Emerging trend toward aerial taut-sheath splicing

In recent years, fiber deployment has expanded exponentially in response to ever-increasing demand for bandwidth

Sep 1st, 2000
Th Bell Atlantic

Once thought to be an unrealistic option, this method can provide access to most aerial cables.

Doug Duke / Alcoa Fujikura Ltd.

In recent years, fiber deployment has expanded exponentially in response to ever-increasing demand for bandwidth. This demand for fiber-based information services has strained the supply of fiber and fiber-optic cable, and has furthered the search for new and more efficient ways to use existing fiber in already-deployed systems and cables. It is, therefore, increasingly useful to find methods to permit effective utilization of unused (or "dark") fibers to meet customer demand.

Locating the video monitor in the front of the splicer, as shown here, is one of several ergonomic-design enhancements that make taut-sheath splicing a realistic option today.
Click here to enlarge image

This need may be especially pressing in high-density industrial, urban, and suburban areas, many of which were established as neighborhoods, and were already "built up" at the time of initial fiber deployments. In such cases, it has been common practice to deploy aerial fiber cable, especially if no underground duct capacity was available.

Fiber cable has been commonly deployed with excess fibers to provision for future growth, and in many cases, efforts have been made to allow for future access to this excess capacity. One common practice is the deployment of cable slack-loops-the periodic spacing of slack-loops at predetermined locations. The slack-loop lets you bring the cable up (from the underground) or down (from the aerial cable), so that you can easily splice a branch cable onto the existing cable in the comfort of an environmentally friendly splicing van or splicing trailer.

But as fiber deployment spreads, the need grows for fiber-drops to customers in locations where using pre-provisioned slack-loops is often impractical.

Aerial-plant considerations

In an aerial-cable scenario, existing splice closures and slack-loops are commonly thousands of feet apart. So, dropping fiber from an existing aerial splice point (and slack-loop) may involve overlashing a new cable over the existing cable for several thousand feet to get into proximity of the customer-premises or termination point. But ideally, you want to be able to enter the existing aerial-cable sheath in close proximity to the needed termination point, and drop a short cable directly to the termination point without overlashing.

Entering an existing aerial-cable sheath at any point other than a slack-loop necessitates entry into the cable in a no-slack, taut-sheath scenario, which presents daunting operational challenges. The absence of a slack-loop implies that the available fiber for splicing will be short. The length of the available fiber on the existing installed taut cable, and how effectively it can be used, depend on several factors:

  • The method of entry. For example, will the fiber be cut at the center of the sheath opening so that both pieces of fiber can be spliced and will be usable on both sides of the splice point? Or, is it acceptable to cut the fiber at one end of the sheath opening, and consider the fiber beyond the splice point to be dead?
  • The splice closure design-particularly the allowable cable-sheath opening;
  • The fusion splicer's ergonomic design;
  • The relationship of the splicer to the cable sheath and splice closure; and
  • Cable type and construction.

In addition to the essentially ergonomic challenges related to the lack of slack fiber, influenced by the factors described above, aerial taut-sheath splicing presents other challenges as well. For example, the taut-sheath method can provide economic benefits by eliminating the labor costs of an overlashing operation, as well as saving on the cost for the several-thousand-foot length of overlashed cable itself. By placing a drop cable and splice closure at the taut-sheath location appropriate for a specific customer, the closure becomes drop-site and customer-specific. If an existing splice-closure location is used, there is no cost for an additional splice closure. So, if you want to realize the cost savings from eliminating cable overlashing, the new splice closure must be low-cost and have a low installation cost (i.e., little installation time).

The environment is another significant consideration for aerial taut-sheath splicing. To be useful, taut-sheath operations must be successful in exposed aerial conditions, which can especially present significant challenges for the fusion-splicing equipment.

Taut-sheath method

Because the length of fiber on the existing taut cable will be limited, taut-sheath splicing is far easier if you can cut the fiber at one end of the sheath opening. In this case, the length of available fiber is basically equal to the length of the sheath entry, and as much as 2 ft of fiber from the taut cable can be available-a relatively easy splicing scenario.

Unfortunately, this approach may not be desirable in many system designs. If the fiber is cut at the extreme end, it will be unusable beyond the splice location. For an interoffice cable running between two telephone exchange offices, for example, it may be desirable to cut the fiber at the center of the taut-sheath opening. Each side of the fiber can be spliced, with each ultimately connected to opposite central offices. This approach may be useful to allow a redundant-ring architecture drop to the customer location.

Another reason to cut the fibers in the center is to use the same fibers for tandem deployments of digital loop carrier systems. While cutting the fibers at the center of the sheath opening dramatically increases the difficulty of taut-sheath splicing by reducing the available fiber length to about 1 ft, this is sufficient to permit splicing operations if other factors are considered (such as the position of the fusion splicer relative to the sheath opening).

Aerial splice closures

Of course, splice closures have been used in the aerial environment for a long time. For fiber cable, most have been sealed types. More recently, "breathable" or non-sealed closures have come into use. These closures are quite similar to their copper-splicing equivalents, and provide protection from rain. But since they are not sealed, moisture can enter the closure and condensation may form. Still, these closures have now gained widespread acceptance for fiber deployment, particularly as closures for storage of protected fusion splices.

The ability to suspend this work platform from the cable strand minimizes the length of fiber that the technician must access.
Click here to enlarge image

In such splices, the fibers have been welded into a continuous piece of glass, so there is no concern that moisture could obstruct the splice point. Furthermore, the splice-protection sleeve provides a hermetic seal over the striped region of the fiber, near the splice point, protecting the bare glass.

Breathable closures suitable for fiber splicing are commonly available from several manufacturers. Recently, versions of these closures suitable for taut-sheath splicing have also become available. These particular closures provide sheath openings of approximately 25 inches. The internal designs of the taut-sheath versions provide versatility in the location of the splice-protection-sleeve organizers, which is helpful when dealing with the short lengths of fiber available in taut-sheath splicing operations.

Breathable closures also meet the economic challenges of taut-sheath splicing. The unit price of this closure type is low, which means this product represents a cost-effective solution that permits use even if the closure will only contain a few splices or a single splice to connect to a single customer. In addition, breathable closures are simple to install, and installation time is short. In fact, 45 minutes is typically sufficient preparation time for taut-sheath splicing, including both the taut-sheath cable entry and preparation as well as breathable-closure assembly and installation. All things considered, this method is quite favorable when compared to the labor required to overlash a cable to an existing splice-closure location some distance away.

Splicers for aerial splicing

In the past, fusion splicers were poorly suited for aerial splicing. Core-alignment and mass fusion splicers were large and heavy, and were principally designed for use in splicing vans or other benign environments. The use of fixed V-groove alignment mini-splicers in outdoor aerial environments for fiber-to-the-curb trials began about 1992. Early mini-splicers were sufficiently small and portable for aerial splicing, and they were generally battery-powered, allowing use at remote locations.

But these early mini-splicers had several deficiencies. Battery power, though necessary in many remote-splicing locations, is often not the most convenient method since alternating-current (AC) power is available in most bucket trucks. Also, while early mini-splicers were simple in design, they lacked the sophisticated diagnostics and splice-quality-assurance features of larger splicers. Experience began to show that this caused problems when the units were used in harsh field conditions. Furthermore, the design of the early mini-splicers was not ergonomically optimized for taut-sheath splicing.

To overcome these deficiencies, a new generation of more-sophisticated mini-splicers was developed and deployed by 1995. The new mini-splicers in use today are typically equipped with an exchangeable power-module system for both battery and AC operation. The ergonomic design has been optimized for operations in aerial taut-sheath splicing conditions, with minimal available fiber slack. The tube heater for the splice- protection sleeves is positioned at the rear of the splicer, close to the cable. The video monitor is located at the front, out of the way. And the wind protector opens toward the front. This allows placement of the splicer just in front of the aerial cable's taut-sheath opening, allowing the maximum number of splice attempts with the minimal available fiber.

The newer splicers also have sophisticated programmability and software features. A dual-axis video inspection system using CCD cameras monitors fiber alignment and cleave quality to help ensure good splicing results. The system also estimates the splice loss and inspects for splice defects (such as bubbles at the splice point). Sophisticated diagnostic self-test capability has been added to ensure splicer performance in exposed outdoor conditions. One of the most important is an arc-power-calibration test to ensure the splicer will apply the proper heat to the fibers during splicing. In addition, these splicers monitor the atmospheric conditions to automatically adjust the fusion-arc power as required for ambient conditions. The latest generation of mini-splicers also features a sophisticated wind-protector design to ensure a stable splicing arc, even in the presence of winds in excess of 30 miles per hour.

By early 1998, mass fusion splicers meeting all of the same criteria had been developed. Today, an operator can purchase a single mass fusion splicer that can perform high-productivity splicing of high-fiber-count cables, and also meet the challenges of aerial taut-sheath splicing. More recently, mini core-alignment splicers have been introduced. Thus, the consistent low-loss splicing capability of core-alignment splicing, regardless of fiber age or quality, and the more-accurate core-alignment loss estimation are available in a splicer suitable for taut-sheath splicing.

Work platform

With a suitable mini-splicer and suitable splice closure, one additional piece of equipment is needed to permit aerial taut-sheath splicing-an aerial workstation to secure the fusion splicer in the proper position relative to the splice closure, the cable-sheath opening, and the available fiber. This is absolutely essential if the fiber is to be cut at the center of the sheath opening, since no relative motion between the cable and splicer can be allowed with only 12 inches of available fiber. Even if the fiber will be cut at one end of the sheath opening, a workstation is extremely useful and greatly eases splicing operations.

Also available from manufacturers today are aerial splicing platforms that can be suspended from, and secured to, the cable strand. These setups let you position the platform and the splicer just below or just in front of the cable opening and splice closure, minimizing the needed length of fiber.

To accommodate various aerial splicing scenarios, make sure the platform is readily adjustable. For example, the splice platform must be adjustable in height relative to the strand and particular splice closure. Also, the strand mounting must allow easy right-to-left movement to maximize the number of splice attempts with the fiber available, on either side of the fiber's center cut.

Field experience has demonstrated that front-to-rear adjustment is also required. In some cases, positioning the splicer just below the closure is optimum for maximum usability of available fiber length. In other cases, however, the presence of another cable just below the cable being accessed does not permit such splicer positioning, so it's best to position the splice platform so the fusion splicer is just in front of the splice closure. Safety straps should always be secured to the cable strand to ensure the aerial platform and splicer will not fall, even if the strand-mounting clamp comes loose.

Cable types

Taut-sheath splicing of aerial cable is possible with all three common aerial-cable types: loose-tube, central core-tube with stranded fiber, and central core-tube with ribbon fiber. In each case, even with a center-cut fiber method, at least three splicing and recleaving attempts are possible if the aerial-splicing workstation is used to properly position the fusion splicer.

Loose-tube cables are generally the most common type found in the embedded base. In some ways, they present more difficult challenges than central core-tube cables because of the oscillating stranding of the tubes around the central strength member. In a taut-sheath operation, fiber access from the tubes is more difficult.

In the best-case scenario, the reverse oscillation lay (ROL) point should be at or near the center of the sheath opening, which will let you easily unwind the tubes from the central strength member from the center position, achieving maximum slack in the tubes. This, in turn, allows the maximum slack in the fiber in any of the tubes that must be accessed for the splicing operation.

Unwinding and separating the loose tubes from the central strength member is also necessary so that the strength member can be cut out in the center and properly clamped and terminated at each end of the closure. This step is strongly recommended for preventing temperature-induced cable loss (TICL)-a one-time aging phenomenon that may result in severe loss due to shrinkage of some of the cable components. Proper termination of both the central strength member and the cable's outer sheath at each end of the closure is necessary.

In some recently manufactured loose-tube cables, the ROL location is marked on the cable sheath, which makes locating the sheath opening simple. In addition, some recently introduced loose-tube cables have a short distance between adjacent ROL points, making it easier to find the ROL point and increasing the resulting slack.

Unfortunately, older loose-tube cables generally do not have the ROL point marked, and the distance between ROL points can be significant, increasing the difficulty of finding the ROL. This makes opening the cable sheath-such that the ROL point will be near the center of the sheath opening-quite challenging. One solution is to initially make only a 6-inch sheath opening, with the hope that the ROL point will be somewhere within this 6-inch opening. In this case, the sheath opening is extended in either direction as required to open the complete 25-inch section, so that the ROL is more or less centered.

If the ROL is not visible within the initial 6-inch opening, you may be able to guess the likely direction in which to extend the sheath opening to find the ROL: If you notice that the spiral angle of the loose tubes is smaller at one end of the opening, then that end is probably close to the ROL. In such a case, extend the sheath opening another 6 inches beyond the initial opening, in that same direction. Continue this process up to a maximum sheath opening of 25 inches.

After the sheath opening is completed, if the ROL point is far off to one side, it still may be possible to sufficiently unwind the loose tubes from the central strength member to permit effective termination and clamping at each end of the closure. If not, it may be wise to "gang" two of the breathable closures together. A coupling element can be used to join two closures end-to-end, effectively doubling the available sheath opening.

Taut-sheath splicing with loose-tube cables is really only practical when they contain a single layer of tubes, with fiber counts typically running to a maximum of 96. With a double layer of tubes, even if the outer layer contains the fibers that you must access, gaining access to the central strength member and properly terminating the cable would be virtually impossible.

Of course, central core-tube cables do not present any issues related to ROL or TICL, but they still require proper termination practices at each end of the closure. Take care when opening the central core-tube to ensure you do not cut or damage the fibers inside when you slit, ring-cut, and remove the core.

In the case of central core-tube cable with stranded fiber, you may have to use extra care when sorting and separating the fiber bundles using the color-coded binder strings. In every case, it is wise to use a fiber identifier to detect tone on the fiber to be spliced, regardless of the cable type.

Entry into a central core-tube ribbon cable follows the same process as entry into a central core-tube stranded-fiber cable. But things change once you enter the cable because dealing with fiber ribbons differs from dealing with stranded fibers. Splicing an entire ribbon to a drop cable for a customer location is straightforward, but splicing a 4- or 6-fiber ribbon sub-unit, or a single fiber from a ribbon, requires that you separate the ribbon. Many ribbons today have a structure that lets you easily peel the ribbon matrix away from the fibers by using a kit. Other ribbons require that you soak the matrix, then scrub the fibers to separate them. You can separate the oldest adhesively sandwiched ribbon simply by peeling off the Mylar tape that forms the ribbon. As another alternative, tools that permit a mechanical separation of the ribbon structure are coming onto the market. But regardless of the method you choose, be certain to avoid sharply bending the fibers during ribbon access to avoid disrupting live traffic.

Aerial taut-sheath splicing presents challenges and often has been regarded as untenable. Recent field experience, however, demonstrates that it is possible to use this method to provide speedy and economical response to new customer-service demands. By using a new-generation mini fusion splicer, an aerial breathable enclosure, and an aerial splicing workstation together in a coherent system, you can meet the ergonomic challenges of splicing with taut fiber. Using the proper methods and procedures, you can use the aerial taut-sheath method to provide access to the majority of existing aerial cables.

Doug Duke is a member of the splicing-applications engineering staff at Alcoa Fujikura Ltd. (Spartanburg, SC).

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