Follow proven practices when installing fiber-optic cables in premises networks

Sep 1st, 1995

Follow proven practices when installing fiber-optic cables in premises networks

Although it is not a fragile medium, optical fiber cable must be carefully and expertly installed in premises networks to accomplish proper connectivity

This information was provided by Berk-Tek Inc., New Holland, PA.

The important aspects of fiber-optic cable installation include general premises network guidelines--for horizontal, ceiling, floor, cable tray and riser runs, for example--and specific procedural instructions that apply to most cable pulls--jacket removal, component handling, fiber stripping and breakout kit usage, for example.

Whenever a building or indoor space calls for fiber-optic cable installation, there are several basic rules to follow. These cost-effective and efficient rules have been culled from nearly two decades of practical experience in deploying fiber-optic cable networks.

For example, fiber-optic cable must always be supported to avoid crushing, stressing and overbending. Every cable is characterized by minimum-bend radius and maximum tensile loading values that must not be exceeded. Moreover, fiber cables should never be allowed to hang freely for long distances or be pressed against building structures.

When fiber-optic cable is being pulled in conduit, all transition points, such as those going from conduit to pull box or exiting the conduit, should be smooth. Sometimes adding a piece of conduit beyond the transition helps keep the cable from resting on a sharp edge.

Flexible conduit can be placed within boxes or at interfaces to prevent pressure or the scraping of rough edges against the fiber-optic cable. It can also be added to frequently accessed areas, such as raised computer-room floors, where numerous cables and devices pose a high risk for fiber-optic cable damage.

Compliance with the cable`s minimum-bend radius is mandatory. Some premises network applications might present conditions where equipment location could cause an overbending of fiber-optic cable if layout precautions are not taken. Conduit bends, pull boxes and joints must be checked to verify that the fiber-optic cable`s bend radius is not violated. Innerduct or flexible conduit can be used to ease or sweep the fiber cable around tight corners.

In practice, the inside radius of conduit bends for fiber-optic cable should be at least 10 times longer than the cable diameter. Cable pulls through tightly bent elbow fixtures should be backfed--that is, instead of being pulled from end to end, the cable should be inserted into and out of an open junction box. Then, the cable should be coiled loosely on a solid surface and fed through the rest of the run.

In tray and rack installations, the cable`s minimum-bend radius must be monitored where the cable is routed around corners or through transitions. Where raceway or rack transitions expose the fiber cable, flexible conduit should be used for protection.

Horizontal cabling

Intra-building conduit runs can be made in ceilings or walls or under floors. In these locations, however, certain precautions should be taken because conduit systems are inflexible. Such conduit systems should be used only where workstation outlet locations are permanent, no wiring flexibility is required and outlet density is low. In-floor conduits are often embedded in concrete, making network adds, changes and moves virtually impossible. Conduit can be made of metallic tubing or rigid polyvinyl-chloride plastic, according to National Electrical Code regulations.

Conduit runs should be limited to 100 feet, with no more than two 90-degree bends between pull points or boxes. The commercial building standard for telecommunications pathways and spaces, Electronic Industries Association/Telecommunications Industry Association 569, details conduit installation and sizing requirements. The NEC lists appropriate conduit types.

Pull boxes are useful for fishing the run and looping the cable for the next length of conduit; they should not be used for splicing cable. Fish tapes or pullcords should al ways be placed in the conduit to ease cable installation. Innerduct is a superb mechanism to protect cable and ease future installations.

Plenum runs or those in dropped ceilings or raised floors can be easy installations. Many dropped ceilings or raised floors have panels that can be removed or opened to provide access. Many new buildings have dropped ceilings, which ease cable installation. Raised floors are usually found in computer rooms. When the area under the floor is used for environmental air handling, the cable must be plenum-rated.

Suspended ceilings generally consist of removable lightweight panels supported by metal frames or grids that are attached to the ceiling using struts or wires. Cables in these ceiling spaces should be supported in organized, easy-maintenance trays, wireways or racks. At minimum, the cables should be supported by I-hooks or bridle rings.

Cable trays

Cable trays or ladder racks provide a convenient and safe location in which to install fiber-optic cables. Trays can be situated in ceilings, below floors and in riser shafts. Tray installation usually precedes pulling the fiber cable, because trays usually carry other types of cables; consequently, a tray-distribution system might already be in place. These routes can be used for installing new cable if they run to appropriate locations.

Fiber-optic cable should always be run in trays to avoid tension, crushing and bending. Tray routes should be inspected for sharp turns, snags (sometimes from other cables) and rough surfaces. To prevent stress, the cable should be installed without pulling under or between heavier or multiple cables. To accommodate moves, adds and changes, the cable should be secured to the tray at least every 3 feet.

Riser cabling

If a premises network requires the fiber cable to be rated for use in risers, use a cable that is NEC-rated as optical fiber, non-combustible, riser, at a minimum. Cable-bend radiuses and tensile loading must not be exceeded. Cables in vertical runs should be supported at a reasonable number of points.

Fiber-optic cables intended for vertical applications have an assigned maximum vertical-rise value. The vertical rise is the distance the cable can be pulled vertically before being supported. It is determined by the weight of the cable and its ability to resist buckling or kinking.

Split wire mesh grips should be used to pull cable vertically. The items, which work like basket or finger grips, support the fiber cable without crushing the core. Cables should be supported by cable ties, straps or clamps in wiring closets. Wherever possible, begin the cable installation at the highest elevation, thereby allowing the weight of the cable rather than additional load to help the pull.

Termination practices

Because specific handling procedures for fiber-optic cable vary, pay close attention to the manufacturer`s instructions. However, some general installation practices for pulling and terminating fiber should be observed. For terminating a multi-fiber cable, a portion of the outer jacket must be removed to expose the fibers. For simplex or duplex cables, whose jackets fit within the connector, the length of jacket removed is specified by the connector manufacturer. Typical values for outer jacket removal for these cables are 1.5 to 2 inches. Outside-plant cables that are to be terminated in trays could have more than 6 feet of jacket removed.

Interconnect cables. Simplex and duplex cable jackets should be removed no more than a few inches from the point of termination; these jackets are easily removed using standard buffer or jacket strippers. Round interconnect cable jackets can be removed using round cable slitters or other tools that will not damage the core.

Distribution cables. Distribution cables are provided with ripcords to ease jacket removal. High-fiber-count, unitized cables have dual ripcords. For cables with ripcords, only the first few inches of jacket need to be removed. Cable slitters or a hook

knife can be used to remove several inches of jacket to expose the ripcords.

Heavy-duty breakout-style cables. All heavy-duty cables contain dual ripcords for jacket stripping and clear polyester tape wraps to maintain core symmetry and fiber protection. Removing several inches of jacket allows access to the ripcords.

Outside-plant cables. Outside-plant cables have ripcords and core wraps to aid in the removal of the rugged outer jacket. Take care to avoid getting the aramid strength members tangled with the ripcords.

Core components

After the jacket has been removed to the required distance, the tape and ripcords can be cut back to the jacket. In cables that have layers of aramid in the core--for example, interconnect and low-fiber-count distribution-style--trim the aramid to the necessary length as specified by the equipment or connector manufacturer. Aramid is most easily cut with utensils sold specifically for the purpose, although razor blades and scissors work.

Central strength members must also be trimmed. Some are cut back to the jacket so they will not interfere with termination, but other applications call for the central strength member to be cut to a specific length and incorporated in the termination (for example, in some breakout kits).

Central strength members made with a fiberglass rod can be severed using a cutting tool. Those made with aramid inners will be easier to cut with aramid cutters. Unitized distribution cables contain aramid yarn within and jackets over each subunit; these jackets must also be cut back. It might be easier to complete the installation of one 6- or 12-fiber subunit at a time to avoid connecting the wrong fiber to the wrong termination point.

Buffer tubes on outside-plant cables can be removed by razor blades or special buffer-tube cutters. Score one side of the tube with the razor--but not too deeply--and bend the tube away from the score. The separated piece of tube can then be pulled off the end of the fiber.

Fiber stripping

Commercial tools are available to remove the buffer and coating from 900-micron tight-buffered fibers or the coating from loose-buffered fibers. Tight-buffered fibers can be stripped in a one- or two-step process. Tools sold for one-step removal will remove the buffer and coating in one action. The two-step procedure requires one tool to remove the buffer and another tool for coating removal. Removing the coating from loose-tube fibers can be done with the same tool used for removing the coating of tight-buffered fibers. Some tools allow blades to be changed for the two functions.

The amount of buffer or coating that is removed depends on the application and the termination procedure. Many connectors come with exact templates for this purpose. Examine the hardware or connector manufacturer`s specific instructions for a detailed procedure.

To remove cable-filling gel, try Hydrasol solvent. Any gel remover that is not carefully used can mar the tube printing, but Hydrasol used in moderation should not affect identification markings.

Breakout kits

Breakout kits are designed to shield optical fibers from damage when the protective parts of the cable have been removed. They provide a layer of support and strain relief for 900-micron tight-buffered fibers or bare 250-micron unbuffered fibers when the cable`s outer jacket, core wrap, aramid strength members and loose tubes have been removed for termination or connectorization. A standard breakout kit contains stress- and abrasion-resistant tubes with aramid strands for tensile strength.

Not all installations require breakout kits. Prior to termination, outside-plant cables can be protected without using them. Many outside-plant cables are terminated in entrance facilities or equipment rooms. The cable`s outer protection is stripped back to expose the fibers, which are wrapped in fiber trays for distribution and spliced into the next link. The tray protects the fiber from environmental and mechanical stress.

Many high-fiber-count premises distribution cables are terminated in telecommunications closets, computer rooms, boxes or other limited-access areas. Standard connectors are available to affix directly to the 900-micron fiber. When the connection does not require many changes (involving pulling the connector in and out) and there are no hazards to the fibers, breakout kits are unnecessary. u

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