In my April Fiber Optics column, “A Comprehensive Scope of Work,” I described how a fiber optic cable plant design and installation could benefit from “midspan access.” I tried to describe the technique, but doing that with just words is challenging. Let’s use the power of the web to add drawings to make the process easier to understand.
Many fiber optic cable plant installations involve splitting the fibers in a cable or dropping a small-fiber-count cable from a large backbone cable at one or more locations. Backbone cables of 144–288 fibers are common, and larger ones are becoming more common, too. Drop cables are often only 2–14 fibers, meaning most fibers are continuing straight through the drop point.
A process called midspan access allows adding drops to a cable without disturbing the fibers that need to pass through the drop point. Midspan access offers major savings in installation time and cost.
Midspan access involves opening the cable by removing the jacket and strength members, separating the tubes of fibers passing through the drop point and opening only the one buffer tube containing the fibers being dropped at that point to splice them. The untouched buffer tubes from the opened cable are carefully rolled up and stored in the same splice closure as the fibers that will be separated and spliced to a drop cable.
For example, consider a 144-fiber loose tube backbone cable with 12 buffer tubes with 12 fibers each at a point where we need to drop a 4-fiber cable. If we had to break the cable and splice all the fibers, we would have to make 144–146 splices to connect a 4-fiber cable. Instead, with the midspan access method, we only need to make 4 splices.
If you are building a star network where every drop links directly back to the origin of the network, you will splice 4 fibers in the cable to the drop cable, leaving 4 splices on 4 fibers (instead of 144 splices if the backbone cable is cut and respliced).
If you are building a ring network, you may only be splicing 2 fibers going to the drop and 2 that are continuing along the ring network. That’s just 4 fibers again, instead of 146 splices if all fibers were spliced.
The basic process of midspan access is simple. Using the example above, we will look at a loose tube cable, but processes exist for ribbon cables also, sometimes involving splitting ribbons to access the drop fibers.
Following the cable manufacturer's directions, remove a short length of cable jacket to find the reversal point for the helical winding of the buffer tubes. The reversal point will be the center of the opening for access to make unwinding tubes neater. Find the ripcords to use for removing a longer length of the jacket.
Note: If you open up on a reverse twist and end on reverse twists, the tubes will enter the closure as straight as an arrow. Where the reverse twist philosophy is ignored, the tubes entering an enclosure will need to be forced into position and will be under bending stress. On some cables, you may be able to find the reversal point by looking at the cable jacket—if the jacket is thin enough that the location of the tubes shows through the cable jacket.
You must remove the jacket of the cable for a specified length according to the cable type and splice closures used. Manufacturers will provide that information. After removing the cable jacket, you remove unnecessary binder tapes, water-blocking tapes and strength members, leaving enough of the stiff central strength member on both ends to attach the cable to the splice closure. Identify the tube with the fibers to be spliced to the drop cable and set aside while carefully coiling the other tubes for storage in the closure.
Separate the buffer tube that will be opened from the tubes that will pass through the splice closure. The pass-through tubes will be secured in the closure, usually under the splice tray.
To open the buffer tube with the fibers to be spliced, you need a midspan-access tube that shaves off a section of the tube to allow removal of the fibers without damaging them. Here is a Miller tool that shaves the tube:
After shaving the tube and removing the fibers, count carefully to ensure you remove all the fibers. You can cut the tube off to have bare fibers only for the length you need to splice on the drop cable. Secure the tube to the splice tray. All these fibers will be placed in a splice tray for safe storage, but only the fibers being dropped will be cut and spliced to the drop cable. This is what the closure will look like, ready for splicing the drop cable.
The closure is ready for the addition of the drop cable and splicing fibers.
With ribbon cables, the ribbons that are passing through will have to be stored in a splice tray. If only a few fibers from one ribbon are to be spliced, that ribbon will be split, and the fibers spliced independently.
For more information, refer to the fiber optic cable’s manufacturer