AMAZON multi-meters discounts AMAZON oscilloscope discounts
cont. from part 1
2.5 Installation in conduits
• Ensure that the cable is sufficiently covered in lubricant before it is pulled into the conduit.
• The cable should be pulled by hand wherever possible.
• If the cable is to be pulled by winch, it is essential that a tension gauge is attached to ensure that the maximum permissible cable tension is not exceeded.
• The ends of cables must be completely sealed and made waterproof before pulling commences. Moisture around the fibers will cause permanent long term damage.
• The larger the surface area of the cable compared to the surface area of the conduit, the more friction that will exist as the cable is pulled through the conduit. To determine the number and size of cables that can be pulled through a given size conduit, a general rule of thumb is to divide the cross- sectional area of the cable by the cross-sectional area of the conduit and compare this calculated percentage figure with maximum permissible percentage figures. Approximate figures used for the X% value illustrated in FIG. 12 are:
- Less than 55% for a single cable
- Less than 30% for two cables
- Less than 40% for three or more cables
FIG. 12 Determining if cables will fit into a conduit.
• It is possible to simply tie the pulling rope to the strengthening member with a knot. This is enough for very simple pulls with very low resistance. But for conduit runs, this is generally considered unacceptable as the knot may catch along the route and may suffer from breakage.
The following is the recommended method for attaching a pulling line to a fiber optic cable using a 'pulling eye'.
• Strip back the cable to expose 15 cm of the strengthening member only.
• Cover the strengthening member with epoxy glue.
• Fill the pipe section of a pulling eye with epoxy glue and fit it to the strengthening member. Allow the epoxy to set before commencing the payoff.
• Cover the end of the cable with tape to ease the transition between strength member and cable. This protects the end of the fibers also and stops ingress of dirt or water.
FIG. 13 Attaching a pulling eye to the cable
• For long cable runs, use intermediate pulling points where the cable is pulled through and coiled up in a figure '8' pattern on the ground and then fed into the next conduit section.
• At intermediate pulling points, reapply lubricant to the cable before pulling through the next conduit section.
• For cable runs greater than 500m, it is advisable that intermediate junction boxes be installed with several meters of cable slack at each entry. The junction boxes should be strategically located to account for possible future extension of the cabling system to other locations.
• Another very popular method of attaching a pulling rope to a fiber optic cable is the 'Chinese Basket' or 'Kellems Grip'. This works most effectively on larger diameter cables (generally greater than 0.75 cm diameter). It consists of a pulling eye with a long cylindrical weave (up to 1 meter) attached to it. The weave is placed over the end of the cable and is glued or taped to the cable. If it is glued, the end of the cable is cut off and thrown away after the pull is complete.
FIG. 14 The 'Kellems' cable grip [Pulling Eye Cable Weave Mesh]
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 3 m along the cable, and using electricians tape, tightly wrap the rope to the cable until approximately 10 cm from the connectors.
• Ensure that protective caps have been placed on the ends of the connectors.
• Place a metal pipe with a sealed end over the connectors and tightly wrap the pipe to the pulling rope with insulation tape. The main purpose of the pipe is to prevent the connector ends from being bent back and the fibers from being broken while the cable is being pulled.
FIG. 15
Attaching a pulling rope to a connectorized cable for conduit runs
2.6 Leaving extra cable
• It is considered mandatory that significant cable slack is left at the beginning and end of every cable run. Cable slack should be left at all termination cabinets, junction boxes, pit boxes, splicing centers, splicing trays, cable vaults and at the end equipment. This cable slack is useful for cable repair, entry into the cable along its length and equipment movement.
• Cable slack is important when a cable requires repairing. If the cable is accidentally cut or dug up, the cable slack can be shifted to the damaged location, necessitating only one splicing point in the permanent repair, rather than two splices that would be required if additional cable were added. This results in reducing costs and less link loss. Generally, 3 to 6 m of cable at each end of a cable run is sufficient for this contingency.
• Additional cable slack at any planned future points of cable system expansion will provide significant cost and labor savings when the new cable drop is required. For this purpose, a minimum of 10 meters of cable slack should be left at these points.
• Additional cable slack will allow relocation of equipment, terminals, hubs and the cable itself with relative ease and without requiring new splices.
• Splicing of cables is a relatively involved process and it requires significant working space to be performed correctly. Splicing cannot be performed in confined spaces or in mid air. Enough slack cable must be left to allow the cable to be taken to a table to be spliced. This may be as far away as 5 m and should be planned for and written into the cabling installation plan before installation commences. In this case, allow approximately 10 m of cable slack.
FIG. 16 Leaving sufficient slack at termination points
• Consideration is necessary as to where the cable slack is to be stored once installation is complete. The location where it is to be stored should have sufficient space so the cable does not suffer from macrobends and should be located where it will not be disturbed and will therefore be protected from potential damage. For external cables, it is often convenient to use round (at least 1 m diameter) jointing pits, and to coil the cables back into the pit after jointing above ground.
• Spare fibers should be coiled up in the splice trays and carefully clipped out of the way.
• Remember to include all cable slack requirements in the cable length calculations.
2.7 Lubricants
• Use standard cable pulling lubricants for installations where excessive friction is anticipated. The coefficient of friction per dry Blyethylene cable sheath in a PVC duct has been measured as about 0.45. This is clearly a function to find out how smooth and clean the ducts are, with no excess glue at joints etc. Using a proprietary cable pulling lubricant such as 'Polywater' values of coefficient of friction as low as 0.1 have been measured if complete coverage of the cable is achieved. Practical field applications have tended to show results in the range 0.15-0.25 due to lesser coverage. As these products are expensive, they are normally only used when essential.
• For long pulls of external cables, water is the best lubricant. This has been shown to reduce the coefficient of friction to about 0.3. Its great advantage is cost so we can ensure adequate covers of the ducts. Flood the ducts with water continuously throughout the pulling process and 'float' the cables in.
• For cables that are to be pulled through conduit, it is recommended to always use lubricants. 7.2.8 Environmental conditions
• Avoid installing cables when the ambient temperature is less than 0°C or greater than 70°C. Beyond these limits, there is a possibility of damage to the cable sheath, and in some cases, to the internal components of the cable and subsequently, the fibers themselves.
• Avoid installing cables when the humidity is greater than 95% for ambient temperatures greater than 60°C.
• Cable suppliers specify a maximum temperature and humidity at which the cable should be stored before installation.
3. Indoor cable installations
• This section of the section examines particular requirements that are associated with the installation of fiber optic cables in indoor environments.
• Rubber floor ducts should be used to protect the cable, if cables are to be installed on to floor areas that people would walk over.
FIG. 17 Rubber floor ducts
• If cables are to be installed under the carpet, ensure the cable type has a strong sheath and is of a loose tube construction. Special cables are available specifically for installing under the carpet.
• If cables are to be laid around the walls of a room near the skirting, ensure that they are taped to the skirting with a high quality strong tape. This will help prevent damage to the cable or its connectors when accidentally pulled up by a passing foot.
• If a cable is to be run vertically up a wall, then cable clips that are screwed to the wall should be used to hold it in place. Wrap electrical tape around the cable before inserting it into the cable clip. The tape will provide more malleable sheath, which will firstly provide a better grip for the clip and secondly, cause less damage to the cable from the clip.
FIG. 18 Cable wall clips [ Screw Electrical Tape Cable Clips Cable]
• Rubber grommets should be used where the cable enters or leaves a plastic or metal cable duct. They protect the cable from sharp edges and from bending tighter than the minimum bending radius.
FIG. 19 Rubber grommets used in cable ducts [Duct Cables Rubber Grommet]
• Often, installers will bolt a second smaller cable tray to the side of the main cable tray for the fiber cables only. This ensures that no other cables will crush or damage the fibers. The trays are generally made of plastic and are often colored yellow. Flexible plastic tubing then runs from the fiber cable tray down to the rack.
• As the majority of indoor installations are only short cable runs, it is often more cost effective to have the cables pre-terminated in the factory before being transported to site for installation. This procedure generally saves both time and money.
• Cables that are installed under raised floors are subject to crushing and kinking. Therefore, cables should be either of a high quality (have a strong sheath and be of loose tube construction) or installed in a conduit or a separate underfloor cable tray. The conduit will aid in the pulling of the cable and will provide valuable protection when the inevitable rearrangement of the copper cables occurs.
• If the cable is to be laid directly into the ceiling (which is often the most cost effective method of indoor installation), care should be taken to avoid cross members, ceiling hangers, sharp edges and corners, sharp screws or nails or metal studs and around areas warranting heavy potential maintenance (e.g. air conditioning ducts, water or gas piping, electrical installations). If the cable is to unavoidably run near these dangers, then install it in conduit, even if the conduit lengths are only of short sections.
• For vertical installations, most tight buffered riser fiber optic cables will self support approximately 100 m of their own weight over the life of the cable. Ensure the bending radius is not exceeded at any vertical transitions and use clamps on the sheath at regular intervals.
• For installations in cable risers or elevator shafts, it is recommended that the cable be tied at every floor of the building. Wrap the cable in electrical tape before a tie is attached and ensure that the tie is not pulled too tight. This will ensure that the cable does not exceed its maximum tensile load and would help prevent cable movement.
• Connection to any data equipment or patch panel should be with a large loop of slack cable (generally about 30 cm).
4. Outdoor cable installations
This section of the section examines particular requirements that are associated with the installation of fiber optic cables into outdoor environments.
• Obtain the relevant permission or permit that is required to run the cable through government or private property that does not belong to the cable owner.
• Carefully plan all installations and carry out thorough cost analysis, so that the final cable route is of minimum cost.
• For outdoor installations, it is vital to use the correct cable. The cable should be chosen to suit the application and the environment in which it is to be used. Do not hesitate to seek professional advice from cable suppliers if required.
• The parameters of cable type, fiber type, sheathing, diameter, moisture barrier, strengthening members, connectors and splicing type all need to be carefully considered.
• It is recommended that all underground cables be installed in conduit. The conduit would provide protection from water, excessive temperature variations, physical stress from cars or trucks that drive over the top of the cable, attacks by vermin and to some extent, from persons accidentally digging through the cable with spades and mechanical diggers. Also, of significant importance, it allows new cables to be laid without having to dig the trench again and to easily replace damaged or old cables.
FIG. 20 Bury underground cables in conduit where possible
• Cables can be buried directly in the ground, but they should have a suitable sheath that provides protection against vermin and serves as a good moisture barrier (preferably jelly filled). The sheaths can be double jacketed, nylon, Teflon and/or metal armored.
• The deeper a cable is buried the less likely it is to suffer from temperature variations, physical stress or attacks from vermin. A depth of 1 to 1.5 m is ideal.
• Allow a minimum of 3-6 m of cable slack at the end of each run to reduce any possibility of undue cable tension, and to allow for the possibility of repairs.
• Place a termination cabinet and patch panel at the end of each cable run between buildings so that the system can be easily reconfigured and maintenance can be carried out as is required.
• At intermediate points where cable is pulled out and stored on the ground before being pulled through the next section of conduit, the cable should be laid on the ground in a large figure '8' pattern. This helps prevention of twists and tangles forming in the cable when it is pulled into the second stage of the route.
FIG. 21 Carrying out an intermediate cable pull
• If pressurized cables are to be used, ensure that the cable pressure is checked before and after installation. With this type of cable, particular care should be taken to ensure that the pulling eye and end cap seals are not broken during installation.
5. Other installation methods
This section briefly looks into two other methods of installing fiber optic cables.
5.1 Aerial installations
Fiber optic cables are often installed as aerial cables hanging from electric power poles. Special cables that have significant internal strength are manufactured and those can be installed hanging directly between two poles. Other cables are designed to be supported along a steel support wire (also referred to as a messenger wire). It is also possible to tie the fiber optic cable to the power cable itself. The use of a support wire is generally preferred because it provides extra strength and puts less stringent strength requirements on the fiber optic cable itself.
Aerial cables are designed to withstand large forces that result from strong winds and extremes of temperatures. The sheath of the cable is made from UV stabilized plastic and is designed for an extended operating life of 10 years or more.
If the aerial fiber optic cable is to be attached to a steel support wire, the cable should be securely tied or taped to the support wire every 30 cm. The ties of tape that is used should be UV stabilized and designed for outdoor weather conditions. At the mid point of each 30 cm span, the cable should have a droop of approximately 3 cm to allow for expansion and contraction of the steel support wire. The support wire often passes through a pulley block on the pole or is attached to a ring bolt on the pole using dead end grips.
FIG. 22 Aerial cable attached to support wire [3cm 30 cm Steel Support Wire
UV stabilized tape or ties]
Most slotted core or loose tube fiber optic aerial cables are compatible with the standard helical lashing or clamping techniques that are normally used with other telecommunications cables.
5.2 Blown fibers
This is a technique developed by British Telecom, which involves the use of fibers installed directly within a 6 mm microduct. The fibers are drawn directly along the duct by the aerodynamic drag of the viscous flow of air from a small blower producing up to 150 psi. This technique can be used over distances up to several kilometers.
Special fibers that have a rough outer coating that is designed to create significant drag in one direction and very smooth in the other is used in this technique. In this way, the fiber is picked up by the forced air through the tube but has minimal resistance, as it is dragged through the tubing.
The microduct is installed as bundles of color-coded tubes within an overall sheath of polyethylene. These tube bundles can be installed into cable ducts using conventional cable techniques. The individual microducts are joined together by push-on fittings to make a continuous leak-free path from one end to the other. Individual tubes can be brought out from the bundle at the intermediate points and diverted off to individual customers.
The advantages of the technique are that there is virtually no strain imposed on the fibers during installation. Accordingly, the fibers do not require extra strength members. Up to six fibers can be installed in each microduct and fibers can be blown into the duct over existing ones at a later date, enabling fiber provision to be deferred.
This technique is becoming more popular throughout the world, particularly in building distribution systems. It is sometimes preferred because the fibers can be installed on an as required basis, making building cabling management significantly easier. The fibers will also have good mechanical protection as they run through risers, over hung ceiling and under raised floors.
6. Splicing trays/organizers and termination cabinets
This section looks into different types of storage units that are used for housing optical fiber splices and end of cable terminations.
6.1 Splicing trays
Splices are generally located in units referred to as 'splicing centers', 'splicing trays' or 'splicing organizers'. The splicing tray is designed to provide a convenient location to store and to protect the cable and the splices. They also provide cable strain relief to the splices themselves.
Splicing trays can be located at intermediate points along a route where cables are required to be joined or at the termination and patch panel points at the end of the cable runs.
The incoming cable is brought into the splicing center where the sheath of the cable is stripped away. The fibers are then looped completely around the tray and into a splice holder. Different holders are available for different types of splices. The fibers are then spliced onto the out going cable if it is an intermediate point or on to pig-tails if it is a termination point. These are also looped completely around the tray and then fed out of the tray. A typical splicing tray is illustrated in FIG. 23.
FIG. 23 A typical splicing tray
The fibers are looped completely around the tray to provide slack, which may be required to accommodate any changes in the future, and also to provide tension relief on the splices.
Each splice joint is encased in a splice protector (plastic tube) or in heat shrink before it is clipped into the holder.
Splicing trays that have patching facilities are available. This allows different fibers to be cross connected and looped back for testing purposes.
6.2 Splicing enclosures
As a general rule, it is always preferred that splicing of fibers is carried out inside the building, and stored within an equipment rack in the building. External splices are difficult to change once they are installed. They are a perennial concern for network maintenance personnel. Unfortunately though, there will be times when external splicing is required.
The splicing trays are not designed to be left in the open environment and must be placed in some type of enclosure. The enclosure that is used will depend on the application. The following are examples of some enclosures used for splicing trays.
Electrical signal characteristics:
• Direct buried cylinders
At an intermediate point where two cables are joined to continue a cable run, the splices can be directly buried by placing the splice trays in a tightly sealed cylindrical enclosure, that is generally made of heavy duty plastic or aluminum. The container is completely sealed from moisture ingress and contains desiccant packs to remove any moisture that may get in. A typical direct buried cylinder is illustrated in FIG. 24.
FIG. 24 Direct buried splicing enclosure
• Outdoor cases
The splicing trays are generally stored in metal sealed cases at outdoor junction points, as the splices need to be protected from environment. Such outdoor junction points are located in pit boxes or manholes. The case has a screw on lid that can be removed to carry out changes or to test the cable. Again, the case is completely sealed from moisture ingress. An outdoor case is illustrated in FIG. 25.
FIG. 25 Outdoor connection boxes
• Indoor connection boxes
At intermediate points or at junction points that are required indoors, the splice tray is placed in a metal or plastic box with a screw on or slide on lid. The boxes are then screwed to the wall or installed into an equipment rack. An indoor enclosure is illustrated in FIG. 26.
FIG. 26 Indoor connection box for splices (cover removed)
• Termination cabinets
At junction points where a lot of cables meet, the splicing trays are stored in a larger wall mounted cabinet (approximately 500 × 500 × 100 mm) with a hinged door. For outdoor use, the cabinets must be sealed against bad weather conditions. FIG. 27 illustrates a splicing tray in a termination cabinet.
FIG. 27 Termination cabinet for splicing trays
• Patch panels and distribution frames
Splice trays can be used in the back of patch panels and distribution frames for connection of patch cords to the main incoming cable. These enclosures are commonly referred to, as fiber optic break out terminals (FOBOT). An example of a FOBOT is illustrated in FIG. 28.
FIG. 28 Patch panel
6.3 Termination in patch panels and distribution frames
There are three main methods of connecting an incoming cable into a patch panel or distribution frame. Firstly, if the incoming cable contains fibers that have a large minimum bending radius, then it is recommended to splice each fiber to a pre-connected fiber pig-tail that has a smaller bending radius. This reduces undue stress on the incoming fibers and introduces only small losses into the link. This also replaces the more fragile glass of the incoming cable with the more flexible and stronger cable of the pig-tails. This particular technique is now by far the most commonly used technique to connect incoming cables into FOBOTs. This is illustrated in FIG. 29.
FIG. 29 Top view of a FOBOT with splicing tray using pig-tails
The second method is to place the fibers from the incoming cable into a breakout unit.
The breakout unit separates the fibers and allows a plastic tube to be fitted over the incoming fibers to provide protection and strength as they are fed to the front of the patch panel. Note here that there are no splices, which therefore keeps losses to a minimum.
The downside is that the connectors have to be fitted by hand, which can introduce variations and the human element of uncertainty in connector quality and losses (which is not seen in the robot produced pig-tail connectors). This is illustrated in FIG. 30.
FIG. 30 Patch panel with breakout box
If the incoming cable contains tight buffered fibers that are flexible and strong with sufficient buffering, then they can be taken directly to the front of the patch panel. Again, there is the introduced human element of uncertainty when the connectors are fitted by hand. This is referred to as direct termination, and is illustrated in FIG. 31.
FIG. 31 Direct termination of cables in a patch panel.