Cable Installation

Installing cable-in-conduit or innerduct can be accomplished in a number of ways.
These include:
1. Pulling cable into the conduit using a pull line or rope
2. Blowing cable into the conduit using specialized equipment that installs the cable
in conjunction with a high volume jet of air
3. Pre-installed in the conduit by the conduit manufacturer (cable-in-conduit)
Pulling Cable into Conduit
The traditional method of installing cable-in-conduit has been to attach a pull line
(or rope) to the cable and pull the cable into the conduit. This placement method
requires equipment to do the actual pulling, to apply lubricants to reduce friction,
and devices that measure the amount of tension being applied to the cable.
Conduit may be supplied with a preinstalled pull line. This line is either a twisted
rope or a woven tape. These pull lines come in a wide variety of tensile strengths
that range from 500 - 6000 pounds-force. Pull lines are also available pre-lubricated
to reduce friction.
Pull tapes are available with sequential footage marks. This type of tape is useful in
determining the progress of the cable pull.
Empty conduit would require a pull line to be installed. Blowing a pull line
directly or blowing a lightweight line through the conduit using compressed air
accomplishes this. This line is then used to pull a pull line or a winch line into the
conduit to pull the cable.
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Duct and Conduit
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A winch mechanism with a take-up reel is used to pull the pull line with the
cable attached. The winch should have a tension meter to monitor the amount of
tension being placed on the cable during the pull. This monitor will reduce the
risk of damaging a sensitive fiber optic cable during the pull. Check with the cable
manufacturer to determine the amount of tension a cable can safely withstand.
The use of cable lubricants is strongly recommended. Cable lubricants reduce the
amount of friction during a pull and therefore allow longer cable pulls and reduce
the risk of damage to a cable during the pull.
When the cable is attached to the pull line, it is recommended that a swivel be
used between the two. This swivel will allow the cable and pull line to move
independently in the conduit during the pull and prevent unnecessary twisting of
the cable or pull line.
On very long pulls the use of mid-assists is common. Mid-assist equipment can be
as simple as a person pulling on the cable midway or it can be a capstan type device
that provides a controlled amount of pulling tension to the cable to reduce the
tension on the cable and increase the possible length of the pull.
If the conduit is in a manhole, protective devices are needed to guide the cable into
the manhole and then into the conduit. These guides protect the cable from scraping
on metal or concrete surfaces that could damage the cable sheath.
Cable Blowing or Jetting
In recent years the practice of pulling cable has frequently been replaced with a
newer method that uses compressed air to blow the cable into the conduit. Cable
blowing requires specialized equipment produced by a number of manufacturers
that utilize high volume air compressors. There are two categories of air-assisted
cable placement: Low Volume/High Pressure, and High Volume/Low Pressure.
In the first case a dart seal is attached to the end of the cable and compressed air is
introduced into the duct building pressure behind the seal, thus forcing the dart
forward and creating a tensile pull on the end of the cable. At the same time, the
cable is pushed into the conduit through a manifold seal using a tractor pusher. The
cable then experiences simultaneous push and pull forces. In the second case, the
cable is tractor fed into the conduit, again through a manifold seal, but this time has
no dart seal. Instead, cable progress is based on the viscous drag of high volume air
alone. In these methods of cable installation, much longer lengths of cable can be
placed than traditional cable pulling methods, and the tension applied to the cable is
significantly reduced.
When blowing cables into conduit, the use of corrugated conduit is not
recommended. Corrugated conduit causes turbulence of the air that disrupts the
flow of air in the conduit and thus reduces the distance a cable can be blown.
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Duct and Conduit
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The conduit should also be capable of withstanding the pressure of the air being
introduced. Generally the maximum pressure used is in the range of 125 psi.
Caution should be exercised when using compressed air to pressurize the conduit as
a loose joint can lead to injury due to the conduit/joint exploding.
Cable Installed by the Conduit Manufacturer (Cable-in-Conduit)
Some producers of conduit have the capability of installing cable while the conduit
is being extruded. Each conduit producer has specific size and length limits, and it is
necessary to discuss with the producer the type of cable you desire to be installed: its
size, type of material and lengths.
Most producers can lubricate the conduit during this process to allow easy
movement of the cable in the conduit for future removal and replacement.
Cable can be tested prior to and following installation to guarantee the integrity
of the cable. Check with the conduit producer for specific information on testing
the cable.