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Source: Optical Communications Rules of Thumb

Cables and Conduits

Chapter

The topics of this chapter range from the sea bed to the home, yet one

theme is retained; all of these rules relate to how cables are run, protected,

and maintained. The ﬁrst telecom ﬁber cable was not lit until April 22,

1977 (between Long Beach and Artesia), and it wasn’t until 1988 that the

TAT-8 crossed the Atlantic, yielding 40,000 good telephone connections

(and over 1000 times provided by the ﬁrst copper cable). 1 Today, for exam-

ple, an advanced ﬁber transmitting 184 wavelength channels at 40 gigabits

per second can carry more than 90 million phone conversations (enough

to satisfy several teenagers).

One of the topics addressed herein is the management of cables in

which large numbers of ﬁbers are protected. Clearly, this is a critical topic

in these days of constant demand for increased bandwidth, regardless of

application. We also include the issue of allowing the ﬁeld worker to recog-

nize different ﬁbers in these dense cables by use of color. Similarly, a num-

ber of rules relate to the problem of pulling cables through ducts and the

size of the cables that can be accommodated. With increasing ﬁber density

a common trend, we have included a number of rules that relate to this

topic, including ﬂat and tube-like installations.

The above topics lead directly to a set of rules related to the problems

encountered when running large numbers of cables in underground con-

duits, particularly with respect to the potential for the collapse of the

conduit. New conduit materials improve this situation, but care must be

taken to ensure that the installation is consistent with local geophysics,

keeping in mind that long runs will cause the conduit to encounter a vari-

ety of conditions. We also include a rule that deals with the thermal man-

agement of dense cables. Overhead cables get attention as well. We have

included a rule that addresses the threat imposed by weather conditions at

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Cables and Conduits

Chapter One

different locations in the U.S.A. as a result of wind and gravity sag. Another

type of environmental threat comes from ﬁber usage in spaces where ele-

vated temperatures are common and high enough to induce connector

damage. One of the larger rules deals with the threat and properties of ele-

vated temperature operation, the types of cables that are most susceptible,

and other details. At the same time, humidity is a problem that cannot be

ignored.

A number of the rules in this chapter relate to optical time domain re-

ﬂectometry (OTDR) and its proper application in the diagnosis of cables

and ﬁbers. This is a particularly challenging topic when one is considering

undersea applications. Another of the larger rules in the chapter deals

with this topic. In addition, we have included rules related to the perfor-

mance of OTDR systems, measured in terms of the accuracy of the location

of ﬁber defects.

Two of the rules deal with the general properties of the signal-to-noise

ratio that is desirable in dense cable systems. This includes some details

about noise sources in household cable applications.

Finally, we have included several rules that deal with installation chal-

lenges. Aimed at avoiding reﬂections that threaten system performance,

they include some details on common installation mistakes that should be

avoided.

The reader will excuse the use of English units in some of the rules.

They are popular with people working in some of the disciplines and were

used in the original reference.

Reference

1. J. Hecht, City of Light, Oxford University Press, New York, p. 181, 1999.

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Cables and Conduits

I NNER D UCTS

Groups of two to four 25-mm dia. ducts are commonly routed through

100-mm dia conduits.

Discussion

A key factor in the technical and ﬁnancial success of underground ﬁber sys-

tems is the reliable installation and maximum exploitation of the duct work

that goes into the ground. A major threat to these systems is excessive ten-

sion applied to the inner ducts. These are the tubes though which ﬁbers are

deployed and which are drawn through the conduit laid in the ground.

The standard for conduits is 100 mm diameter, although 150-mm units

are becoming popular. In addition, directional drilling has allowed new

ﬂexibility in the installation of these conduits. Directional drilling comple-

ments the other installation methods, including trenching and installa-

tions above ground. In all cases, the driving factor in the evolution of the

technology is the need to manage costs. Of course, a cost factor is the

eventual performance of the installed system, since a failure of the duct or

ﬁber inside can be very expensive.

As designs evolve, not only the installation factors and packing density

are at issue, but also the selection of the materials used in the components.

In addition to mechanical properties, duct work must exhibit suitable resis-

tance to other environmental factors such as temperature, humidity and

moisture, and (in above-ground applications) UV radiation. Persistent ex-

posure to ozone can be a risk as well.

Reference 1 also points out that seals from section to section of the duct

must be air tight to ensure that air-assist placement systems can be used

and to ensure that debris and water do not enter the duct. Finally, it is obvi-

ous that uniformity of outside diameter and wall thickness is critical if the

desired packing density is to be achieved.

A typical deﬁnition of packing density ( P ) is

--------- 100 %

()

where w = ribbon stack diameter

T ID = tube inner diameter

References

1. R. Smith et al., “Selection and Speciﬁcation of HDPE Duct for Optical Fiber

Applications,” Proceedings of the National Fiber Optic Engineers Conference, 1998.

2. J. Thornton et al., “Field Trial/Application of 432-Fiber Loose Tube Ribbon

Cable,” Proceedings of the National Fiber Optic Engineers Conference, 1997.

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w 2

T ID

Cables and Conduits

Chapter One

C ABLE - TO -D UCT R ELATIONSHIPS

1. The maximum diameter of a cable to be placed in 31.8-mm (1.25-in)

inner duct is generally considered to be 25.4 mm (1.0 in). 1

2. Historically, a maximum cable diameter of 25.4 mm (1.0 in) has been

used as a “rule of thumb” for cable installations in 31.8-mm (1.25-in)

ducts.” 2

3. 25.4- through 38.1-mm (1.0- through 1.5-in) inner ducts are commonly

pulled as multiples of two to four ducts into 88.9-mm (3.5-in) square or

101.6-mm (4-in) round conduits.” 3

Discussion

Obviously, getting the maximum number of ﬁbers into a duct is a good

idea. Lail and Logan 2 also comment that a cable diameter of 25.4 mm

(1 in) ﬁlls 64 percent of a 31.8-mm (1.25-in) duct. 2 It seems like, generally,

a cable can be added to a duct if the cable does not exceed about 70 per-

cent of the area of the duct.

Cables intended for installation in these inner ducts must have an out-

side diameter that is not only less than 1.25 in but also small enough to ne-

gotiate the bends and length of the conduit route. Until now, cable

manufacturers have only met this speciﬁcation of 1 in (25.4 mm) maxi-

mum diameter with cables containing 432 or fewer ﬁbers. This, in turn,

has limited service providers to 1296 ﬁbers in a single 4-in conduit struc-

ture.

The small diameter of ﬁber optic cable compared to copper cables

makes possible a means of multiplying the duct space. By placing multi-

ple 31.8-mm (1.25-in) inner ducts in the existing 88.9- or 101.6-mm (3.5-

or 4-in) conduit structures, the effective duct capacity can be increased by

two or three times. The outside-plant challenge then becomes the inside

diameter of the inner ducts rather than the number of available conduit

structures.

References

1. E. Hinds et al., “Beyond 432 Fibers: A New Standard for High Fiber Count

Cables,” Proceedings of the National Fiber Optic Engineers Conference, 1998.

2. J. Lail and E. Logan, “Maximizing Fiber Count in 31.8-mm (1.25-inch) Duct

Applications—Deﬁning the Limits,” Proceedings of the National Fiber Optic Engi-

neers Conference, 2000.

3. R. Smith, R. Washburn, and H. Hartman, “Selection and Speciﬁcation of HDPE

Duct for Optical Fiber Applications,” Proceedings of the National Fiber Optic Engi-

neers Conference, 1998.

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Cables and Conduits

C OLORED R IBBON C ABLES

Colored ribbons provide a number of advantages in optical applications.

They enable high ﬁber count, bulk fusing, and quick identiﬁcation

through the use of color.

Discussion

Colorization must be done properly, since the introduction of a coloring

agent to the matrix material can “affect the cure performance, modulus,

and glass transition temperature of the material.” 1

Clearly, the main advantage of a colored product is to reduce the time

required to identify particular ribbons in the ﬁeld. Figure 1.1 shows the ad-

vantage of colorization.

This new capability is not achieved without some cost. The peel and sep-

aration performances were veriﬁed through standard tests, including use

of the midspan access peel kit and visual inspection of ﬁber surfaces after

separation. Additionally, ﬁber-to-matrix adhesion has been quantiﬁed

through the development and use of a high-resolution test method. This

test measures the critical fracture energy of the ribbon matrix material.

Data from this test method and the equation used are shown in Fig. 1.1.

Through an understanding of the different process and material variables

that control adhesion, the ability to manufacture ribbon with a speciﬁed

adhesion value is obtained.

Reference

1. K. Paschal, R. Greer, and R. Overton, “Meeting Design and Function Require-

ments for a Peelable, Colored Matrix Optical Fiber Ribbon Product,” Proceedings

of the National Fiber Optic Engineers Conference, 2000.

350

Clear

300

250

200

150

100

Color

100

150

200

250

Fiber Count

Figure 1.1 Recognition time as a function of ﬁber count.

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