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Duct and Optical Fiber Cable Laying Technique

Duct and Optical Fiber Cable Laying Technique: This article provides details of available infrastructure deployment of duct and optical fiber cable laying techniques. More than one technique can be used in the same network based on the specific circumstances of the network building. Duct laying technique is the most traditional method of underground cable installation and involves creating a duct network to enable post-installation of a optical fiber cable using a pulling, blowing or floating technique. A traditional duct infrastructure can be constructed in many ways.

1. The main duct system consists of small, hard flexible tubules for individual cable installations.
2. Large diameter duct allow the cable to progressively move along with the network's growth.
3. Small diameter sub-ducts also available for the single cable installation.

Duct laying technique provide the duct infrastructure for the telecom network and it allows for additional access network development and reconfiguration.Duct laying technique provide the strong support for the future network extension.
Underground duct is one of the major cost items of the network. It is important -that new duct or duct additions be well planned. Insufficient duct will result in early additional cost; too large a duct structure will represent uneconomical investment of money and material.

Important Points for Duct Planning and Laying

If, underground duct structure will be very costly, like in mountainous places, another alternative s can be studied to be used as aerial network.

For identification of ducts number and dimensions, the primary network cables maximum requirements should be taken into consideration beside the usage of the new technologies to reduce the ducts size.

Certain locations such as bridges, railways and major road way crossings will require consideration of over sizing to meet expected ultimate needs. These cases are created similarly to over sizing near exchange buildings. Over sizing consideration should be at least 50% of the required minimum number of ducts.

Duct routes should conform to the fundamental plan for the Exchange. They should be triangular in design. That is to say there should be one main collector system with branches normally running at 90 degree from the collector. Parallel routes should be avoided.

The economies of alternate locations must be compared relative to permanence, safety and utilization. The routes should be selected to avoid any future relocation. Locations should be chosen that are free of danger and also provide a safe working environment. There will be times when the safety aspect alone will have the greatest bearing upon the selection of one route over another.

Locate the trench line so that it agrees with the Ministry of Municipal and related government authority. If that location is already occupied by another utility locate the trench line so that the duct structure will be free as possible from disturbance of other under- ground users. In so doing the following should be considered.

Duct Laying in Cities

Under the sidewalk, especially if the construction precedes the laying of the sidewalk, is usually an excellent location.
Usually provides a organized location between the pavement and the building line.
Between the sidewalk and the curb is not the best location because of tree planting or possible street widening.
In the street near the curb; if this location is used every effort should be made to locate the manholes under the sidewalk.
In parkways, duct may be placed along the edge of the park if permission can be obtained.

Duct Laying in Rural Areas and Highway

The above mentioned locations are applicable but the designer should assure himself that the duct is not likely to be involved in street widening.
Must be avoid placing duct on main highways unless the highway has reached its maximum growth, then there may be a location on the side that would be acceptable.
Obtain all available information relating to the location of existing or proposed structures of other utilities. Special care should be taken to work in the vicinity of other utilities. In some areas, ministry of municipal and rural affairs may be able to provide information regarding the location of other underground structures.
The minimum recommended separation between foreign structures and the telephone duct systems. Must be maintain separation between underground duct and optical fiber cable & other utilities. As per below given table-
Separation table for duct and OFC laying
Separation table between duct,OFC and other utilities

Where locations are not reassigned, all avail able street and road plans should be reviewed, including plans concerning future road construction or widening projects.

The soil type and nature should be accurately analyzed to identify whether it is normal or not e.g. to avoid location with ground water or rocks.

The standard duct structures based on the duct being placed in stable soil with 1000 mm (minimum) depth of cover. These conditions are of particular concern because of their effect on forces which produce stress in the structure.

The main consideration in concrete encased duct structures is the tensile stress resulting from two forms of vertical load. These loads are generally classified as live load and dead load. Live loads have greater effect on the structure than dead loads.

Live load is the pressure resulting from a combination of the mass and velocity of an object, such as a motor vehicle, moving over the structure. Live loads are assumed to be transmitted in a conical shaped section, with the point of the cone at the top. Therefore, the greater the depth of the structure, the greater the area over which the load is spread. Live loads can be neglected at points that are 2 m or more below surface grade.

Dead load results from the combined mass of the structure and the backfill over the structure. Forces resulting from the dead load will increase with the depth of cover, but this is not as critical as live load considerations.

It is the designer's responsibility to be aware of any unusual depth requirements that are established for underground structures. This particularly applies to crossing main service.

In an area where final grade has not been established, the designer should make every effort to obtain the grade from the proper authorities. to achieve standard cover should be shown on the work plans.

When the available maps, plans and input from outside sources do not yield sufficient data to develop a proposed plan, it may be necessary to undertake a detailed land survey of portions of the route.

Ducts Type Used in Telecom Network

Generally types of ducts used in Telecom Network are as follows :-
Steel Ducts.

Fiber Reinforced Epoxy (FRE).
Polyvinyl Chloride (PVC)
Polyethylene (PE).
Polyethylene Corrugated (PEC).
Sub-Ducts & Mini Ducts.

Steel and PVC ducts are supplied in pieces 6 meters long, while PE & PEC ducts are supplied in rolls of (100 – 500) meters long. 110 mm OD x 90 mm ID, W/ 5-33 mm OD Sub-ducts, Orange color or any other color, one sub-duct black, 500 m reel length. Used for Optical Fiber Cable installation.

110 mm OD x 90 mm ID, W/ 3-42 mm OD Sub-ducts, Orange co lour, one sub-duct black, reel length 500 m. Another type PECD 110 mm OD x 90 mm ID, Without Sub-ducts, Orange, reel length 500 m.Generally used for copper cable.

PECD type used for Optical Fiber Cable, 77 mm OD x 58 mm ID, W/ 3-27.2 mm OD Sub-ducts, Orange color, one sub-duct black, reel length 1200 m. 

PECD type used for FOC, BSWs & small copper cables,77mm OD x 58 mm ID, without Sub-ducts, Orange, reel length 1200 m.
PEC duct used for the construction of network
PEC Duct used in telecom sector

Multiple cables can be installed in a single duct, but need to be put in place simultaneously or alternatively, multiple draw ropes need to be pre-installed. However, a single duct system can limit the number of cables that can be installed. Entanglement of the cables and high friction to allow space for new ducts between the cable jacket, it can be difficult to remove the old cable from the full ducts. 

It is normal to be located under the old cable duct. Rigid sub duct reduces the total number of cables that can be installed but also involves the need to remove the older cables. This technique includes both cable blowing as well as pulling cable, as it helps to create an airtight connection to the sub duct. 

Flexible textile sub ducts maximizes the total number of cables which can be installed in a duct and at the same time allows older cables to be removed easily. In general, flexible sub ducts tripled the number of cables which can be installed in the main tube.

Main duct sizes which can contain rigid sub ducts vary from about 60mm up to 110mm. Main duct sizes for single cable use are smaller, with a typical internal diameter of between 20 and 40mm.

Optical Fiber Cables can be installed into the ducts by the pulling technique, blowing technique or floating technique. If they are pulled, then the duct must contain a pre-installed draw rope or if this is not the case then a rope would have to be pushed through using a rod. If cables are to be blown in or floated through the duct and any connections between sections of duct, the entire system needs to be airtight.

The inner wall of the duct or rigid sub duct is manufactured with a low friction coating to ensure low friction with the cable sheath. Alternatively, the duct or rigid sub duct may have a low friction extruded profile or in some cases, special duct lubricants are used. Flexible sub ducts are prelubricated during manufacturing to achieve low friction.

A number of factors control the continuous length that can be pulled or blown, which consists of coefficient of friction, bends in the duct route (vertical as well as horizontal), the strength and weight of the cables as well as the installation equipment used. Fill ratios should be calculated as part of the planning process as should the size of cable in relation to duct. In the case of existing networks, the condition of the ducts should be checked for any existing damage and for suitability of space and capacity for future cabling.

Optical Fiber Cable  Laying Technique

Short sections of Optical Fiber cable can be pulled by hand, (less than 300 meters). Due to the maximum allowable pulling tensions of 2 kilo Newton, and the possibility of laying long sections (up to 4 kilometres) to avoid too many joints, the Engineer may request placing of a reel length bidirectional from a pull-through manhole.

When cables are pulled into a duct, a pre-existing draw-rope must be in place or one installed prior to cable winching. The cable should be fitted with a swivel allowing the cable to freely twist as it is installed; also a fuse is required which is set at or below the cable’s tensile strength.

Long cable section lengths can be installed if the cable is capable of taking the additional tensile pulling load, or by “fleeting” the cable at suitable section mid-points to allow a secondary pull operation, or by using intermediate help pullers. Fleeting involves laying loops of fiber on the surface using figure of eight loops to prevent twisting in the cable. If spare ducts or sub ducts are installed, then further cables can be installed as the need arises.

Cable lubricants can be used to reduce the friction between the cable and the sub-duct, thus reducing tensile weight. The minimum bend diameter of optical fiber cable represents the smallest coil for Optical Fiber cable storage within a cable chamber MH/HH. Suitable lungs and guidance instruments should be used to ensure that minimum dynamic turning radii is maintained during installation. If the cable exceeds 75% of the diameter of the outer diameter duct, then the length of the pull may be less.
Optical fiber laying by manual technique
Manual cable laying

Unidirectional Pulling of Optical Fiber Cable

If the pulling wire is to be used, verify that it is continuous throughout the pull.At the reel-end manhole, install the cable conduit feeder. Attach the pulling wire or winch line to the cable using the pulling hardware configurations.Verify that the set screws are well tightened on the central strength member of the cable.

Start pulling at low speeds. When the cable enters the duct and it is verified that there are no obstructions, pulling speed can be gradually increased to 20 meters per minute. When the cable reaches a pull-through manhole, reduce the pulling speed until the cable has entered the next manhole section then gradually resume the pulling speed.

At the reel end of the pull, apply the cable lubricant on the cable as it enters the cable chute.
At pull-through manholes, apply the lubricant to the cable once the pulling attachment has entered the duct in the opposite wall.Do not enter the manhole until the pulling hardware has entered the duct on the opposite wall.

Bidirectional Pulling of Optical Fiber Cable

This technique of cable laying requires equipping the cable with a pulling eye at each end.Locate the cable reel at the designated pull through man- hole. The length of cable remaining on the reel is uncoiled and laid on the ground in a large figure of eight (8). Site engineers must be confirm that the minimum bending radius of Optical Fiber Cable is not exceeded.

Move the pulling truck to the other end of the section pull.The cable must be pulled at a lower speed (10 meters per minute) to facilitate an easy feeding of the cable into the duct without twisting.Keep enough slack at the ends of the pulled sections to allow racking of the cable at the pull through manholes. Pull the slack back by hand at each manhole. Leave enough cable at the end manholes to allow jointing and testing.

Testing equipment may be mounted in a van or truck. The loop left in the end manhole must be long enough to reach.

Optical Fiber Cable Laying by air blowing

Optical Fiber cables are now laying by the air blowing technique blow (if the duct infrastructure was designed for this action). This system can be quicker than pulling, and air blowing technique allow longer continuous lengths for the laying, thus reducing the amount of cable jointing. If spare ducts or sub ducts are installed, then subsequent cables can be installed as the need arises.

When the cable is blown into the duct, it is important that the duct network is airtight with its length. This should be a case for new-build, but it may be necessary to check for existing ducts, especially if they belong to a legacy network.

There should be a balance between the inner diameter of the duct and the outer diameter of the cable. If the outer diameter of the cable is more than 75% of the internal diameter of the duct, air pressures higher than those provided by conventional compressors are required or the blow length may be reduced.. Even so, good results have been achieved between 40% - 85% fill ratio. If the cable is too small, then installation can cause difficulties, especially if the cable is too flexible in such cases, a semi-open shuttle connected to the cable end can solve this difficulty.

In Optical fiber cable blowing technique a cable-blowing head end is required to both blow and push the cable into the duct. The friction between the cable ends and duct becomes more in the first few hundred meters, and the cable is removed from the drum. A suitable air compressor is attached to the blow-head. To ensure a proper flow of air through the ducts, ducts and connections should be tight enough. Hydraulic pressure on the flying head should be strictly controlled so that there is no harm Cable.
Air blowing technique used for optical fiber cable laying
Cable Laying by air blowing technique

Optical Fiber Cable Laying by floating

We know that most outdoor plant underground cables are in contact with water in a major part of their life, floating is an alternative way that can be used for the laying of optical fiber cable. Floating can be arranged using machinery designed to blow originally: the air is simply replaced with water. Compared to blowing, floating makes it possible to keep cable in the duct for a long time without an intermediate access point.
water floating technique
Floating technique for for cable laying

In many situations floating over-laying can prove to be very efficient for cable. Keeping the cable with outer diameter more than 75% of the diameter of the duct reduces the performance of the process. Nevertheless, good results have been achieved for high filling factors; For example, a 38 mm cable floated more than 1.9 km in a corps with the internal diameter of 41 mm (Internal factor filled 93%).

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