FOA Guide

Fiber Optic Splice Closures

Splice Trays and Closures

Once fibers are spliced, they need to be protected. For protection against the outside plant environment and damage, splices require placement in a protective enclosure, usually called a splice closure. Splices are generally placed in a splice tray which is then placed inside a splice closure or integrated into a fiber pedestal for OSP installations.  For premises applications (indoors) splice trays are often integrated into patch panels or wall-mounted boxes to provide for connections for the fibers.

There are hundreds of different designs and options on splice closures. Some are designed for concatenation of long distance cables where two identical cables are spliced together. Some closures are designed for connecting several smaller cables to a larger one for breaking out the larger cable to several destinations. Closures for FTTH preterminated cables (plug & play) may have connector mating adapters inside the closure to create a patch panel for the factory made drop cables. Closures can be used for midspan access, where the cable jacket is stripped but most of the buffer tubes are coiled inside without opening, while one or more tubes will be opened and fibers spliced to other cables.

Some splice closures have all cables entering into one end, usually called dome closures or sometimes called a butt closure, while some have cable entries on both ends, sometimes called inline closures. Inline closures are used in applications where two identical cables are spliced and an inline closure saves space or when making repairs to damaged cables. Dome closures with all the cables coming into one end are more popular since they are easier to handle when splicing and storing service loops and the single end seal can be more reliable.

dome closure underground dome closure aerial
Dome closures underground and aerial

closure  inline closure

The photos above show the advantage of the dome closure (left) over inline closures (right) - note how much neater fiber management is in the dome closure, especially when re-entering the closure for moves, adds and changes.

There are splice closures designed to be buried, mounted on walls, hung from cables or poles. Some are small pedestals themselves. Each type has a particular application and probably every application has a special closure. Special hardware may be necessary for handling different cable or splice types, so make certain you have the right hardware before using the closure. It is recommended that you work with vendors to find the best closure for your applications then follow their instructions.

Splice trays in patch panel.  rack

Patch panels often have splice closures built-in, especially when the patch panel has many connections. Special splice trays are in the back of the rack or on sliding trays for access. Often large numbers of fibers must be spliced so splice trays can be stacked high.

Closure FTTH pedestal FTTH

Another type of closure is a hybrid of splices and a patch panel. These are often used with fiber to the home (FTTH) networks where drop cables to individual subscribers are factory made preterminated cables and just require plugging in connectors - no splicing required. Here is two examples of fiber drop closures that use SC/APC connectors. Some use weather-resistant connectors on the outside of the closure to more simply connect cables without opening the closure.

Preparing cables for splice closures involves several steps that should be followed in the exact sequence specified by the manufacturer to ensure the cables are properly secured with adequate strain relief and the closure will seal. The cable jacket (or sheath) and strength members of the cable design may require special hardware from the closure manufacturer to ensure the cable is secure and sealed.  

For every splice closure, it is important to follow the manufacturer’s instructions on stripping the cable to ensure proper lengths of strength members to secure the cable to the closure and proper lengths of buffer tubes to connect to the splice trays. The proper length of fiber is needed to allow splicing and then neatly storing fiber in the splice tray. Inside splice closures and at each end, cables with metallic shielding or strength members must be properly grounded and bonded.

Care should be taken when arranging fibers and splices in splice trays and buffer tubes in the splice closure to ensure all fibers are safely stored. Closures usually have spaces to secure buffer tubes from loose tube cables or fiber ribbons. Fibers should be carefully placed in the splice tray and to prevent stress on the fibers or pinching when trays are stacked or covers placed on the trays. Arranging fibers inside splice trays may require twisting the fiber but following the closure manufacturer’s instructions will minimize the stress on the fiber. Often the fibers are broken as the trays and closure are assembled or re-entered for troubleshooting and repair. Covers on splice trays sometimes pinch fibers and cause breaks which can only be found with a VFL test - the break can be too close to the splice to find with an OTDR so it just looks like a bad splice.

Special closures are required for hard ribbon cables (also called matrix ribbon) since the ribbons only bend in one direction, requiring care in routing ribbons in the closure and splice trays. Newer flex ribbons where the fibers are intermittently bonded do not require the same consideration since they flex in all directions.

Cables must be secured to the splice closure and sealed properly. Generally loose tube cables will have the tubes extending from the entrance of the closure to the tray, where they are secured, then approximately 1 meter of bare fibers on each side of the splice are organized in the tray after splicing.

Before using a closure for the first time, read and/or watch the manufacturers application notes or videos to understand the use of that closure. Directions provided by the manufacturer on the lengths of cables, buffer tubes and fibers should be followed carefully to ensure proper - and neat - storage of all the fibers. The manufacturers directions also provide guidance on the sequence of procedures required to properly use that individual closure.

Care must be taken to properly bond electrical conductors such as the armor on some cables or center metallic strength members to the closure and at each end.

Gel-filled cables are much less common than dry water-blocked cables today, but they require special care to prevent gel leakage into the closure. The generally recommended solution is to seal cables and buffer tubes with silicone sealant to prevent gel leaks.

All closures must be capable of protecting the splices and fibers from water damage. Some aerial or above ground closures are free-breathing while most underground closures are sealed to prevent moisture entry. Sealed closures may need to be pressure tested. In the photo below the closure was pressurized and a soap solution sprayed around all seams to look for leaks. After testing the pressure is released by the valve on the closure, Check the manufacturer's instructions for proper inflation pressures and procedures.

pressure testing closure

Closures must be properly secured, with the location being determined by the installation type, and excess cable service loops properly coiled and stored. This may be in a pedestal or vault, on a pole or tower or buried underground. Some aerial closures, particularly those used on electrical utility transmission towers, may have metallic (even bulletproof) covers for protection.

Whether closures are placed underground, aerial or indoors, service loops will be required for the tech to do the splicing work nearby (often in a van, so excess cable may be 10m/33 feet or more on every cable) and access the closure for future moves adds and changes. These service loops should be stored neatly, coiled inside handholes or manholes, on wall fixtures indoors or lashed to messengers with plastic "snowshoes" managing the ends of the cable loops on aerial cables.

Remember that one must be careful to follow guidelines for minimal bend diameter for the fiber optic cable to prevent damage to the cables.

Service loops  dome closures in manhole

Closures underground with neatly coiled service loops can get crowded and messy over time if additional cables are added. When adding more cables, the installer should try to keep the manhole or handhole as neat as possible. 


Underground closures can also get very dirty when left for long periods of time, making entry for repairs difficult. Some installers cover the closures with plastic bags to keep them clean.

Service loops
Aerial closure secured to the messenger with service loops secured with "snowshoes" on each end (left) and service loops secured to utility pole (right)..

Service loops  
Wall mounted rack with service loops on wall with holders.

Choosing A Splice Closure

The long term survival of a network depends on the integrity of splice closures to protect the splices and cables at that location. Choosing a splice closure is a matter of solving the problems of protecting the splices and installing the closure plus choosing a design that the tech knows how to use. Here are some guidelines to choosing splice closures.

Number of cables/fibers/splices: the first consideration is how many cables with what number of fibers are to be joined with splices at the closure.

Location of the closure: Outdoors: aerial, underground or integrated into a pedestal, Indoors: wall/rack mount or integrated into patch panel.

Special needs: Many options, including cable types (armored requires grounding), adding other components like splitters for PON networks, hard ribbon cables requiring  splice trays that are thicker, breakouts to connectors for FTTH drops, etc.

Closure Quality: The ruggedness and sealing of the closure are of primary concern since moisture/water, dirt, etc. entering the closure can compromise the quality and lifetime of the cables, fibers and splices. Most closures will be rated IP68 according to IEC 60529 or Telcordia GR-771-CORE, although GR-771 has not been updated since 2008.

Recommendation: Like other components in a network, splice closures come in a variety of quality levels and pricing, and have been subject to copying by cheap suppliers. Due to the importance of closure integrity, it is recommended that you purchase brand name products from trustworthy suppliers to ensure the future of your expensive fiber optic network.

More Topics On Fiber Optic Installation

Table of Contents: The FOA Reference Guide To Fiber Optics

(C)2023, The Fiber Optic Association, Inc.