Underground Fiber Optic Cable

What is Underground Fiber Optic Cable?

There are 3 most common outdoor fiber optic cable installation way in fiber optic industry. Pole line aerial installation and underground cable installation. Underground Fiber Optic Cable can be buried directly underground or placed into a buried duct.

Geometrical Characteristic

Fiber Count 2-36 38-60 62-72 74-96 98-120 122-216
Cable Diameter (mm) 12.5 12.5 14.5 16.5 17.5 19.0
Tolerance Range (mm) ±0.5, or customized
Cable Weight (kgs/km) 200 220 255 300 350 400

Mechanical Characteristic



Long term 1000
Short term 3000


Long term 1000
Short term 3000
Min. Bending radius

(D=Cable diameter)

Dynamic 20xD
Static 10xD
Temperature Range  (℃) -40~+70

Fiber Type

Fiber Type SM9/125, MM50/125, MM62.5/125, OM3, OM4

How to install Underground Fiber Optic Cable?

Underground Fiber Optic Cable is the common use for long cross-country installations. This kind of installation is very fast and easy. The most used Underground Fiber Optic Cable are steel armored outdoor fiber cables.

Underground duct installation:

Fiber optic cables can also be pulled through underground ducts, which protect the cables from harsh environment. Underground duct installation also provides opportunity for future expansion without the need to dig. This is the most common practice in urban areas. Another benefit is that fiber cables without armor can be used which makes the installation even easier.

Preparation before underground cable installation:

1. Obtain proper right-of-way permits
2. Identify existing underground utilities such as buried cables, pipes
3. Investigate the soil condition in order to determine the installation depth, whether duct should be used, the type of fiber cable should be used and plowing equipment needed

Cable Burial techniques:

1. Plowed under
Plowing in a fiber cable is faster than trenched. But this process has to be carefully monitored to prevent from damaging the cable.

2. Trenched
The fiber cable can also be trenched. This process is slower than plowing in; however, it allows a much more controlled cable installation. Trenches can be dug by hand or by machine.

3. Directional boring
Directional boring technique is used where the surface cannot be worked on such as high, railway crossings, etc. Instead, a boring machine is used to bore a several inches diameter hole underneath the surface, a duct is pulling through the hole and then fiber cables pulled into the duct.

Important factors:

1. Cable installation depth: up to 40 inches
The depth that fiber cables should be buried is determined by the soil conditions and surface usage. In colder areas, fiber cables are typically buried below the frost line to prevent the cables from being damaged by ground frost heaves.

2. Keep the trenches as straight as possible
Fiber cable trenches should be straight, and the trench bottom should be flat and level. Backfill can also be used to distribute the cable load evenly and protect the cables.

3. Fiber cable warning labels should be buried directly above the cable
Fiber optic warning cables are typically buried directly above the cables to warn future digging operations. Or concrete slabs can be buried below the warning label but above the cables to provide even more protection.

4. Whenever conducting armored fiber cables are used, they should be properly grounded
In direct burial installations, steel armored fiber cables are often chosen for their strength and protection from rodent bites. A very important thing to keep in mind is to properly ground the cables. They should be grounded at all termination points, splices and building entrances. The cable steel armor should be connected to a ground wire using a compression-type clamp. The cable ground wire is then bolted to a nearby ground terminal strip.

5. Fiber optic cable minimum bending radius should always be observed during installation.

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