Choices in Fiber Optic Construction
 

CABLE TYPES
General

Cables are categorized by their:

    • Buffering mechanism.
    • Function

Common functions on customer premises include:

    • Interbuilding backbone.
    • Intrabuilding backbone.
    • Horizontal distribution.
    • Patch cords and equipment cables.

The two types of buffering mechanisms for optical fiber cable are:

    • Loose buffered (outside plant and some inside plant cables).
    • Tight buffered (inside plant and underground plant cables).

Loose-buffered Cables

Loose buffered cables are constructed so the fibers are decoupled from tensile forces that the cable may experience during installation and operation. Loose-buffered cables have the following characteristics:

    • More robust than tight buffered cables for outdoor applications.
    • Optimized and proven for long outdoor runs.
    • Less expensive than indoor cable per fiber-meter, specifically at fiber counts above 24.
    • Have high fiber counts.
    • Have better packing density.

Loose-buffered cables generally have a 250 um coating and not a 900 um fiber buffer as do most inside cables. Loose-buffered cables are available in:

    • Armored constructions.
    • All-dielectric constructions.
    • Riser-rated constructions.

Armored Cables

Use armored cables in direct-buried applications for:

    • Rodent protection.
    • Added crush resistance.

Building fire codes require that outdoor metallic cable elements be grounded at each building entrance.

All-dielectric Cables

All-dielectric cables have no conducting materials within the cable. Use all-dielectric outside plant cable for:

    • Duct applications.
    • Aerial applications.

Using all dielectric cable eliminates the need for grounding or surge suppression at each building entrance. All-dielectric cable can be placed in proximity to electrical service cables without danger of induced voltages. All-dielectric cables do not conduct lightning strikes into buildings.

Riser-rated Cable

Relatively new to the industry, riser-rated (OFNR) loose-tube cables provide the excellent outside performance of standard loose-tube cables, but are also rated OFNR for use within the building. They typically differ from the standard outside plant cables in that they have a flame-retardant jacket instead of PE and they use either a water-blocking tape or powder rather than a water-blocking gel

Tight-buffered Cables

Tight-buffered fiber generally have a 900 um plastic coating applied directly to the fiber.

Some applications for tight buffered cable are:

    • Intrabuilding backbone
    • Horizontal distribution.
    • Patch cords and equipment cables.
    • Interbuilding underground (below the frost line).

These cables are usually more sensitive than loose buffered cables to:

    • Adverse temperatures.
    • Outside forces.

Tight buffered cables are desirable because of their:

    • Increased physical flexibility.
    • Smaller bend radius for low fiber-count cables.
    • Easier handling characteristics in low fiber counts

The two typical constructions of tight-buffered cables are:

    • Distribution design, which has a single jacket protecting all the tight buffered fibers.
    • Breakout design, which has an individual jacket for each tight-buffered fiber.

NOTE: The distribution design cables are recommended for typical installations because of lower cost and smaller diameter. Typically, large fiber count distribution cables (greater than 24 fibers) are constructed in a unitized design in which an inner jacket is placed around units of 6 or 12 fibers.

Indoor cables are usually:

    • Unfilled
    • Constructed of materials that meet building fire code requirements

Patch Cords and Equipment Cables

Patch cords and equipment cable are one- or two-fiber indoor cables used as jumper cables between:

    • Optical devices.
    • Patch panels and equipment.
    • Workstations and work area outlets.

Patch cords and equipment cables that meet all fire codes are available, including cables for air-handling plenums.

CABLE TYPE SELECTION

Introduction

Consider the cable’s operating environment when selecting a cable design.

Indoor Environments

Selecting indoor cable involves:

    • The application.
    • The number of fibers required.
    • Whether or not an extra strength member is needed.
    • Environmental concerns (plenum, riser and general purpose).

In general, a tight buffered cable is recommended for indoor use because of its:

    • Ease of connectorization.
    • Ability of meet fire codes.

General considerations for indoor environments are that:

    • As compared to loose buffered cables, tight buffered cables require less hardware (e.g., fanout kits) and time to field connectorize.
    • Tight buffered distribution design cables, rather than breakout design, are normally recommended for intrabuilding backbone and horizontal cabling because individual sheaths on individual fibers are not necessary except in very harsh environments.

Outdoor Environments

In outdoor environments, use loose buffered cables because they:

    • Are rugged.
    • Are specified to operate over a wide temperature range.
    • Allow higher fiber densities per sheath size than tight buffered designs. Where duct space is limited, this becomes a significant factor.

Loose buffered cables are typically unlisted because of the filling compounds used within the cables. The NEC allows the use of exposed outside plant cable for the first 15m (50ft) at the building entrance. If the cable must run to a point farther away than 15m the most cost-effective solution may be to enclose the cable. Raceway, in accordance with Chapter 3 of the NEC, should be used for non-conductive cables. Rigid conduit, grounded in accordance with the NEC, should be used for conductive cables. Local building codes should be referred to for exact requirements.

Mixed Environments

When a cable is exposed to indoor and outdoor environments in the same run (such as between two buildings):

    • Loose buffered cable should be used in aerial and direct-buried situations as well as in conduit above the frost line.
    • Loose buffered or tight-buffered cable, either alone or in combination, may be used in conduits below the frost line. Tight buffered cables are not recommended for use above the frost line because they are subject to damage from freezing water or moisture.

Recent advances in cabling technology have produced loose buffered cable designs which meet the requirements of the National Electrical Code (see previously referenced riser-rated loose-tube cable). These designs make it unnecessary to place tight buffered cables in outdoor environments.

When planning cable requirements in a mixed environment, choose one of the following:

    • All loose-buffered cabling.
    • Riser-rated loose-buffered cabling.
    • A mix of tight-buffered and loose-buffered with a splice or interconnect point.

Depending on the application, each of the three methods may be acceptable and cost-effective. Primary factors to consider are the:

    • Number of fibers.
    • Length of the runs.
    • Complexity of the conduit runs.
 
 
 

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