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Introduction
Installation of fiber optic cables is not the delicate type of operation that one might first think. As was discussed in Section 4, fiber optic cables are extremely strong and can be used in very harsh environments. Generally, fiber optic cables are considered easier to install in cable trays and in conduits than are copper cables because of their comparatively small size and lightweight.
The rules and procedures that apply to installing fiber optic cables are very similar to those that apply to installing coaxial cables. However, there are a number of important differences that need to be carefully considered. These are discussed in detail in this section.
This section is broken into two main sections. Firstly, it looks into the specified installation procedures that apply to fiber optic cables. It then provides a methodology for planning and carrying out an installation. It examines the requirements associated with indoor and outdoor installations, along with a detailed description of all the precautions that need to be taken into account in order to carry out a successful installation.
Secondly, the section examines some of the techniques and equipment that are used to organize and store cables and splices at their end points and at junctions along the cable route.
1. Initial preparation for a cable installation
The successful execution of a fiber optic cable installation requires careful planning of the project and contingencies to be allowed for, before the installation commences. The following details some of the preliminary requirements for the installation of fiber optic cable installations.
1.1 Site survey
Before the planning of a cable system begins, it will already be known what kind of services the fiber optic cable is meant for and the locations where the cables will have to be laid. With this objective in mind, the first requirement has to be carried out with a comprehensive site survey of the location where the cables are to be installed. The site survey should focus on determining the following factors:
• The most appropriate route for each cable. This could be with regard to the existing cable runs or in respect of the proposed newly installed cable housings. It will generally be more cost effective to use existing infrastructure, but the decision depends on the amount of space in the existing housing and on the condition of the housing. Also, an existing cabling route may take a longer path and the extra cabling costs associated with this may exceed the costs of installing a new route.
• The need to run the cables in cable trays, underground, in roof tops or as aerial cables.
• The condition of existing cable housings. (Whether costly maintenance is required before new cables are installed.) Is there any potential danger that could cause damage to the cable because of the poor condition of housings? For example, are the housings prone to be affected by flooding?
• Whether there are any locations that need special attention. Should tradesmen with special skills be deployed to carry out the job? Are there any locations that could subject cables to extreme temperatures? If so, provision is necessary to use fire or explosion-proof cables.
• Are there locations that could subject cables to possible physical damage? If so, provision is necessary to provide appropriate steel armored cables.
• Would the cable route run near high power cables? If so, ensure that the fiber optic cable does not contain any metal (strengthening member or sheath).
• Would the cable route run near areas of high transient voltages (for example, lightning)? If so, ensure the fiber optic cable does not contain any metal.
• Ensure that the installation adheres to all existing electrical and fire codes of the country to which the installation is planned.
• Obtain all required council and government permits before commencing any civil works on public land.
• Will there be sufficient room to use the cable pulling equipment? If not, what equipment needs to be moved to carry out the installation without hindrance.
• Will cars or trucks be driving over the cable, people walking over it or heavy objects laid across it? If so, plan to take the necessary precautions to protect the cable (for example, conduits) and/or to use the correct sheathed cables.
• Locate all the intermediate points from where the cable is to be pulled and where junction boxes are to be located.
• Identify appropriate locations for installing termination cabinets and splicing trays.
• Determine the exact locations for each data equipment hub.
• Talk to local employees to determine if there are any foreseeable problems that may arise during the installation that can be averted now by careful planning.
• All these particulars should be carefully noted during the site survey and then officially and completely documented after the survey is done. These findings would be useful while designing the cable system.
1.2 Designing the cabling system
The cable layout should be designed and a cable pulling plan developed, using the findings obtained during the site visit. The proposed cable layout should be drawn on to an existing cabling diagram of the site if it is not a new site installation. The cabling diagram that is used should include all existing cabling and cable housings. For example, all cable trays, conduits and pole lines should be illustrated. For the purpose of orientation, it is essential to incorporate outlines of buildings, roads, and fixed machinery in the diagram. The new fiber optic cable routes should then be drawn over the top of this with a dark pencil. Termination cabinets and fiber node points containing splicing trays and patch panels should also be drawn on to the diagram in pencil.
A typical building cable network layout is shown in FIG. 1. In some countries, according to their local fire prevention codes, outdoor cables that are filled with jelly should be spliced to non-flammable indoor cables close to the cable entries. Alternatively, the fibers can be cleaned and enclosed in protective sleeving e.g. 'zero cable', and taken to the patch panel or optical fiber distribution frame (OFDF) directly. The cross connection arrangements and distribution hardware needs to be specified for each cable.
FIG. 1 illustrates a typical cable layout diagram. Note that the diagram includes the cable fiber sizes (the number of strands in the fiber) to be installed, the locations for new and old pit boxes, the requirement for new conduit and for fiber optic termination cabinets.
If a fiber ring is being formed, the cables are normally cut in the pit, both ends are taken into the building where they are either spliced through or pig-tails are connected to the fibers before taken to a patch panel. Often, there is a combination of spliced fibers (which are more secure compared to those on a patch panel) and fibers with pig-tails taken to a patch panel. Taking them to a patch panel allows the rings to be made or broken as required, but leaves them free to accidental removal. Compare the length of each cable run with the length of fiber optic cables on the reels that are to be used. Using this information, determine the location of any additional intermediate splicing locations that are required.
Once the cable layout diagram is complete, a cable installation program should be drawn up. This document will be used by the contractor's installation team and therefore, it should contain precise but lucid detail of all the installation procedures and requirements. It should contain a thorough description of all the considerations and potential problems that were noted during the site survey.
The installation program should include a detailed description of the following information:
• The logistics of pulling the cable.
• Where the pulling equipment and cable reels should be located during the installation for each separate pull.
• The precise location where the pit boxes, termination cabinets and splicing trays are to be located.
• Which fibers are to be spliced and which fibers are to be taken through to a patch panel
• Each separate cable pull and the cable size and type to be pulled.
• Each separate conduit installation and the size and type of conduit to be installed. Specify which conduit is to be used over each section.
• Which cable trays are to be used.
• The routes to be taken for cable runs through the roof space.
• All the cable trays, conduits or other housings that will need replacing.
• An installation schedule that would minimize traffic congestion while carrying out road works during peak hours.
• The setting up of 'no parking' areas where installation equipment is to be located. This should be carried out the day before the installation begins. This requirement should cover all pit boxes and manholes.
• All observations that were made during the site visit.
• The specific responsibilities of each member of the installation team should be defined.
When the installation is complete, document all the changes made during the installation and produce final 'as installed' drawings. This will help to ensure that the cables have been installed correctly and that future fault finding and any system upgrades will be hassle free.
FIG. 1 Example of cable layout schematic
2. General installation rules and procedures
The following section provides general installation rules and procedures that should be followed when installing fiber optic cabling systems. They are broken into related subsections for the convenience of reference.
2.1 Cable bend radius
• The rules and general cable specifications applied to minimum fiber bending radius that were discussed in Section 4 would apply here also.
• The most important consideration when installing fiber optic cables is to ensure that during an installation, the cable radius is always not less than the recommended minimum-bending radius of the installation.
• Avoidance of sharp bends along the installation route is absolutely essential. Sharp edges in cable trays or in conduits can cause macrobends or microbends in the fiber, which will significantly affect signal attenuation.
• Ensure that the conduit or the cable tray is constructed with no sharp edges. Use curved construction components and not right angle or T piece components.
• Ensure that the cables are laid on to a flat surface, and that no heavy objects will be laid on to the cables in the future.
FIG. 2 Avoid use of conduit or cable trays that have T connection of 90°
angles
• Avoid putting kinks or twists into the cable. This is best achieved by pulling the cable directly off the reel and entrusting a member of the installation team with carefully watching any cable slack for possible formation of kinks.
• Cable manufacturers will specify a minimum bending radius that applies during the installation of the cable and a minimum bending radius that applies to the long term final installed cable. The long-term radius is significantly larger than the installation radius. Once the cable has been installed and the tension has been released, ensure that the cable radius is not less than the long term installed radius at any point along the cable.
• For any single cable pull, whether it be through conduit, cable tray or otherwise, there should be no more than three 90° changes of direction. If there are more than three 90° changes, then cable should be pulled through to an intermediate point, straight after the third 90° change of direction and the use of back feeding must be performed.
FIG. 3 A cable with three 90° bends using an intermediate pulling point
• As a general rule of thumb, a fiber optic cable that has a diameter equal to 2 cm or less than that will not exceed its minimum installation bending radius if it is limited to a minimum bending radius of 30 cm during installation.
FIG. 4 Approximate minimum bending radius
FIG. 5 Pull the cable on the long side of a bend
2.2 Cable tension
• The rules and general cable specifications applying to maximum allowable cable tension, as were discussed in Section 4 would apply here.
• Although modern fiber optic cables are generally stronger than copper cables, failure due to excess cable tension during installation is more catastrophic (i.e. fiber snapping rather than copper stretching).
• A general rule of thumb used sometimes is that the maximum allowable cable tension during installation is approximately the weight of 1 km of the cable itself.
• When pulling the cable during installation, avoid sudden, short and sharp jerking. These sudden forces could easily exceed the maximum cable tension. The cable should be pulled in an easy smooth process.
• When pulling the cable off a large drum, ensure that the cable is smoothly rotated by one team member to feed off the cable. If the cable is allowed to jerk the drum around, the high moment of inertia of the drum can cause excessive tension in the cable.
• It is very important to minimize cable stress after the installation is complete. A slack final resting condition will help to ensure that the fiber optic cable has a long operating life.
• When pulling the cable through bends, it is recommended that the pulling be performed on the side of the bend where the cable is longer. This reduces the tension in the cable because the majority of the weight is being pulled directly. The bend has the effect of multiplying the tension, so it is better to multiply the small tension at the source rather than the larger tension at the end of the pull.
• If there are many bends in the cable route, it is recommended that as many intermediate junction boxes as possible be used to reduce cable tension. The cable is pulled through at these points, laid out in a large figure '8' pattern on the ground, and then pulled into the next section. Laying the cable in a figure '8' pattern naturally avoids kinking and twisting of the cable. Block systems may be used in the junction boxes. It is recommended that slack be left in the junction boxes at the completion of the installation to reduce overall stress in the cable.
FIG. 6 Running cables through junction boxes
• Most cable manufacturers provide a maximum cable tension value for installation and a maximum cable tension value for the long-term final installed cable. This is of relevance to cables that are installed in cable risers.
Regular tying of the cable along its length will help to alleviate this problem.
• Another important type of cable tension that can cause severe damage to the fibers and quite often overlooked is that of torsional twisting forces. Cable twisting can be caused by using incorrect installation techniques or from forcing cables through tight conduits. When using an ordinary layed-up rope it would twist considerably as tension is applied and it might also twist the (b) Use blocks in Junction boxes if permissible cable. This should be avoided by using a swivel connection to the cable. To help prevent this problem, lay all cables in a figure '8' pattern onto the ground at intermediate pulling points, and always have a member of the installation team manually guiding and watching the cable, as it is fed into the conduit or cable tray.
2.3 Cable reels
• Every cable installed should be given a separate number that is noted on the cabling diagram during installation. Cable suppliers usually place a serial number on the side of the reel, which can be used for this purpose.
• Each fiber on the reel should be tested for attenuation figures before commencing installation.
• Cable manufacturers normally leave the end of the cable that is on the inside of the reel protruding out so that it can be used for testing. After each reel of cable is installed, a second attenuation test should be carried out on each fiber to ensure that there has been no significant damage incurred during installation. The results of these tests should be recorded with the results of the pre-installation tests.
• The cable end that is on the inside of the reel should be taped firmly to the side of the reel so that it does not catch on the outgoing cable during payoff.
Cable reel Tape
FIG. 7 Tape the inside loose cable end to the reel
• In order to minimize damage and unnecessary handling of the cable during installation, it is advisable to payoff directly from the reel. This can be achieved by holding the reel on a rod and directly unreeling it as you walk along the cable tray or trench, or by placing the reel on a payoff stand at the beginning of the cable run and directly payoff from there. This method of payoff also helps to prevent unwanted twisting and torsional tension of the cable.
FIG. 8a Laying the cable by direct payoff from the reel
FIG. 8b Pull the cable directly in with the reel on a payoff stand
2.4 Installation in cable trays
• Laying the cable directly on to the cable tray from the reel will cause the least stress and damage to the fibers. This is often very difficult because of space restrictions around the cable trays and the tray hangers. Refer to FIG. 8a.
• If the cable cannot be laid directly, then it should be pulled in. Ensure that it is not pulled against hard sharp bends. Have a second person to pull cable slack into the bends or set up a system of temporary blocks. Refer to FIG. 8b.
• Ensure that the cable does not cross any cable tray hangers.
• Ensure that the cable is laid flat in the tray and not over any uneven cables.
• It is recommended to lay the cables loose in the cable tray and not tied to other cables or to the tray itself.
• It is possible to simply tie the pulling rope to the strengthening member with a knot. This is enough for very simple pulls with low resistance. But generally, it is not advisable as the knot may get entangled along the route and may break. Tape over the joint to reduce similar risk to the cable and prevent ingress of dirt or water into cable.
FIG. 9 Attach pulling rope to cable strength member with knot
The following is the recommended method for attaching a pulling line to a fiber optic cable:
• Strip back the cable to expose 15 cm of the strengthening member only.
• Cover the strengthening member with epoxy glue.
• Place the pulling rope 30 cm back from the stripped end, and tightly tape the rope to the cable with insulation tape moving from the end of the rope to the end of the cable.
• Continue to tape the cable until a smooth transition is reached between the strengthening member and the cable.
FIG. 10 Attaching a pulling line for cable tray runs
If the fiber optic cable has pre-terminated connectors, the following is the recommended method to attach a pulling line:
• Do not use any epoxy glues.
• Place the pulling rope 1 m along the cable, and using insulation tape, tightly wrap the rope to the cable until 2 cm approximately from the connectors.
• Ensure that protective caps have been placed on the ends of the connectors.
• Carefully wrap the connectors and pulling rope with tape. Ensure it is smooth but not tight.
• Place several small pieces of wood, bamboo or basket weaves about 10 cm in length around the connector ends, and smoothly but not tightly wrap these to the cable with tape. The reason for doing this is to prevent the connector ends from being bent back and the fibers from being broken while the cable is being pulled.
FIG. 11
Attaching a pulling rope to a pre-connectorized cable for cable tray runs [Connecterized Cable]