Lesson Seven, Iowa State Telecom Cabling Installation Course: Planning your next cable splicing operation is important to your future cabling jobs.
Planning is critical to any splicing operation. You must carefully consider cable placement, support structures for the splice and selection of the closure. Equipment rooms, telecommunications closets, main terminals, entrance facilities and cable trays are usually locations for intrabuilding splice construction. Generally, the engineer or designer plans for the development of the prints, specifies cable splice locations and other splicing details in work-order prints and ensures the parts are ordered. The cable splicing techniques described here are for in-building applications using single sheath cable (yet to be activated), and fire-retardant, 25-pair connector modules in a two-bank, in-line configuration.
Copper cable. You may splice intrabuilding copper cable only in backbone cable - never for horizontal cable (horizontal cable extends between the telecommunications closet and work area). Backbone cables usually have expanded polyethylene-polyvinyl chloride (PVC) insulation, and are normally Communications Riser Rated (CMR).
Bonded to a fire-retardant sheath, a core wrap and an overall shield envelops the 24AWG copper conductors in a CMR cable. This NEC requirement mitigates the spread of fire from floor to floor. Designers developed insulated cable, called plastic insulated conductor (PIC) cable, for ease of cable pair identification. PIC cable is bundled in 25-pair color-coded binder groups with color-coded pairs in each binder. Intrabuilding PIC backbone cables range in size from 25 to 1,800 pair. The first binder group of the cable is normally near the center of the cable, and successive groups are in layers toward the cable sheath.
Where the National Electrical Code applies, you may use only "listed" cable within a building structure, except for an entrance cable within 15m (50 ft) from its point of entrance. An exception is to extend unlisted or outside cable in rigid or intermediate metallic conduit. This allows 15m (50 ft) of exposed cable where the cable emerges from the rigid metal conduit or intermediate metal conduit. Listed cables have identifying markings on the outer sheath such as CM (general use), CMR (riser-rated), and CMP (plenum rated). CM means communications media.
You must position the cable, rig the support structure and use proper materials for all splices. If you place conductors in a closure without adequate splice banks, you may stress the wires. Reentry and churning conductors without proper planning can lead to premature failure or poor performance.
Once you set up the cables, prepare the area based on the type of splice you'll use. The number of banks within the splice and size of closure dictates how much sheath to remove. Once the sheaths are open and you've bonded the cable shields together, installed the closure endplates and marked the binder groups, you can choose the splicing method to join the cable pairs.
Two common modular splicing techniques are in-line and foldback.
With the in-line splicing method, you place wire in a straight-across arrangement, providing for little wire slack. The in-line method should receive minimum handling. The foldback splicing method allows you to fold the conductors into the splice, which in turn provides for maintenance, rearrangement and transfer of the conductors. The foldback method typically requires you to store more cable within the splice, increasing the size of splice closure.
You can splice with one or two widely used types of equipment: MS2 by 3M or type 710 by Lucent Technologies. Both use IDC or insulation displacement contacts. Do not bend the cable such that it twists or kinks. As a rule, the bending radius of the cable should not exceed 10 times the cable diameter. Follow the cable manufacturer's guidelines when bending cables.
The bending radius is the essential factor in determining the size of the splice box or maintenance hole into which you will place the splice. The following guidelines may be useful in considering space for the splice.
- Overhead splice: 686 mm (27 in.) between closure and nearest overhead obstruction [813 mm (32 in.) in hard hat areas]. 635 mm (25 in.) between closures where positioning does not permit closure to be lowered.
- Horizontal splice: 635 mm (25 in.) above the floor. (You can also find a temporary position used only for splicing.)
- Vertical splice: Limited by space required to position cable, closure and supporting structure.
After splicing the conductors, and prior to installation of the splice closure, prepare them using a polyethylene wrap. The splice wrap should be snug, yet not too tight, so that modules and cable pairs aren't lodged in the splice closure seams. Once you wrap the splice, install a splice closure over all intrabuilding splices.
An intrabuilding splice closure is a strong, lightweight, fire-retardant covering that protects nonpressurized splices. The closure shields the splice against humidity and moisture and may even resist temporary immersion in water. When you install the closure, properly support it, ground it and test it for air leaks according to manufacturer's recommendations. Affix labels to all cables entering the splice, indicating cable number and pair counts. Take care to clearly designate the "In" and "Out" for the spliced cables.
Optical fiber cable. It's usually best to avoid unnecessary fiber splices by installing a continuous length of cable. However, you can't always avoid splices. Factors such as cable plant layout, length and raceway congestion typically warrant a splice. Other factors are requirements for a transition splice between non-listed OSP (outside plant) cables and listed cable at the building entrance point and unplanned requirements such as damaged cable during the installation or in cable dig-up.
The two major categories of field-splicing methods for optical fibers are fusion and mechanical. Both single- and mass-fiber (typically 12 fibers) splicing methods are available. Both of these methods are field-proven and have excellent long-term reliability when completed according to manufacturer's instructions.
For outside plant and inside building splice locations, splice closures usually protect and secure splices and stripped cables. In both cases, splice trays or organizers hold the splices in place - typically in groups of six, 12, 24 or more fibers per splice tray or organizer.
Splicing occurs between two optical fiber cables - loose-tube cables containing 250 mm coated fiber and tight-buffer cables containing 900 mm buffered fibers. Mechanical or fusion splice methods perform 250-mm-to-250-mm splicing, 250-mm-to-900-mm splicing or 900-mm-to-900-mm splicing. Typically, multimode fibers are 50/125 mm to 62.5/125 mm, while single-mode fibers are 8-9/125 mm. While it's possible to physically splice multimode to single-mode fibers, the splice loss is unacceptable.
Based on the splice hardware you use and the type of cable you're splicing, determine the proper amount of each cable element to strip (outer jacket, strength members, buffer tube or subunit jacket) before beginning the splice. This is a critical element, because you must properly secure the cables to the closure or enclosure and the buffer tubes or correctly route subunit elements from the splice hardware entrance point to the splice trays. Leaving too much buffer tube or subunits may cause difficulty in storage of these elements; leaving too little may cause difficulty in providing an acceptable work area for completing the splice.