LuxHow a fiber splice closure works
Closures
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What a closure does, and its two shapes
A splice closure is the sealed box where fiber cables meet. Inside it, fibers from one cable are joined to fibers in another, split to serve homes, or carried through untouched. This guide opens one: the shell, the trays and sleeves, the splitter, the ports, and the color code that tells a splicer which fiber is which.
A closure has three jobs. It keeps water, dirt and insects away from bare glass. It holds each cable firmly, so a pull on the cable outside never reaches the splices inside. And it keeps the fibers in order, so the next technician can find any one of them years later.
Dome closures
A dome closure, also called a butt closure, has all of its cable ports in one end, the base. The fibers rise from the base into a stack of hinged trays, and a dome slides down over them and locks to the base with a clamp ring and a gasket. Domes are common in handholes, in vaults and on aerial strand. A cable that is not cut can loop into the base through one oval port and back out of the same port.
Inline closures
An inline closure, also called a horizontal closure, is long and narrow, with ports at both ends. The cable enters one end and leaves the other, so the closure lies along an aerial strand or the wall of a vault. The trays sit inside along its length. The parts are the same as in a dome; only the layout differs.
Trays, sleeves, and spliced vs pass-through fibers
Inside the closure, each cable is stripped back and its strength member is clamped to a bracket. The buffer tubes run to the trays. A splice tray is a flat holder with two jobs: it stores a few loops of slack fiber, and it holds a row of splices in slots. Many trays hold 12 or 24 splices.
A fusion splice joins two fibers by melting their ends together in an electric arc. The bare joint is fragile, so it is covered with a splice protection sleeve: a short heat-shrink tube with a steel or glass rod inside, shrunk over the joint and pressed into the tray.
Not every fiber in a closure is cut. Each one is either spliced or passed through:
Spliced fibers are cut and joined to a fiber in another cable, or to a splitter. Each one takes a sleeve and a slot on a tray.
Pass-through fibers, also called express fibers, are not cut. They continue to the next closure, either inside their unopened tube or as loops stored on a tray. A fiber left whole adds no splice loss here and stays free for later use.
The figure on this page switches between three closures: one that splices every fiber, one that passes most fibers through, and one with a splitter.
Splitters, ports and the color code
Splitters
A splitter divides the light in one fiber among several fibers. A 1:8 splitter has one input and eight outputs, and each output carries the same signal at lower power. Splitters in closures are usually small planar lightwave circuit (PLC) modules that sit on their own tray. The input is spliced to a fiber from the cable, and each output is spliced to a drop cable or wired to a port.
Ports
Cables enter through sealed ports. A round port takes one cable; an oval port takes both legs of an uncut cable. Many closures also have hardened connector ports on the outside, so a technician can plug in a home's drop cable without opening the closure. Every port not in use stays plugged and sealed.
The TIA-598 color code
Fibers and buffer tubes are colored to the TIA-598 standard, in a fixed order of twelve: blue, orange, green, brown, slate, white, red, black, yellow, violet, rose and aqua. A splicer names a fiber by its tube color first and its own color second. Orange tube, green fiber is the third fiber in the second tube, which is fiber 15 of the cable. When a cable has more than twelve tubes, the colors repeat on the next set with a stripe or dash mark.
Worked example, key terms and questions
Worked example: one closure, eight homes
A 12-fiber cable with one blue tube reaches a closure that serves eight homes and continues down the street. The splice plan says:
Fiber 1, the blue fiber, is cut and spliced to the input of a 1:8 splitter. That is one splice.
The splitter's eight outputs are spliced to eight drop cables, one for each home. That is eight more.
Fibers 2 to 12, orange through aqua, pass through uncut to the next closure.
The closure holds nine splices in all, on two splice trays beside the splitter tray, plus eleven fibers stored whole. Choose Splitter under the figure to see this closure.
Key terms
Splice closure: A sealed housing that protects fiber splices and holds the cables that enter it.
Splice tray: A flat holder inside a closure that stores slack fiber and holds splices in slots.
Fusion splice: A permanent joint made by melting two fiber ends together in an electric arc.
Splice sleeve: A heat-shrink tube with a strength rod that protects a finished fusion splice.
Pass-through fiber: A fiber that runs through a closure without being cut. Also called an express fiber.
Splitter: A passive device that divides the light in one fiber among several fibers.
Hardened port: A sealed connector on the outside of a closure for plugging in a drop cable.
TIA-598: The US standard for the colors of fibers and buffer tubes.
Common questions
Is a splice closure the same as a splice case?
Usually, yes. Splice case, splice enclosure and splice closure are names for the same kind of sealed housing. Which name a company uses is mostly habit.
How many fibers can one closure hold?
It depends on the model and its number of trays. A small closure for a few drops may hold a dozen splices, while a large dome closure can hold hundreds. The closure datasheet gives its tray count and splices per tray.
Why leave fibers uncut instead of splicing everything?
Each splice adds a little optical loss and a little labor, and every joint is a place that can fail. Fibers that are not needed at this location stay whole, so they reach the next closure with no added loss and remain free for later use.
How Lux does this work
Lux draws a splice sheet for every closure from the network design, so each tray, tube and fiber on the sheet matches the design. See the splice and riser demo.
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