Installation of Mini-Cables

Installation of Mini-Cables: Factors that Influence the Installation Performance of Mini-cables

Ralph Sutehall Martin Davies

Prysmian Cables & Systems UK Ltd [email protected] +44 797 432 5714 [email protected] +44 797 112 9603

Abstract Using practical measurements and a computer model, developed by Prysmian Cables & Systems Ltd, based upon the Griffioen1 theory, a greater understanding of the key parameters that influence the successful blowing of mini-cables into mini-ducts has been gained. The following key parameters have been considered:

Cable Design Mini-duct Design Route Design Quality of Mini-duct Installation Installation Equipment

This paper highlights the primary variables and by considering the whole system gives guidance on how to optimize a blown mini-cable installation.

Keywords: Optical fibre cable: mini-cable; installation.

1. Introduction Using compressed air to install optical fibre cables is a well proven technique. Considerable experience has been gained in understanding the installation performance of optical fibre cables being installed using compressed air. The vast majority of these cables are installed into sub-ducts with a cable/sub-duct fill factor of below 45%. With the introduction of mini-cable systems (typically 6.5mm diameter cables being installed into mini-ducts with a bore of 8.0mm) the cable/mini-duct fill factor increases to typically above 60%. With the reduction in air flow through the mini-duct that this increase causes, this paper re-evaluates the overall system focussing on:

Cable Design Coefficient Of Friction Of Sheath Material Diameter Stiffness Weight

Mini-duct Design Coefficient Of Friction Of Inner Bore Diameter Mini-duct Configuration

Route Design Length Quality Of Design

Quality of Mini-duct Installation Undulations (vertical and horizontal)

Installation Equipment Blowing Head Cable Pay-Off Compressor Lubricant

For the computer modelling a route in excess of 1000 mts was considered necessary. The route configuration should be neither too long (as the reduced air flow would dominate the results) or too tortuous (as the route difficulty would dominate the results). Experience had been gained in installing cables into the test route detailed in figure 1 and subsequently into live networks.

Figure 1. Theoretical route

The route length is approximately 395 mts but for the modelling it was decided to use 4 laps of the above route, total 1575 mts. For the practical measurements it was decided to use the route under consideration by IEC2 and detailed in figure 2:

Figure 2. Blowing trial route

100 mts

Multiple ducts

Total route length – 1100 mts

1 mt

25mts

15mts

180mts

110mts Start

International Wire & Cable Symposium 108 Proceedings of the 59th IWCS/IICIT

Prior to any blown cable installation trial or live cable installation it is crucial that a ‘crash test’ is undertaken to determine the maximum push force that can be applied to the cable from the blowing head. This test involves using the blowing head to drive the cable (at the recommended cable installation speed) through a length of the mini-duct (typically 10 mts). At the end of the mini-duct is a tube end stop. When the cable hits the end stop the blowing head should stop the cable in a manner that does not cause any damage to the cable. This is then repeated with an increase in push force until cable damage is witnessed. The maximum push force of the cable is then taken to be the highest push force figure which resulted in the cable being undamaged when it hit the end stop. This procedure ensures that the cable will not be damaged in the event of the cable hitting a blockage in the mini-duct during the installation process.

2. Cable Design

Figure 3. Typical cable design

2.1 Coefficient Of Friction Of The Cable Sheath During the installation process the cable will come into contact with the mini-duct walls therefore the cable sheath must have a low coefficient of friction to reduce the level of ‘drag’ when this happens. If the coefficient of friction is too low then the level of push-force that can be applied by the blowing head may be reduced due to slip within the cable pushing mechanism.

2.2 Cable Diameter The diameter of the cable will affect the fill factor of the cable/duct system as detailed in figure 4.

0

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60

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80

90

5 5.2 5.4 5.6 5.8 6 6.2 6.4 6.6 6.8 7

Cable Diameter (mm)

Fill

Fact

or (%

)

8.0mm Bore

10.0mm Bore

Figure 4. Cable/duct fill factors

This will in turn determine the level of air-flow through the duct at a defined pressure as detailed in figure 5. The lower the fill factor between the cable and mini-duct is, the more free space for the air-flow resulting in an increase in the maximum blowing distance.

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0 200 400 600 800 1000 1200

Distance Installed (mts)

Flow

Rat

e (l/

min

)

14 bar Pressure - 10mm Bore

10 bar Pressure - 10mm Bore14 bar Pressure - 8mm Bore

10 bar Pressure - 8mm Bore

Figure 5. Reduction in air-flow during the installation of a

6.5mm cable into a 12/10mm and 10/8mm mini-duct A common perceived approach to improving the maximum distance a cable can be installed is by reducing the diameter of the cable. In order to achieve this, the central strength member and optical fibre tubes may have to be reduced in size. This will in turn reduce the stiffness of the cable which may reduce the maximum push-force that can be applied to the cable. It has been found that this reduction in push-force can be as much as 30% which will have an affect on the maximum installation distance as shown in figure 6.

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0.1 0.11 0.12 0.13 0.14 0.15 0.16

Coefficient Of Friction

Dis

tanc

e (m

ts)

300N Push Force

150N Push Force

225N Push Force

Figure 6. Affect of push force on installation distance

(Theoretical).

Additionally, it is important to confirm that cable diameter reduction does not have a negative impact on other cable performance requirements, especially at low temperatures during thermal cycling .Another common way of reducing the diameter of the cable is to reduce the cable sheath thickness. This will reduce the contact area in the ‘cable pushing mechanism’ in the blowing head and reduce the level of compression that can be safely applied by the blowing head as seen in figure 8. However the reduction in radial sheath thickness will also reduce the level of shear force that can be applied to the cable. These factors will also have the affect of reducing the maximum push-force that can be applied to the cable.

POLYMERIC SHEATH

POLYMER TUBE CONTAINING OPTICAL FIBRES

NON-METALLIC CENTRAL STRENGTH MEMBER


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0 0.05 0.1 0.15 0.2 0.25 0.3

Amplitude (mts)

Dis

tanc

e (m

ts)

6.5mm Cable In 12/10mm Mini-Duct CoF - 0.1. Push Force 300N

5.7mm Cable In 10/8mm Mini-DuctCoF - 0.1. Push Force 200N

Figure 7. Performance of a 5.7mm cable into a 10/8mm mini-duct with a reduced push force and a 6.5mm cable

installed into a 12/10mm mini-duct (Theoretical). As can be seen from figure 7 if a reduction in cable diameter is being considered then it is important to minimise the affect it has on the ability of the cable to accept the push force. A smooth sheath will maximise the air-flow passing over it but a sheath showing the stranding formation (reflective) will increase the effectiveness of viscous drag of the air. See figure 8.

Figure 8. Different types of mini-cable sheath finishes.

2.3 Cable Weight The weight of the cable will affect the ability of the air to carry the cable along the mini-duct. The lighter the cable weighs the further the maximum blowing distance of the cable will be.

2.4 Cable Stiffness The cable must not be too stiff so that it cannot go around the bends in the route but must not be too flexible so that the level of push-force being applied by the blowing head is reduced.

3. Mini-duct Design 3.1 Coefficient Of Friction Of The Bore Liner The majority of mini-duct manufacturers provide mini-ducts with a co-extruded low friction liner in the bore of the mini-duct. This liner material varies from supplier to supplier resulting in variations in mini-cable installation performance as detailed in figure 9.

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1400

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0.1 0.11 0.12 0.13 0.14 0.15 0.16

Coefficient of Friction

Dis

tanc

e (m

ts)

5.7mm Diameter Cable

6.5mm Diameter Cable

Figure 9. Affect of coefficient of friction between cable

and mini-duct (Theoretical). During the installation process the cable will come into contact with the mini-duct walls therefore the bore liner material must have a low coefficient of friction to reduce the level of ‘drag’ when this happens. This coefficient of friction typically varies from 0.1 to 0.16 (measured by the Inclined Plane method as seen in figure 10) for different mini-duct suppliers.

Figure 10. Inclined Plane test apparatus

The coefficient of friction of the bore liner will also affect the level of air-flow through the mini-duct. Those with a lower the coefficient of friction having a higher flow rate of air through them compared with those with a higher coefficient of friction value.

3.2 Internal Diameter Of The Mini-Duct Providing the compressor has an output suitable to fill the mini-duct then the internal diameter of the mini-duct will govern the level of air-flow through it. This will in turn affect the installation performance of the cable.

Smooth Reflective Highly Reflective


0

1

2

3

4

5

6

0 200 400 600 800 1000 1200

Distance (mts)

Push

For

ce (b

ar)

10/8mm Mini-Duct

12/10mm Mini-Duct

Figure 11. Push force required to maintain an installation

speed of 60m/min for a 6.5mm diameter cable

As can be seen from figure 11 the increase in air-flow, achieved by using a larger bore mini-duct, has a considerable affect upon the ease with which the cable can be installed. Whilst the cost of the 12/10mm mini-duct is more expensive than the 10/8mm product, typically 30%, in the overall cost of a project the increase in cost is minimal and this cost increase can be reduced or eliminated by the reduction in mini-cable installation time.

3.3 Mini-Duct Configuration Recent innovations in mini-duct cable designs have led to the introduction of mini-ducts configured in a horizontal formation. The main drive for this is the ease of access to the mini-ducts for route ‘drop-offs’. However it has been found that the reduction in stiffness of the mini-duct cable has resulted in the cable ‘following’ any undulation in the trench floor or ‘over-laying’ of cables in ducts. This can reduce the installation performance of the cable.

Table 1. Stiffness values (Nm2) for 10/8mm mini-duct cables

4. Route Design 4.1 Length Of Route The length of the route will determine the level of air-flow through the mini-duct as detailed in figure 12. The higher the level of air-flow through the route the easier it will be to install the cable. The will also result in an ability to overcome undulations in the route caused by poorly installed mini-duct cable.

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500

Route Distance (mts)

Flow

Rat

e (l/

min

)

200 mts 1000 mts800 mts600 mts400 mts

12 b

ar

10 b

ar

14 b

ar

10 b

ar

10 b

ar

10 b

ar

10 b

ar

12 b

ar

12 b

ar

12 b

ar

12 b

ar

14 b

ar

14 b

ar

14 b

ar

14 b

ar

Figure 12. Level of air-flow through 10/8mm mini-duct

4.2 Quality Of Design A cable route goes from A to B and as such there is little that can be done with the design of the route however consideration should be given to the number of bends and how the bends are configured. It is better to have 2 x 45o instead of a single 90o bend or ‘sweeping’ bends. When considering the affect of bends it should be remembered to consider the route in three dimensions. Loops of mini-duct in manholes for future joint provision will reduce the installation distance due to the increase in route difficulty and should be avoided if possible.

5. Quality Of Mini-Duct Installation This is a process which is often overlooked however it has been found to be, both theoretically and practically, one of most important factors that govern the installation performance of the system. A poorly installed mini-duct installation will potentially drastically reduce the maximum installation distance and probably cost the most to rectify.

Figure 13. Poor installation

Problems such as those in figure 13 are obvious, however as mentioned in section 3 the ability of a mini-duct cable to follow undulations, vertical and horizontal, whether they be as a result of poor trenching, mole-ploughing or overlaying of mini-ducts within a congested main-duct will have an affect on the maximum

Cable Type.

152.3 48.7 11.3 1.86 Stiffness

Trench Floor


installation distance of a cable. Another potential cause of undulations is the ‘set’ in the mini-duct cable caused by the manufacturing process and storage on the drum. If a mini-duct cable is installed into a larger duct using a winch then the problems will probably be reduced as the tensile force on the cable will keep it in a reasonably straight line. If the mini-duct cable is being installed into a trench then during the backfilling operation care should be taken to ensure that it is as straight as possible. As can be seen from figures 14 and 15 these problems can be reduced by increasing the air-flow through the mini-duct as a result of increasing the mini-duct size or by using a mini-duct with a low coefficient of friction value.

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0 0.05 0.1 0.15 0.2 0.25 0.3

Amplitude (mts)

Dis

tanc

e (m

ts)

20 mts Pitch, Push Force 300N




Figure 14. Affect of installation of a 6.5mm cable into a

10/8mm mini-duct (Theoretical).

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0 0.05 0.1 0.15 0.2 0.25 0.3

Amplitude (mts)

Dis

tanc

e (m

ts

5mts Pitch, Cof 0.16 20mts Pitch, Cof 0.16

5mts Pitch, Cof 0.16 20mts Pitch, Cof 0.1

Figure 15. Affect of installation of a 6.5mm cable into a

12/10mm mini-duct (Theoretical).

Guidance and monitoring of the installation of the mini-duct cables can reduce these problems and should not be considered as an extra expense as it potentially will save time and money when it comes to installing the mini-cable at a later date.

6. Installation Equipment 6.1 Blowing Head The main function of the cable blowing head is to pull the cable from the drum and deliver it into the air stream within the mini-duct at zero tension. At this point the air propels the cable forward through the route. As the installation continues the cable requires more push force due to the fact that the level of air-flow is reducing and the number of bends in the route is increasing. This means that not only should the blowing head be able to deliver a

high level of push force, the cable should be able to accept it without being damaged in the event of the cable hitting a blockage in the mini-duct. For smaller diameter cables this may require configuring the cable drive mechanism (belts or wheels) differently. Care should be taken to minimize air leaks around the ‘air-box’ within the blowing head.

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Amplitude (mts)

Dis

tanc

e (m

ts)

5.7mm Cable In 10/8mm Mini-DuctPitch - 20mts




Figure 16. Performance of a 6.5mm cable into a 12/10mm

mini-duct (CoF – 0.1) against a 5.7mm cable into a 10/8mm mini-duct (CoF – 0.1) assuming 300N push force

(Theoretical).

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Dis

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Figure 17. Performance of a 5.7mm cable into a 10/8mm

mini-duct (CoF – 0.1) with different push forces (Theoretical).

From figure 17 it can be seen that the higher push force results in the ability of the cable to overcome installation problems. Figures 14 to 17 show that for mini-ducts with a high coefficient of friction this will have a significant effect on the installation performance of the cable along with the quality of installation of the mini-duct cable.

6.2 Cable Drum Pay-Off The cable drum pay-off is an often forgotten piece of equipment but with the reduction in the diameter of the cables and the subsequent reduction in the central strength member within the cables the requirement for the cables to be dispensed from the cable drum at a constant low tension has increased. The use of small lightweight drums is recommended.


6.3 Compressor The compressor should deliver clean, oil-free and water-free air. Due to the high cable/duct fill factors it is recommended that 15 bar compressors should be used in order to maximize the air-flow through the route.

6.4 Lubricant The practice of adding a small quantity of a wet lubricant prior to the mini-cable installation has shown increases of up to 20% in the maximum blowing distance. This process involves adding a quantity of lubricant and then inserting a sponge into the mini-duct. Using compressed air this is then blown through the route with the result being that the inside of the mini-duct is coated with an extra lubricant along its’ entire length. Whilst this may add an extra process to the installation practice the benefits far outweigh the small extra cost. The type of lubricant is critical and should be specifically designed for use with mini-cables.

7. Conclusions What this investigation has shown is that systems providers should have a holistic understanding of the interactions of the key parameters that make up an installed mini-cable route. By understanding these interactions they can install networks most cost effectively. What has been shown is that the installation of the mini-ducts and the coefficient of friction of the mini-duct/mini-cable are the primary factors that influence the installation performance of mini-cables. The other factors are seen as secondary.

8. Acknowledgments The authors recognise the valuable assistance of colleagues within Prysmian Cables & Systems Ltd, Eden Ltd, Factair Ltd and Radius Systems.

9. References [1] W. Griffieon “Installation Of Optical Cables In Ducts”

Plumettaz, PTT Research 1993 [2] IEC 60794-1-2 Ed. 3.0 (draft) Method E24 (86A/1340/CD)

10. Authors

Ralph Sutehall is a Principal Engineer with the Communications Division of Prysmian Cables & Systems in the UK, where he is responsible for installation & applications development. He has been working with optical fibre cables for 35 years and has numerous patents and conference papers in this field of study.

Martin Davies is Chief Engineer (Telecoms) with Prysmian Cables & Systems in the UK, where he is responsible for product design and development. He has been an active member of a number of standardisation bodies, including BSI, ETSI and IEC and is a Fellow of the Institute of Engineering and Technology.


Installation of Mini-Cables

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