EoW January 2008

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see Figure 7 . They are easily mounted to the duct assembly without any cutting or splitting. The fitting is very gentle to the ducts due to the length and shape of the wedge and provides for quick and easy installation. For each type of duct assembly suitable fittings have been selected and tested. 3.5 Blown fibre units and micro cable A blown fibre unit, EPFU (Enhanced Performance Fibre Unit), consists of a number of fibres or ribbons encapsulated in two layers of UV-curable acrylates, see Figure 8 . The outer layer accommodates small glass beads on the surface to enhance the blow performance. For details see [1] . The EPFU is used as a ‘drop cable’ from the end user termination point to a distribution point. To minimise installation time and total cost of installation, the EPFU is delivered pre-terminated with optical connectors, factory-fitted in one end. The pre- terminated EPFU is delivered on small polystyrene reels and is available in different lengths. The micro cable, Figure 9 , is mainly designed for use in access and metro- politan networks. Here, the micro cable is used as ‘feeder cable’ from the distribution point. The micro cable consists of up to eight compact fibre units (CFU) with either 4 or 12 fibres per unit. This enables a fibre count from 4 to 96 fibres. For details see [3] . The versatility of this cable design has enabled a very compact and small fibre cable that can be installed in ducts with a diameter of 7 or 10mm.

Figure 12 : Micro cable blowing

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Snow, ice, temperature differences and wind will affect the cable or, in the worst case, all of those at the same time. The span length for the air-blown concept is typically 30-60 metres, but limited to a maximum of 60 metres. One reason is that the fibres are installed with no excess length in the tube, which gives fibre strain at any extra cable load (in contradiction with traditional optical cables). However, there is no fibre strain at the installation load case. Another reason is that for longer spans the ‘figure-8’ shape tends to be more sensitive to galloping effects at certain wind speeds. Due to the relative short span lengths the concept is best suited for the access network. The concept has been installed in three continents under three different environmental conditions. Load calculations, including parameters such as maximum wind and ice loads, temperature changes and span lengths, show that the fibre strain in these conditions reaches a level corresponding to a decreased fibre life time.

However, if unexpected loads occur and the fibre is damaged, eg due to a falling tree, it is easy to remove the fibre and replace it with a new one. This aerial air-blown system is easy to install. The combination of lightweight duct assemblies and quick install fittings allows the system to be installed with very few tools, see Figure 10 . The dielectric design of the duct assemblies enables installation along existing power distribution lines. It can be installed as standard ADSS cables, pulled out on installation rollers. Since it is often quite short distances and low weight it can also just be pulled out on the ground and afterwards lifted up pole by pole. Due to the low weight the duct assemblies can even be tensioned by hand. Ordinary tensioning tools can, of course, be used as well. 4.2 Installation of main distribution cabling The main multi-duct assembly, as described in section 3.1, is installed along a street. In selected poles it is terminated in joint closures. After the tube ends are installed in the closure, a temporary tube is connected between the terminated duct ends which make it possible to install the micro cable a longer distance, see Figure 11 . The micro cable is installed by blowing technique, using compressed air supported by additional mechanical pushing, see Figure 12 . The temporary tubes in the joint closures are then removed immediately giving the correct over length of micro cable for mid-span access . One or more of the fibre units (CFUs) are branched at this point.

4. Installation 4.1 Aerial air-blown installation

Generally, aerial installed cables are the cable type subjected to the toughest conditions after installation.

Figure 9 : A 24-fibre micro cable

Figure 10 : Installation of aerial multi-duct assembly ▼

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Figure 11 : Temporary connection tube attached to duct ends in fibre joint closure ▼

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EuroWire – January 2008