In case you missed it: Let there be light

Clive Kessell’s article in issue 122 (December 2014) introduced us to the techniques of fibre blowing. So when Emtelle, a leading manufacturer of blown fibre products, staged an open day for the rail industry at its Hawick factory and head office site, Rail Engineer was naturally keen to learn more about production and installation processes and the benefits that this developing technology can provide.

Emtelle itself has grown over the past three decades from humble beginnings as a minor manufacturer to its status today as a global provider of fibre networking infrastructure. Under the brand name FibreFlow, it produces an impressive range of ducting systems and specialised fibre optic cables. As well as manufacturing sites in Hawick and nearby Jedburgh, the company also operates factories in Denmark and India, with a total workforce of over 800.

Micro and Mini

Emtelle produces a broad range of fibre optic cables, some of which are specifically designed to be blown into its duct systems using compressed air.

Microcable fibre units are intended for use in low friction 5mm micro-tube assemblies (micro-ducts). The bare buffered fibres, either multi-mode or single-mode, are firstly protected within an extruded resin coating. This in turn is surrounded by a low-friction polymeric sheath. With fibre counts ranging from 2 to 12, Microcable ranges in diameter between 1.1mm and 1.6mm. It has a maximum blowing distance of 1200 metres.

The Minicables range offers similar advantages to the Microcable series, but they are used when a higher fibre count is required. They range from 12 to 288 fibre cores, with corresponding diameters of 5.8mm to 10.5mm. The fibres are contained within gel-filled loose tubes, typically with 12 fibres per tube. A central strength member is provided and the cable is protected by an extruded low friction outer jacket.

The Emtelle ducting systems and fittings, as used by Network Rail in Scotland, were described in Clive’s article. They are manufactured from scratch at the Hawick factory, starting with the extrusion of the inner polyethylene tubes. Six extrusion lines work continuously to meet ongoing customer requirements. A low-friction inner lining is extruded concurrently with the tube outer wall. Up to 24 individual micro ducts can then be bundled together to form the finished product, incorporating fillers, metallic moisture barriers and a variety of extruded jackets.

The versions used by Network Rail so far are of two types – direct burial and direct install. In the direct burial type. a PVC filler surrounds the tube bundle. This is then surrounded by a 125μm aluminium water-barrier that has an adhesive coating on its outer surface. An extruded sheath of flexible black polyethylene bonds with this adhesive and a tough outer jacket of HDPE is then added.

The direct install micro duct system, intended for use within existing troughing, is similar in construction but does not have the tough outer HDPE layer. As polyethylene has a high coefficient of expansion, two longitudinal strength members of glass reinforced plastic are embedded within the jacket. These stabilise the expansion and contraction properties of the micro duct over a large temperature range.

How is it blown?

So what about the fibre blowing technique? This was demonstrated by Emtelle engineers during the open day and it appeared to be very straightforward. Once the micro duct system is in place, it’s a simple procedure to connect up a fibre blowing head. Push-fit connectors can be used to extend the Microduct or Miniduct to a convenient position for the fibre cable reel, portable air compressor and fibre blowing head to be set up. Expendable O-rings and gland seals are used to make everything air tight and the cable can then be fed into the duct.

A system of motor-driven rollers built into the blowing head feeds the cable into the duct at a constant rate, usually at about 50 metres per minute. As the cable enters the duct and friction increases, the feed rate starts to fall. With perhaps 300-400 metres of cable within the duct, the air supply to the blowing head is turned on and the feed rate immediately rises again. The flow of air passing over the entire length of cable produces a fluid drag that gently carries the cable along the duct.

In this way it’s possible to blow up to 2000 metres of cable, requiring an air pressure of 10 to 15 Bar, depending on the duct size, and a typical air flow of 100 litres per minute. This maximum stage length matches up nicely with the standard duct reel sizes of 1000-metres or 2000-metres. Longer distances can be blown, of course, by fleeting the emerging cable and then onward blowing it. The fibre blowing head can also be used to blow a cable out of a duct if need be.

Applications and installations

Emtelle’s engagement with Network Rail began four years ago, when the concept and benefits of air blown fibre systems were discussed. Various iterations of designs were proposed, culminating with the installation of lineside fibre as part of the Edinburgh-Glasgow Improvement Programme (EGIP) phase 1 in 2014. This 11km trial installation between Sighthill East Junction and Greenhill Junction signal box uses a Microduct system containing three 5mm tubes accommodating a 4-core fibre optic Microcable. In this application, the fibre cables are used solely for VoIP (Voice over Internet Protocol) telecoms, with the fibres being spliced into BOP (Box on Post) lineside enclosures. The sub-duct runs within the same lineside troughing as the existing FTN cable, which has not been disturbed.

Following the success of the EGIP trial, a similar, but larger, duct system has now been installed on the Borders Railway project between Edinburgh and Tweedbank. Here the ducting has been directly buried by using a combination of mole ploughing and open trench digging. The duct has three 12mm sub- ducts, one of which accommodates a 24-fibre Minicable that is used for signalling purposes. There are also three 5mm Microducts, each containing four-fibre Microcables for VoIP telephony.

A similar technique will also be used on the EGIP phase 2 project between Greenhill and Winchburgh. On this section there is currently no FTN network, so an Emtelle duct system will accommodate node to node links in the form of Minicables. Lineside services will make use of four-core Microcables.

Chatting with Roy Brader of Linbrooke Services, he was full of praise for the Emtelle fibre blowing concept and the use of fibre optics generally on the national rail network. On electrified routes, any systems relying on copper cables need to be immunised against inductive interference. Roy estimates that, on the EGIP scheme alone, the use of fibre optics has saved approximately £400,000 in immunisation costs.

Blown-fibre systems seem set to create substantial savings in other ways too. This quick and easy method of installing secure and robust lineside fibre cables makes the use of IP (Internet Protocol) technology a cost-effective proposition. On the Borders Rail Project, the Edinburgh SSI (solid state Interlocking) signaling is interfaced through MPLS (Multi-Protocol Label Switching) routers in conjunction with IP technology, making use solely of the lineside fibre optic backbone.

In view of the cost savings and versatility offered by this technology it seems clear that it will now be deployed elsewhere on the national rail network. The Borders Railway Project is very much seen to be leading the way on the concept of a fibre only railway. Ease of installation, reliability in service and the simplicity of adding or removing cables at will make it a no-brainer.

So will the entire UK rail network one day rely solely on fibre optic cabling for data and voice communications? Almost certainly, the answer is yes.

Article was first published in Rail Engineer May 2015 Issue 127