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Almost hand-in-hand with fiber
optic cable has come the development of innerduct. This author
offers general guidelines for the installation of the corrugated
version.
By Richard G. Kraft
Innerduct, or sub-duct as it is often
called, is a product designed to provide a clean, low friction
environment for the installation of fiber optic cable in the
underground telephone plant. Innerduct evolved as a product
when the need to install small fiber optic cables in large
conduits presented the problem of efficient space utilization.
Innerducts provide a means of increasing the number of paths
available in crowded subways.
Two Types
Two basic types of innerduct are available:
smoothwall and corrugated. Smoothwall was the first innerduct
product developed and is essentially water pipe. Smoothwall
is a round tube available with various inside diameters and
wall thicknesses. Smoothwall is heavy and stiff, making it
somewhat difficult to work with in the manhole after initial
installation. A variation of smoothwall innerduct is ribbed
innerduct, which has longitudinal ribs. Ribbed innerduct has
the same general characteristics as smoothwall.
Annular, corrugated innerduct is a relatively new product
developed in the late 1970s for use in the underground plant
environment. It is available in a number of sizes, all of
which are measured according to their minimum inside diameter
(ID). The sizes available are 1, 1-1/4, 1-1/2, and 2 inches
ID. The 1-1/2 and 2 inch sizes are not normally used in conduit
installations. Rather, they were developed for direct burial
applications where a large outside diameter (OD) is needed
for low friction cable pulls. These products are generally
produced from High Density Polyethylene (HDPE). Innerduct
is available in colors to facilitate the identification of
cables in the conduit system.
Annular, corrugated was developed with the primary objective
of reducing the friction encountered when pulling fiber optic
cables into the underground plant. Corrugated innerduct reduces
the friction encountered when pulling cable, because it has
little or no reel memory. This allows it to lie flat in the
conduit after installation, in contrast to smoothwall which
spirals during installation due to reel memory.
Corrugated innerduct has high cross sectional strength,
giving it a greater ability to resist ovalization especially
when wound on a cable reel for shipping. Corrugated innerduct
specifications call for maximum ovalization of five percent,
versus 10 percent for smoothwall and ribbed innerduct. In
addition, corrugated innerduct installs in the underground
with relative ease due to its lighter weight and flexibilty.
Annular, corrugated innerduct provides the user with another
major advantage: it increases in inside diameter when pulled
to near its tensile strength. Smoothwall and ribbed type products
tend to neck down at random points when pulled to near tensile
strength. This necking, if undetected, is serious, because
it can cause a cable to jam in the innerduct during a pull.
Although corrugated innerduct has a tensile strength of
more than 600 pounds versus 1,200 pounds for smoothwall, its
lighter weight means lower pull tensions during installation
and its elasticity more than compensates for its lower tensile
strength. The greater flexibility of corrugated innerduct
makes it much easier to work with in the manhole after initial
installation.
Installation Methods
When installing innerduct, smoothwall or corrugated, it should
be treated the same as cable. The tension applied to corrugated
innerduct is important and should be limited to 450 pounds maximum
to prevent excessive elongation of the innerduct. When pulling
it into a conduit, a guide must be used to prevent damage when
passing over manhole edges, past other cables and racking in
a manhole. Failure to do this may result in damage to the innerduct
and its subsequent loss in the conduit during installation.
Lubrication is needed to reduce friction and ease the installation.
The following information is provided to assist users in
the engineering of projects and product installation. Specific
instructions for use of the various types of hardware and
lubricants mentioned should be obtained from the manufacturers
of those items and followed closely.
The number of innerducts that can be installed in one conduit
is a function of the ID of the conduit and the OD of the innerduct
being used. A fill factor (the ratio of innerduct ODs to conduit
ID) of 67 percent is considered the norm. But one must also
consider the condition of the underground system.
If the conduit system is old, it is likely to have dropped
sections or other breaks that will reduce the ID of the conduit.
In this case, fewer innerducts or smaller innerducts must
be installed. Another factor to consider when calculating
the tension that may be encountered in a pull is the number
and radius of bends in the conduit run. As a rule of thumb,
each 90 degree bend of five feet in radius and each offset
of three feet in less than 10 feet will add 110 pounds of
tension to a pull. This can be significant when maintaining
a 450 pound maximum tension limit.
Combinations of innerduct sizes may be installed in conduit
in order to obtain the maximum number of innerducts in a conduit
run. If combinations are used, it is important to note that
the likelihood of the inerduct jamming or wedging in bends
in the conduit during a pull increases significantly.
Guidelines for the number of innerducts than can be installed
in a clean, straight, obstruction free conduit are as follows:
Conduit # of 1" # of 1-1/4"
ID Innerducts Innerducts
4" 3 0
4" 0 3
4" 2 1
4" 1 2
3.5" 3 0
3.5" 0 2
3.5" 2 1
For smaller or larger conduits use the 67 percent fill
factor to determine the total count of innerducts that can be
installed. Increasing the number of innerducts may result in
wedging and increased pull tensions that may exceed the tensile
strength of the innerduct, which may lead to cable installation
problems later.
If the conduit system includes severe and/or numerous bends,
has obstructions such as dropped or partially collapsed sections
or has an accumulation of debris or mud, the above recommendations
are not valid. The use of lubricants will facilitate the installation
by reducing pulling tensions, but they will not overcome obstructions.
Pulling Tensions, Lengths
Annular, corrugated innerduct has a tensile strength of at least
600 pounds, but 450 should be the maximum limit. The reason
for the limitation is the amount of stretch that will occur
in the innerduct and the possibility of thinning the innerduct
walls. Although the thinning of the walls is not by itself objectionable,
it may result in tearing of the innerduct if sharp edges are
encountered in the conduit. Thinning also may cause holes to
develop in the innerduct, which will allow water and silt to
collect inside during the pull.
General practice is to pull no more than 1,500 feet of innerduct
at one time. If the conduit system is relatively obstruction
free and clean, this length can be pulled without the use
of tension metering equipment. If the pull is longer or the
condition of the conduit system is questionable, then tension
metering should be used with the maximum tension not exceeding
450 pounds. If the tension reaches the limit, the pull should
be stopped and the innerduct allowed to relax. After it has
recovered from the stretching, the pull can be resumed. At
this point, the pull tension will likely be below 450 pounds.
Pulling Equipment
There is a variety of hardware for attaching the pulling rope,
tape or winch line to the innerduct for installation. No matter
which hardware is used, it is important to follow the manufacturer's
directions carefully to prevent the loss of the innerduct during
a pull due to a seperation of the innerduct from the hardware.
Basket or Kellems Grips, which install over the innerduct
and grip tighter as tension increases, are one type of pulling
hardware. When basket grips are used, itis important that
the innerduct be plugged with a wooden dowel to prevent it
from collapsing when the pull tension is at its peak. Basket
grips create a problem of an increased bundle size, which
can restrict the pull if the conduit has obstructions and
bends.
Pulling eyes are also used, and most types do not increase
the size of the bundle at the head end of a pull. This type
of hardware is available in a number of forms. The most common
pulling eye is the same type used on cables. Pulling eyes
grip the outer surface of the cable or innerduct via crimping
or other means of tightening. When this type of pulling eye
is used, the innerduct must have a dowel inserted into it
to prevent the innerduct from collapsing when the pulling
eye is tightened to create the grip.
Other pulling eyes for innerduct are inserted into the innerduct
and hold it via self-tapping threads or expansion of the pulling
eye itself. The threaded type work. However, if the innerduct
ID varies, the threaded type may not grip well enough to complete
pulls with high tensions. Some types of expansion pulling
eyes are not recommended for corrugated innerduct. This type
uses a tapered cone and gripping pieces that press outward
on the innerduct as the tension of the pull increases. The
internal pressure they create can cause splitting of the innerduct
and loss of the innerduct in the conduit. Other expansion
types, which can be adjusted to grip internally without variable
internal pressure based on pull tension, may be used with
corrugated innerduct.
Epoxy corrugated pulling plug kits, such as one manufactured
by Hysol, is another type of pulling attachment. The kit performs
three main functions:
1. quick setting epoxy poured into the end of the corrugated
innerduct forms a very strong grip. The epoxy sets up around
the eye bolt assembly within 10 minutes.
2. the sealing of the end of the innerduct, preventing any
debris or water from entering hte innerduct during the pull.
3. the plug can be left in place until it is time to install
cable, thus keeping the innerduct sealed.
A swivel must be installed at the beginning of a pull bundle
to allow the innerduct to turn freely as it is pulled. Although
corrugated innerduct does not spiral during installation as
the innerduct goes around 90 degree bends, the bundle often
turns over, creating a twist in the innerduct. If multiple
bends are encountered, the use of a swivel is very important.
Polyethylene corrugated innerduct can be supplied with a
pull rope or tape threaded through the innerduct during the
manufacturing process. This eliminates the need to blow a
line through the innerduct after its installation, thus expediting
cable installation in the innerduct.
When installing innerduct with pre-installed pull line or
rope, it is very important to release the end of the pull
line prior to the start of a pull. This is critical, because
the amount of pull line in the innerduct is always slightly
less than the length of the innerduct itself. If the pull
line is not released, it will result in the pull line breaking
or the innerduct becoming jammmed in the conduit during installation.
The craftsperson must release the pull line at the end being
connected to the pulling eye or grip and be sure that there
is sufficient slack stuffed into the end of the innerduct.
The amount of slack should be five percent of the length
to be pulled. Slack may be obtained by cutting back the innerduct
to the point that exposes enough pull line to provide the
excess five percent or by splicing additional pull line to
the installed line using knots or splicing methods approved
by the pull line manufacturer. If adequate slack is not provided
and the pull line is released, the end of the pull line will
withdraw into the innerduct during installation and make the
pull line unusable.
Lubricants, Connectors
When innerduct is being installed in a conduit, it should be
treated like a cable. This applies to the use of lubricants
as well. Lubricants, such as Polywater "J," Hydralube Blue or
other non-detergent or non-soap based lubricants will reduce
the pulling tensions and allow longer pulls that stay within
the 450 pound tension limit. Lubricants are only effective when
used in an environment that is clean and dry. Excessive water
will wash away the lubricants and the presence of dirt or debris
cannot be overcome by lubricants. If the conduit contains dirt
and debris, it should be flushed immediately prior to the innerduct
pull.
Sections of corrugated innerduct can be joined using a number
of different connectors.
Threaded metal connectors are the oldest and most common
type. They are made from metal tube stock and have internal
threads that are half left hand thread and half right hand
thread. The two pieces of innerduct are held against either
end of the connector and a pipe wrench is used to turn the
connector onto the innerduct. This type has limited tensile
strength and may be used to join sections of corrugated innerduct
but not to pull the joined sections. The threaded metal connector
can only be installed prior to cable installation.
The newest type of connector, the Endoclip made by Endot
Industries, is a two piece snap-on connector that fits into
the outer annular corrugations and joins two sections of innerduct.
The joint created is stronger than the innerduct itself, thus
allowing joined sections to be pulled. The Endoclip can be
installed on innerduct witha cable installed, which allows
repair of innerduct opened for cable inspection or repair.
The innerduct must be manufactured with exactly 36 corrugations
per foot or the Endoclip will not fit the innerduct. The clip
is made from polyethylene, will not corrode and has a very
low profile, allowing its use in pulling multiple innerducts
in a conduit.
A plastic internal threaded connector is also available.
This connector has tapered external threads that install like
the metal connectors but on the inside of the innerduct rather
than on the outside. This type has a grip that is less than
400 pounds and should not be used to join sections that are
going to be pulled. This type does have and advantage over
others in that it creates a nearly air tight joint. However,
it can only be installed before cable installation.
Polyethylene innerduct is not rated for any level of flame
retardance. It should never be used in a building, cable vault
or other location where fire hazards exist or where building
codes and electrical codes require low flame spread, low smoke
density and high oxygen index rated raceways or conduit.
Endocor/PL is available for these special installations.
Endocor/PL is the first UL Listed Plenum Raceway meeting
the UL Standard 2024 and to comly with National Electric Code
Article 7770 for fiber optical raceways. Endocor/PL
is produced from PVDF and provides a clean low friction raceway
for cables in buildings where riser (OFNR) or plenum (OFNP)
rated materials are required. Endocor/PL does not eliminate
the need for plenum rated cables.
At the cable vault or building entrance, the innerduct should
be terminated and the conduit end and the innerduct end sealed
with approved materials to prevent the entrance of of hazardous
gases or other foreign matter into the vault or building environment.
Tips on Racking
When innerduct is pulled through a manhole without a break and
cable is subsequently pulled into the innerduct, corrugated
innerduct provides enough flexibility to allow it to be manipulated
off to one side of the manhole and racked without having to
cut the innerduct.
When the innerduct is not continuous or has been cut, a
special form of innerduct is available to cover the cables
pulled through innerduct but left exposed in the manhole.
Split duct, a very flexible form of corrugated innerduct which
can be opened and slipped over the cable, is generally used
to provide a mechanical protection for the cable. This material
is colored bright orange for rapid visual identification of
fiber optic cables.
Innerduct is often thought of as a simple plastic tube that
cannot be damaged. This article points out that the subject
is far more complex. Proper planning and the use of the right
equipment when placing innerduct will ensure a clean, low
friction environment for your optical cables. These factors
will reduce the risk of damage to the glass fibers and ultimately
minimize job costs.
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