fiber optic splice closure structure
Wiki & Tutorial

What is a Fiber Optic Splice Closure?

Fiber optic splice closures, also called fiber optic closures, are widely used everywhere, including in harsh environments, since they’re designed to keep fiber optic splices and joints safe and sound against a hostile environment, such as dirt, dust, wind, moisture, rain, insects, even intentional sabotage, etc. There’re varieties of designs of fiber closures corresponding to different placements and requirements. When you plan to deploy your cabling system, it’s better to learn some basics of fiber optic splice closure, as it plays an essential role in the entire wiring.

What is a fiber optic splice closure?

It provides space and protection for sensitive spliced fibers from environmental hazards, like pressure, extreme temperatures, insects and moisture, and so on. The fiber optic splice closure is rugged, hermetically sealed, waterproof, weatherproof, and with high tensile resistance.

Figure 1: fiber optic splice closure
Figure 1: fiber optic splice closure


As mentioned earlier, a fiber optic closure needs to meet strict demands to protect spliced fibers and joints. It must be:

  • durable, rugged
  • High tensile strength
  • Waterproof and efficient sealing to effectively resist surrounding environmental hazards.
  • Versatile, adaptable to different placements, fiber types, fiber counts, etc.

Types of fiber optic splice closures

Currently, there’re three kinds of fiber optic splice closure, including horizontal type, vertical type, and hybrid type. Though there’re various configurations in terms of splice trays, capacities, and fiber types, they all use similar materials like long-wearing plastics and similar technologies like sealing mechanisms with seals made of gels, gaskets, or pull-and-shrink tubing. We’ll go into them respectively.

Horizontal fiber optic splice closure

This is the most common type of fiber splice closure, very likely because of its versatility. It is pretty flat or cylindrical in terms of appearance. Either mounted aerially or underground, horizontal fiber closures could differ in the number of splice trays and capacities, as a high-capacity one could hold much more fiber counts. Generally speaking, standard numbers for tray capacities include 12 and 24.

Designs may be diverse, but usually, they follow the same specifications. For instance, they must be waterproof, airtight, and resistant to outside forces. In a word, they must efficiently protect the sensitive splicing joints within away from harm. If the horizontal fiber splice closure is about to be attached to a pole or hung from wiring,  then the same argument applies to this case. Thus the closure needs to be held firmly in place in case not to impede the link performance.

Figure 2: Horizontal fiber optic splice closure
Figure 2: Horizontal fiber optic splice closure

Vertical fiber optic splice closure

Vertical fiber closure looks like a dome in shape, also called fiber dome closure, designed for buried applications, yet can be used above grade as well. Like the horizontal variety, the vertical variety also has various configurations in numbers of splice trays, capacities, etc. Needless to say, the high-capacity version, as its name indicates, has much more capacity, meaning more inlet and outlet ports.

In addition, some vertical fiber closures are specially designed for easier re-entry or much more repeated re-entry than others. Meanwhile, as required by underground applications, seals and waterproof technology are what count since prevalent moistures, dirt, and insects pose severe risks to the cabling inside.

Figure 3: Vertical fiber optic splice closure
Figure 3: Vertical fiber optic splice closure

Hybrid fiber optic splice closure

Besides the two mentioned above, there is a hybrid variety of fiber optic splice closure. A hybrid version combining the advantages of both horizontal and vertical types has come in to meet the above- and below-grade applications demand.

Its main draw lies in all-around versability, allowing for wall, aerial, or cell installations regardless of weather, temperature, or location. Even pedestal mounting and below-grade applications are feasible for a compact design of hybrid variety.

Likewise, it meets the demands required by the same protection level as the other two varieties of fiber splice closure.

Figure 4: Hybrid fiber optic splice closure
Figure 4: Hybrid fiber optic splice closure

How to choose the fiber optic splice closure?

Fiber optic splice closures are traditionally made from high-strength plastic or stainless steel, specially designed to minimize installation cost or improve repeated access, etc., as required. They may differ much from each other in designs to meet specific demands. For example, repeated access is vital for the network distribution system’s distribution stage and drop lines, while this is not the case at the start.

Therefore, before buying fiber optic splice closures, careful considerations should be given to these aspects, including cable compatibility, entrance capacity, termination system, strength members, types of splices, bonding and grounding, placement,  hardware, and accessories, cable management, and accessibility.

1.   Cable compatibility

Good fiber optic closure should be compatible with any fiber optic cable specified in the bid documents. Therefore, first check the compatibility of your cabling system, including cable types, adapters, and other hardware. Then determine your placement and capacity of fiber optic splice closure.

In addition, you can choose universal designs of fiber splice closures to maximize the compatibility of the entire wiring. Otherwise, hybrid fiber closures providing interchangeable grommets to allow for varying fiber counts and types of drops are also alternative options.

2.  Entrance capacity

The number of inlet and outlet ports, also called entrance capacity, means its capacity to deal with the number of fiber cables. For example, the capacity of a splice tray refers to its capacity for splicing with fiber cables. Thus, a practical approach is to increase the capacity of a fiber splice closure and minimize the size of ports to hold more ports.

3.  Termination system

Since sufficient mechanical strength between fiber cables and fiber splice closures is necessary, the cable termination system must be capable of filling this need. Besides, due to the effects of relative motion between cable components, which resulted from the fact that fibers are made of materials susceptible to thermal expansion and contraction, the materials used for fiber splice closures shall minimize or negate the said effects of relative motion.

4.  Strength members

The strength members must be tightly in place, terminated near the cable opening, and tied to the sheath to contribute to reducing the effects of thermal expansion and stress. Plus, for conductive elements, ensure the strength members are properly bonded and grounded to function accurately. The closure’s clamping mechanism will hold these parts secularly against pulling out during handling.

5.  Types of splices

There’re three kinds of splice methods: fusion splice, ribbon splice, and mechanical splice. The fusion splice approach offers the largest available space for the fiber splice closure. In contrast, ribbon or mechanical splices are physically larger, meaning less empty space for the fiber splice closure.

Another thing worthy of attention is to ensure these splice trays always hold the splice firmly and provide a protective cover between it and other trays in the stack.

Finally, no matter which type of splice you choose, never twist it, lest it stresses any fibers.

6.  Bonding and grounding

The proper bonding and grounding of the conductive elements of the optical network are indispensable to the safe deployment and operation of the network.

7.   Placement, hardware & accessories

The placement of fiber splice closures has to be taken into consideration. For instance, extra hardware is needed if the fiber splice closure is to be aerially positioned, like hanging via messenger wire or attached to the pole. Also, the hardware or accessories shall meet the demand for environmental resistance. FYI, the international standard rating of IP68 occupies the top of the IP rating scale to guarantee total protection against solid ingress and water ingress.

8.  Cable management

From the cable management perspective, it’s important to keep a low bend radius of fibers so that it won’t impede the data transmission performance. Besides, the fast installation also helps. Thus, when selecting fiber splice closure, these things also need to be considered.

9.  Accessibility

Finally, it’s better to opt for universal designs unless the cabling system has a specific requirement. For example, some designs may be relatively difficult to gain internal access to, as they may require special tools, such as C-cement, and specialized sealing tape, to access the closure. In this case, choosing the closure that can open and close using basic tools is preferable. The same argument applies to the locking mechanism.


With these aspects to consider in mind, it’s not difficult to find the best appropriate fiber optic splice closure for your project. It shall be rugged, all-around, waterproof, and anti-tensile, capable of filling your needs, no matter where its placement or design is. It can be designed for either above-grade or below-grade applications, easy re-entry, repeated access, high-capacity, etc.

The last thing to remember, only an experienced technician with careful hands should install fiber optic closures.

Leave a Reply

Your email address will not be published.