5 - Specifying Fiber Optic Cable, TELEKOMUNIKACJA, eng, --Światłowody (koksa)
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C
HAPTER
5
S
PECIFYING
F
IBER
O
PTIC
C
ABLE
ERIC PEARSON
CABLE PARAMETERS AND TYPICAL VALUES
In order to completely specify a fiber optic cable, you need to define at least 38
specifications. We divide these cable specifications into two subgroups, installa-
tion specifications and environmental, or long-term, specifications. Most of these
specifications have a standard test technique by which the parameter is tested.
Note that not all specifications apply to all situations. You will need to
review your application to determine which of the specifications in this section
are needed. For example, cable installed in conduit or in protected locations will
not need to meet crush load specifications.
INSTALLATION SPECIFICATIONS
The installation specifications are those that must be met in order to ensure suc-
cessful installation of the cable. There are six such specifications:
1.
Maximum recommended installation load, installation load, or installa-
tion force (in kg-force or pounds-force, or N)
2.
Minimum recommended installation bend radius, installation bend radius,
short-term bend radius, or loaded bend radius (in in. or mm)
3.
Diameter of the cable
61
62 CHAPTER 5 — SPECIFYING FIBER OPTIC CABLE
4.
Diameter of subcable and buffer tubes
5.
Recommended temperature range for installation (in degrees centigrade)
6.
Recommended temperature range for storage (in degrees centigrade)
Maximum Recommended Installation Load
The maximum recommended installation load is the maximum tensile load that
can be applied to a cable without causing a permanent change in attenuation or
breakage of fibers. This characteristic must always be specified. It is particularly
important in installations that are long, outdoors, or in conduits; it is of lesser
importance when cables are laid in cable trays or installed above suspended ceil-
ings. We present typical and generally accepted values of installation loads in
Table 5-1. Choose the value that best fits your application.
If you believe that your application will require a strength higher than those
typically specified, then you will want to specify a strength higher than those in
Table 5-1. The cost increase of specifying such a higher strength is a small per-
centage, typically 5 to 10 percent, of the cost of the cable.
Minimum Recommended Installation Bend Radius
The minimum recommended installation bend radius is the minimum radius to
which cable can be bent while loaded to the maximum recommended installation
load. This radius is limited more by the cabling materials than by the bend radius
of the fiber. This bending can be done without causing a permanent change in
attenuation, breakage of fibers, or breakage of any portion of the cable structure.
This bend radius is usually, but not always, specified as being no less than 20
times the diameter of the cable being bent. Specifying the bend radius is impor-
tant when pulling by machine or hand through conduit, or in any long pulls.
Table 5-1 Typical Maximum Recommended Installation Loads
Application
Pounds Force
1 fiber in raceway or tray
67
1 fiber in duct or conduit
125
2 fiber in duct or conduit
Multifiber (6–12) cables
250–500
Direct burial cables
600–800
Lashed aerial cables
>300
Self-support aerial cables
>600
CHAPTER 5 — SPECIFYING FIBER OPTIC CABLE 63
In order to determine this value, you need to examine the locations in which
you are to install your cable in order to determine the bend radius to which you
will bend the cable during installation. Conversely, you can choose the cable and
specify the conduits or ducts in which you are to install the cable so that you do
not violate this radius.
Diameter of the Cable, Subcable, and Buffer Tubes
The cable must fit in the location in which it is to be installed. This is especially
true if the cable is to be installed in a partially filled conduit. It will not be impor-
tant if the cable is directly buried, installed above suspended ceilings, or in cable
trays. If the diameter is limited by the space available, the diameter limits may be
the only factor that determines which of the five designs of the cable you must
choose. If cable diameter must be limited, the ribbon designs will be the smallest.
The diameter of the subcable and the buffer tube of the cable can also
become a limiting factor. In the case of a “breakout” style cable, the diameter of
the subcable must be smaller than the maximum diameter of the connector boot
so that the boot will fit on the subcable. In addition, the diameter of the element
must be less than the maximum diameter that the back shell of the connector will
accept.
Recommended Temperature Ranges for Installation and Storage
All cables have a temperature range within which they can be installed without
damage to either the cable materials or the fibers. It is more important for out-
door installations or in extreme (arctic or desert) environments and not impor-
tant for indoor installations. In general, the materials of the cable restrict the
temperature range of installation more than do the fibers. Note that not all cable
manufacturers include the temperature range of installation in their data sheets.
In this case, the more conservative temperature range of operation can be used.
In severe climates, such as those in deserts and the arctic, you will need to
specify a recommended temperature range for storage (in degrees Centigrade).
This range will strongly influence the materials used in the cable.
ENVIRONMENTAL SPECIFICATIONS
The environmental specifications are those that must be met in order to ensure suc-
cessful operation of the cable in its environment. There are 21 such specifications.
1.
Temperature range of operation
2.
Minimum recommended long-term bend radius
3.
Compliance with the NEC or local electrical codes
64 CHAPTER 5 — SPECIFYING FIBER OPTIC CABLE
4.
Long-term use load
5.
Vertical rise distance
6.
Flame resistance
7.
UV stability or UV resistance
8.
Resistance to damage from rodents
9.
Resistance to damage from water
10.
Crush loads
11.
Resistance to conduction under high voltage fields
12.
Toxicity
13.
High flexibility/static versus dynamic applications
14.
Abrasion resistance
15.
Resistance to solvents, petrochemicals, and other chemicals
16.
Hermetically sealed fiber
17.
Radiation resistance
18.
Impact resistance
19.
Gas permeability
20.
Stability of filling compounds
21.
Vibration
Temperature Range of Operation
The temperature range of operation is the temperature range within which the
attenuation remains less than the specified value. Typical ranges of operation are
given in Table 5-2 for various types of applications. In general, there are very few
applications in which fiber optic transmission cannot be used solely for reasons
of temperature range of operation. In fact, some fibers have coatings that will
survive continuous operation at 400°C. For operation at such high temperatures,
fibers are usually, but not always, incorporated into a cable structure consisting
of a metal tube. For operation at exceedingly low temperatures, cables are con-
Table 5-2 Typical Temperature Ranges of Operation
Temperature Range
Application
(°C)
Indoor
–10 to +60, –10 to +50
Outdoor
–20 to +60,
–40 to +50,
–40 to +70
Military
–55 to +85
Aircraft
–62 to +125
CHAPTER 5 — SPECIFYING FIBER OPTIC CABLE 65
structed of plastic materials that will retain their flexibility. For cables used at less
severe temperatures (80–200°C), fluorocarbon plastics such as Teflon, Tefzel,
Kynar, and others are used.
There are two reasons for considering the temperature range of operation:
the physical survival of the cable and the increase of attenuation of the fiber when
the cable is exposed to temperature extremes.
All cables are composed of plastic materials. These plastic materials have
temperatures above and below which they will not retain their mechanical prop-
erties. After long exposure to high temperatures, plastics deteriorate, become
soft, and, in some materials, crack. Under exposure to low temperatures, plastics
become brittle and crack when flexed or moved. Obviously, under these condi-
tions, the cable would cease to provide protection to the fiber(s).
The second reason for considering the temperature range of operation is the
increase in attenuation that occurs when cables are exposed to extremes of tem-
perature. Optical fibers have a sensitivity to being handled. This sensitivity is seen
when the fibers are bent. This bending, which results in an increase in attenua-
tion, is referred to as a “microbend-induced increase in attenuation.” When a
cable is subjected to temperature extremes, the plastic materials will contract and
expand at rates much greater (100 times) than those rates of the glass fibers.
This contracting and expanding results in the fiber being bent on a micro-
scopic level. Either the fiber is forced against the inside of the plastic tube as the
plastic contracts, or the fiber is stretched against the inside of the tube as the plas-
tic expands. In either case, the fiber is forced to conform to the microscopically
uneven surface of the plastic. On a microscopic level, this is similar to placing the
fiber against sandpaper. This microscopic bending results in light escaping from
the core of the fiber. This escaping light results in an increase in attenuation. This
type of behavior means that the user must determine the temperature range of
operation in order to ensure that there will be enough light for the system to func-
tion properly.
Minimum Long-Term Bend Radius
The minimum recommended long-term bend radius is the minimum bend radius
to which the cable can be bent for its entire lifetime. It is important for cables
installed in conduits designed for electrical cables. It is usually, but not always,
specified as being no less than 10 times the diameter of the cable.
Compliance with Electrical Codes
Fiber optic cables used in indoor applications must meet the requirements of the
NEC and applicable local electric codes, some of which are more stringent than
the NEC. Consult your local fire regulation authorities for those codes to which
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