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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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