, by cutting away the stiff outer jacket to expose the relatively more flexible buffer tube, and coiling that instead.

The process of coiling the cable into loops as described above causes the cable to twist along its axis. (References to "twist" herein shall include a "full twist" of substantially 360 degrees about the cable's axis, a "half twist" about substantially 180 ■ degrees, or other portion thereof, as the context permits.) If at least one end of the cable is not fastened or secured (in the sense that it is free to rotate about its axis), the twist can be transmitted through the cable and out at the free end. However if both ends of the cable are fastened or secured, then each coil created will introduce an axial twist of 180 degrees into the cable which will be retained in the cable. Thus a coil comprising two wraps or loops will introduce a 360 degree twist into the cable, and so on. Excessive torsion on the cable causes kinking and creates stress, and can result in optical performance degradation as well as mechanical failure, affecting fibre reliability over the lifetime of the cable.

In a blown fibre application, the two ends of both the fibre tube and the fibre unit itself are usually effectively secured. As described above, the fibre tube is initially installed, and fastened within e.g. a tube tray at each end, ready for population by the optical fibre at a later date. An overhead blow fibre cable overhead installation typically comprises up to 500m in a single span. In an installation where the cable is to be secured by clamping or coiling to protect a weak point located at or near one end of the cable, the suspended section of the fibre tube and/or cable will be effectively secured due to its length. The other end of the fibre tube, in addition to being fastened on a tube tray, could also travel through e.g. customer premise walls, where the hole through which the cable passes is plugged up so that it is impossible to rotate that end of the cable as well. The fibre unit, when blown through the fibre tube, is not secured within tube, but its length makes it difficult if not impossible, to rotate about its axis within the tube.

There is therefore a need to protect the cable, particularly at a section or a point along which it is especially vulnerable to stress and tension such as at a termination splice or connection point, in a way that does not unduly compromise the integrity or performance of the cable, tube or fibre over the lifetime of the same.

According to a first aspect of the invention, there is provided a method of protecting a weak point along a cable, the cable being subject to stress, comprising the steps of

(i) selecting a section of the cable between a source of the strain and up to and including the weak point, (ii) using the section of the cable to form a cable configuration comprising a pair of coils, by - putting a twist in the cable by winding the cable in a first direction to form a first coil, and taking the twist out of the cable by winding the cable in a second direction to form a second coil, and (iii) securing under tension the cable configuration formed.

By anchoring a part of the cable between the source of stress (which can be any mechanical force propagated or transmitted along the cable such as compression, tension, twisting, vibrations and the like) and the weak point (which could comprise a splice or connection point, or a section of such weakness) in this manner, potentially harmful twisting is avoided. The source of stress is typically located at the end of the cable, but the method ,can be used at any other point along the cable although its effectiveness may decrease.

According to a second aspect of the invention, there is provided a device for protecting a weak point along a cable, the cable being subject to stress, comprising guiding means for guiding a section of the cable to form a cable configuration comprising a pair of coils, the guiding means adapted to put a twist in the cable when wound in a first direction to form a first coil, and to take the twist out of the cable when in a second direction to form a second coil, and securing means to secure under tension the cable configuration formed.

This device can be used to protect a weak point or section along the cable, which could be any type of cable which is vulnerable to torsion in the immediate case or in the long run.

According to a third aspect of the invention, there is provided a method of using a device according to any one of claims 14 to 21 below, comprising the steps of positioning the device at a section of the cable proximate to an end of the cable, - forming a cable configuration comprising a pair of coils, by using the guiding means to put a twist in the cable by winding the cable in a first direction to form the first coil, and to take the twist out of the cable by winding the cable in a second direction to form the second coil, and securing under tension the cable configuration formed.

Using this method to form the coils or loops in this manner results in a zero net bend in the cable configuration created.

According to a fifth aspect of the invention, there is provided a method according to claim 22 below wherein the cable comprises fibre tube, further including the steps of releasing and un-forming the cable configuration, installing an optical fibre through the fibre tube, re-forming the cable configuration, and re-securing under tension the cable configuration.

This aspect of the invention is for use where the optical fibre or fibre unit is installed into pre-installed tube e.g. by the blown fibre technique.

According to a fifth aspect of the invention, there is provided an installation for protecting a weak point along a cable, the cable being under stress, comprising a first device according to any one of claims 14 to 21 below at a first end of the cable, a second device according to any one of claims 14 to 21 below at a second end of the cable, one or more utility poles defining a route between the first end and the second end of the cable, and - - . . • I ^ I l_ (.UUf

wherein the cable is suspended from the one or more utility poles.

The methods and devices of the invention can be advantageously deployed in an overhead or aerial installation.

Systems, methods and apparatus embodying the present invention will now be described by way of example only, with reference to the following drawings, wherein:

Figure 1 is a schematic diagram showing an overhead installation between a distribution point and customer premises;

Figure 2 depicts the cross section of a blown fibre droptube;

Figures 3A and 3B show a fibre node without and with its cap;

Figures 4A, 4B and 4C are depictions of a cable anchoring device according to the invention; Figure 5 illustrates a principle used in the invention;

Figure 6 is a view of another embodiment of the cable anchoring device;

Figure 7A and 7B are views of another cable anchoring device according to the invention; and

Figure 8 depicts an installation including a cable anchoring device of the invention at the customer premises.

The method and apparatus of the present invention address the above problems and issues especially, but not exclusively, in the provision of FTTP connections to customers, in particular those customers currently served by copper telecommunications connections arriving at their premises by an overhead feed. In particular the invention seeks to address issues relating to the integrity of the fibre unit and cable and the protection of vulnerable points and sections of the fibre unit or cable.

Figure 1 is a schematic overview of the apparatus and methods used in delivering a telecommunications services to customer premises by an overhead feed, using the blown fibre technique. A blown fibre droptube (2) is initially provisioned on utility or telephone poles (4) along a path between customer premises (6) and a distribution point, here represented by a joint box (8). Within the joint box is an optical fibre node (10). The node can be located up the pole (4) as well, but is here shown to be located The methods and devices of the invention can be advantageously deployed in an overhead or aerial installation.

Figure 2 depicts the cross section of a blown fibre droptube; Figures 3A and 3B show a fibre node without and with its cap;

Figures 4A, 4B and 4C are depictions of a cable anchoring device according to the invention;

Figure 5 illustrates a principle used in the invention;

Figure 6 is a view of another embodiment of the cable anchoring device; Figure 7 A and 7B are views of another cable anchoring device according to the invention;

Figure 7 shows a network termination including a cable anchoring device of the tfwewfeft; and

Figure 1 is a schematic overview of the apparatus and methods used in delivering a telecommunications services to customer premises by an overhead feed, using the blown fibre technique. A blown fibre droptube (2) is initially provisioned on utility or telephone poles (4) along a path between customer premises (6) and a distribution point, here represented by a joint box (8). Within the joint box is an optical fibre node

(10). The node can be located up the pole (4) as well, but is here shown to be located in the underground joint box. Optical fibres cables of blown fibre from the exchange 8

in the underground joint box. Optical fibres cables of blown fibre from the exchange arrive at the joint box and are terminated in the node, typically in the form of a splice, although it is possible to effect termination by using an optical connector.

Tubing - which could be BFD (2) or another kind of fibre tube - is installed and secured at one end within the joint box, and then led up a telephone pole (4). Here, a blown fibre tube manifold (12) may be provided where a number of customer drops (e.g. more than three) are anticipated. The BFD is suspended along poles along the route to the customer premises, where it is secured to the wall of the premises with a clamp (7), then terminated at a customer termination (9), which can be located inside or outside of the customer premises. When the decision is made to populate the tubes with optical fibre, a fibre unit (typically comprising a bundle of individual optical fibre such as that described in EP521710) is blown through using pressurised air, from one end or the other of the route described by the tubes between the joint box and the customer premises. The optical fibre is then terminated, typically in the form of a splice, at each end, one within the node (10) and one at the customer termination (9).

Figure 2 is cross sectional view of a BFD (2). It comprises a bore (13) which includes a low friction liner along its inner walls, to ease the progress of a fibre unit being installed through it. Strength members (14) are included in the walls, which provide a measure of stiffness to the cable to protect the fibre unit when installed, from excessive bending. This permits the BFD to be directly suspended from the telephone poles without need e.g. to be coupled to a separate messenger wire.

Figures 3A and 3B are representations of a fibre node (10) which in Figure 1 is shown to be sited within the underground joint box (8). The node includes a number of trays (16) which serve as splice points for optical fibres (20). The trays are pivotally arranged along a spine (18), allowing for access to each tray. The fibres are broken out of fibre tubes which enter the node from the bottom and which travel to its destination tray via the spine. Figure 3B shows the node with its cap (22) on.

Figure 4A shows a cable anchoring device according to the invention. In this embodiment, the device is configured as a tray (32) being part of a tray assembly (30) suitable for use in an optical fibre node of the type shown in Figure 3A. The tray assembly comprises three sections: a cable anchoring tray (32), a splicing tray (42) and a lid (not shown).

The cable anchoring tray (32) includes a anchoring channel (36) which has comprises two loops in a figure-eight configuration, suitable for receiving a transport tube (34) therein. The transport tube within the tray assembly can include a fibre unit within it or not, depending whether the tube has yet been populated. Unlike fibre tubes, transport tubes are not internally lined with a low-friction liner, as fibre is not blown through them. It provides the fibre unit protection and guides it from the node base to its tray assembly and the fibre anchoring tray (32).

The minimum bend radius for singlemode fibre, which is commonly deployed in the UK, is about 25mm. Typically however, bend management apparatus is set to about 30mm radius, and the loops of the anchoring channel (36) each have a radius of about 30mm. When the transport tube is installed along the channel, it will follow the figure-eight path, as will any fibre unit within the tube. The channel receives and guides the fibre tube through two-and-a-quarter turns from entry to exit of the transport tube from the anchoring channel section.

The transport tube then enters a side channel (37) which guides the tube along the periphery of the anchoring tray as shown, which is then led to the slicing tray (42) which incidentally serves as a lid for the anchoring tray (32), which discourages the tube from escaping from the anchoring channel of the anchoring tray after installation.

The anchoring tray includes a cut out to accommodate a water blocking connector (40). Water blockers help prevent water from migrating to the fibre splice. This is required as the node is located underground, up on a telephone pole or the like, so that it is susceptible to water ingress. At the termination at the customer end (9 in Figure 1), the tube is additionally gas-blocked for health and safety reasons, before entering the premises.

Figure 4B shows how the transport tube (34) is arranged within the fibre unit anchoring tray (32). A transport tube (34) enters the tray in the direction, of arrow "X". The transport tube passes under and then up into the tray as shown.

(The skilled person would understand that references to movement used herein such 10

as "passes", "enter", "exit", "arrive" and "leave" have no literal significance in descriptions of essentially static installations such as fibres disposed on node trays, and so are used only by convention and for convenience.)

The cross section of the anchoring channel (36) in this tray is wide and deep enough to frictionally receive the transport tube (34). Starting from point "A", the fibre tube is wound initially in a counter-clockwise direction following the arrow "Y" on the left-hand loop for three-quarters of a turn (270 degrees). At point "B", the fibre tube is wound in a clockwise direction (again following arrow "Y") for a full turn (360 degrees) until the fibre tube once again reaches point "B". The direction of winding changes again after the crossover point "B" so that the tube is again disposed in a counter-clockwise direction until it reaches point "C". By this time, the fibre tube has undergone a total of two-and-a-quarter turns (810 degrees). At point "C" the fibre tube leaves the anchoring channel. The configuration of the tube thus obtained is secured and stored under tension by confinement of this section of the tube and any fibre within in the anchoring channel.

The transport tube then is guided along the side channel (37) along the edge of the tray, where it is connected to another transport tube via. the water blocking connector (40). It serves to transport the fibre unit up to the splicing tray (42). A pair of hinges (46) couple the entire tray assembly to the spine of the node, and another hinge (44) couples the fibre unit anchoring tray (32) to the splicing tray (42) and the tray assembly lid (not shown).

Figure 4C is a view of the fibre splice tray (42) being part of the tray assembly (30). Again, the lid of the assembly is not shown. The fibre unit within the transport tube (34) is led up from the anchoring tray (32), then broken out from the transport tube for splicing (using a separate device). The splices formed are stored in the splice holder slots (48).

The tray assembly and component trays can be made from polycarbonate or an ABS plastic compound, and has dimensions of about W150mm x D120mm. The node it is to be used in is about 350mm tall, and can accommodate anything between two and 12 such tray assemblies. 11

Turning now to the use of such a tray: when a suspended BFD experiences environmental loading in the form of wind, ice and the like, it will elongate. When this happens, the fibre unit within the BFD is likely to experience a measure of tensile stress as well. This stress will be transmitted to the cable ends as a pulling force, specifically to the nodes where the fibre is spliced. If the pulling force is sufficiently great, the fibre may be pulled completely out the splice. This is of course fatal to the connection. As fibre is able to withstand only about 1% tensile strain, any more will compromise optical performance even if the splice or connection remains mechanically intact.

Thus the securing of the fibre in the securing tray (30) serves to protect the delicate splice from the possibility of high tensile forces along the length of the fibre, by mechanically isolating it from the stress experienced along the cable. The effects of the strain are instead absorbed by the secured section of fibre running through the anchoring channel (36).

As was discussed above, friction between a cable or fibre unit and the object against which the cable/fibre unit is bearing such as the inner section of the tube, has the effect of reducing relative movement. Thus can movement the optical fibre within the tube be discouraged. In the formula for what is sometimes known as the capstan effect:

μθ

Ti = T₂ e

where

T₁ is the pulling force T₂ is the restraining force μ is the coefficient of friction between the surfaces θ is the angle of wrap (in radians)

it can be seen that the restraining force required to counteract the pulling force is related to the friction level between the surface of the fibre and the tube, as well as the number of turns of the tube.

Thus, the gripping force obtained from the friction generated can be increased or 12

decreased by changing the type of tube, the type of fibre unit, and the number of wraps or turns of the cable. For example, transport tube is deployed within the securing tray in the shown embodiment, which advantageously improves the friction within the tube. However it would of course be possible to use any tube, even low-friction flown fibre tube. The applicants have determined that in the UK, wrapping tubes having the dimensions of 2.0mm inner diameter/3. Omm outer diameter, or 2.5mm inner diameter/4. Omm outer diameter through a total of two-and-a-quarter turns would achieve the desired securing effect. This is based in part on external information sources, such as wind data from the Meteorological Office. The cable could of course be wrapped though further turns, but this would take up more cable and space. Other applications in e.g. other countries and/or using other tubes and fibres having different friction levels, would require a different number of turns to achieve the desired anchoring effect.

Advantageously, the anchoring channels are configured so that the tube and optical fibre are looped in a figure-eight configuration.

"Straight coiling" by looping a cable in the same direction coil after coil, introduces an axial half-twist (180 degrees) for every coil formed into the cable. Thus, every two turns introduces a full axial twist (360 degrees); an excessive amount of torsion can degrade optical performance, and introduce mechanical stresses into the fibre. Where at least one end of the cable is free to rotate about its axis, the axial twist can be removed. However this is not possible in e.g. a blown fibre installation, where both ends are substantially secured through actual fastening, and/or by dint of the length or other property of the tube or fibre as the case may be.

By disposing the tube, and the fibre in a figure-eight configuration however, the half- twist introduced into the fibre when forming the first loop is taken out upon the creation of the second loop when the change of direction (e.g. from counter-clockwise to clockwise) is made during the winding.

Thus, when a tube is initially installed along the path from the distribution point to the customer premises, the empty fibre tube is secured in the securing tray. The end of the tube leading to the suspended BFD is actually or effectively secured, and the other end leading to e.g. the customer premises through the premises wall is also incapable 13

of rotation. When the time comes to blow the fibre unit through the section of fibre tube in the securing tray, the tube is carefully removed from the anchoring channels which frictionally grip the slightly pliable tube. When the tube is removed, it lifts out neatly so that the loop on the right hand side unfurls at the crossover portion (point "B" in Figure 4B) and the tube straightens out without a twist. The fibre is blown through the entire length of the installation, then the newly populated tube is re-inserted into the anchoring channel, so that the initial half-twist is negated by the creation of the second loop on the right hand side.

It is trite that the loops can be formed in any order, from the left hand side to the right hand side, from top to bottom and so on; that the loops can be wound by starting in a counter-clockwise direction and then clockwise, or the other way around. Also, the loops need not be of the same size as long as the angle of wrap is the same. What is essential to the invention is that a twist introduced into the cable, tube or fibre has to be taken out again, so that there is zero net bend in the cable. Thus, it would be within the scope of the invention to form a number of pairs of loops as shown in Figure 5 where a cable wound in the direction of the dotted arrows, as each second loop formed takes out or negates the twist introduced by the first-formed loop. Another embodiment of the invention is shown in Figure 6, where the cable is coiled using a known technique known as "over/under coiling", involving the steps of twisting the cable in one direction to make the first coil, un-twisting it to make the next, and repeating this until the end of the cable is reached.

The skilled person would also recognise that there are a number of ways of guiding or forming the tube and/or fibre into the required securing configuration and to secure the configuration when formed. For example, clips disposed in a correct layout could be used, as could bands or some other such confining means and method. The use of a anchoring channel has the advantage of being easy to use, requiring only that the operator press the tube into the pre-formed channels, and further imparts some gripping effect onto the outside of the tube itself.

Figures 7A and 7B depicts alternative embodiments of the invention. These are devices for securing a smaller number of blown fibre tubes, e.g. a single tube containing a fibre unit comprising four individual fibres, and for splicing the same. As such this version of the invention is ideal for use at the customer premises and can be 14

located e.g. within the customer termination (9 in Figure 1) that could be located on the external wall of the premises, or alternatively within the premises.

In both embodiments, the cable anchoring device (60) is protected by a housing (62). The housing can be made weather proof and be strengthened against accidental and deliberate damage if it is to be deployed outside the premises. A gas-blocking (41) connector can be seen in Figure 6A as this is the located at the customer end.

A pair of bollards (64) is provided within the housing, which serves as the means for securing the fibre against movement. Fibre tube (34) in the case of Figure 7A, and bare fibre (66) (which has been broken out from the fibre tube) in the case of Figure 7B, is wound around the bollards in a figure-eight arrangement. The surface of the bollards in contact with the tube or fibre are preferably made of a resilient material such as rubber, to improve the grip between the respective surfaces. ^• .

The surface area in contact between the bare fibre and the bollards (Figure 7B) are reduced compared to the engagement of the fibre and the tube (Figure 7A). It is anticipated that, according to the capstan effect formula, a greater number of turns or windings is required to obtain the desired level of friction exists to reduce or prevent movement of the fibre unit.

It would be realised by the skilled person that the fibre tube and the bare fibre need not be wound around a pair of bollards of the specific construction and dimensions shown in Figure 7A and 7B. For example, the bollards need no comprise cylindrical structures - they may be oval, or comprise just a section of a circle resembling a pair of facing capital letter "Ds". They may comprise simple a number of pegs within the housing defining the configuration to be followed by the fibre. Depending on the specific design of the bollards, it is possible to increase or decrease the size of the securing configuration as required and relevant for the particular application, size and other properties of the tube and the optical fibre. One of the many alternative implementations available, would be the possibility of using a securing tray much like that depicted in e.g. Figure 4A for the securing of bare fibre within the node.

Figure 8 shows yet another embodiment of the anchoring device of the invention. This can be used at the network termination in conjunction with the fibre node (e.g. within 15

the joint box 8 or on the pole (4, in Figure 1) and/or at the customer premises (6 in Figure 1). The device here is opened up permitting a view of the anchoring device comprising a pair of bollards (64) around which fibre unit and or fibre unit in a tube can be formed into a figure-eight configuration.

Figure 9 shows an installation using the method and device of the invention at the customer premises (6), which is represented by the external wall of the premises. An customer termination which maybe part of the ONU (10) including the anchoring device comprising bollards (64) is installed either internally or externally on the wall of the premises. It is of possible to provide a anchoring device separately to the rest of the ONU, and also to provide the same within the premises.

The apparatus, methods and configurations described above and in the drawings are for ease of description only and not meant to restrict the invention to any particular embodiments. It will be apparent to the skilled person that various sequences and permutations on the apparatus and methods described are possible within the scope of this invention as disclosed. The invention can also be applied to applications outside blown fibre installations, wherever an elongate member secured at each end includes a weak point along its length, which experiences a pulling force transmitted through the cable at the weak point, and is susceptible to the effects of twisting.

Claims

16Claims:

1. A method of protecting a weak point along a cable, the cable being subject to stress, comprising the steps of (i) selecting a section of the cable between a source of the strain and up to and including the weak point,

(ii) using the section of the cable to form a cable configuration comprising a pair of coils, by putting a twist in the cable by winding the cable in a first direction to form a first coil, and taking the twist out of the cable by winding the cable in a second direction to form a second coil, and

(iii) securing under tension the cable configuration formed.

2. A method according to claim 1 wherein the source of stress is a tensile load.

3. A method according to claim 1 or claim 2 wherein the step of forming a cable configuration comprises forming more than one pair of coils.

4. A method according to any preceding claim wherein the step of forming a cable configuration comprises forming a single pair of coils comprising a substantially figure eight configuration.

5. A method according to any preceding claim wherein the cable comprises an optical fibre substantially fastened at each end.

6. A method according to claim 5 wherein the cable comprises an optical fibre disposed within a tube, substantially fastened at each end.

7. A method according to claim 5 or claim 6 wherein the weak point is an optical splice.

8. A method according to any one of claims 5 to 7 wherein the step of forming a cable configuration comprises winding the cable into coils each having a radius of at 17

least 25mm.

9. A method according to any preceding claim wherein the step of forming a cable configuration comprises winding the cable through 810 degrees.

10. A method according to any preceding claim wherein the forming step and the securing step comprise winding the section of the cable around a pair of bollards.

11. A method according to any one of claims 1 to 10 wherein the forming step and the securing step comprise installing the section of the cable into a pre-formed channel.

12. A method according to any preceding claim, wherein the forming step is performed using the section of cable proximate to the weak point.

13. A method according to any preceding claim wherein the weak point is an end of the cable.

13a. A method of installing an overhead optical fibre installation including a method according to any one of claims 1 to 13.

14. A device for protecting a weak point along a cable, the cable being subject to stress, comprising guiding means for guiding a section of the cable to form a cable configuration comprising a pair of coils, the guiding means adapted to put a twist in the cable when wound in a first direction to form a first coil, and to take the twist out of the cable when in a second direction to form a second coil, and securing means to secure under tension the cable configuration formed.

15. A device according to claim 14 wherein the cable comprises an optical fibre substantially fastened at each end.

16. A device according to claim 16 wherein the cable comprises an optical fibre disposed within the optical fibre tube, substantially fastened at each end.

17. A device according to any one of claims 14 to 16 wherein the guiding means 18

comprises a pair of bollards.

18. A device according to any one of claims 14 to 17 wherein the guiding means comprises a anchoring channel defining a path along which, in use, the section of cable is installed to form the cable configuration.

19. A device according to any one of claims 14 to 16 further including a tray including a anchoring channel defining a path along which the section of cable is installed to form the cable configuration, and wherein the anchoring channel is adapted to frictionally receive the tube.

20. A device according to any one of claims 14 to 19 further including optical splicing means.

21. A fibre node including a device according to any one of claims 14^' to 20.

22. A method of using a device according to any one of claims 14 to 21 comprising . the steps of positioning the device at a section of the cable proximate to an end of the cable, - forming a cable configuration comprising a pair of coils, by using the guiding means to put a twist in the cable by winding the cable in a first direction to form the first coil, and to take the twist out of the cable by winding the cable in a second direction to form the second coil, and securing under tension the cable configuration formed.

23. A method according to claim 22 wherein the cable comprises fibre tube, further including the steps of releasing and un-forming the cable configuration, installing an optical fibre through the fibre tube, - re-forming the cable configuration, and re-securing under tension the cable configuration.

24. An installation for protecting a weak point along a cable, the cable being under stress, comprising a first device according to any one of claims 14 to 21 at a first end of the cable, 19

a second device according to any one of claims 14 to 21 at a second end of the cable, one or more utility poles defining a route between the first end and the second end of the cable, and wherein the cable is suspended from the one or more utility poles.