Building blocks of a modern Network

A modern optical fibre network has four fundamental building blocks:

1. the optical fibre cables
2. the connectivity to joint and route them
3. the active equipment to ‘light’ the fibres and
4. a duct or microduct system to contain and facilitate installation, repair and removal/replacement of the cables.

Historically large ducts with internal diameters around 100mm were used to contain a mix of copper and fibre cables. However, as fibre cables are smaller and lighter than their copper predecessors it has become clear that a better approach was required. Optical fibres, whilst otherwise durable do not tolerate tensile loads (pulling) well so the installation by blowing technique increased in popularity.

The need to protect fibres cables and the need to install them by air-blowing has given rise to the popularity of ‘microducts’. So what exactly is a ‘microduct’?

Example of Direct Bury Microduct

A single microduct is a flexible (usually plastic) pipe with an external diameter between 3 and 16mm designed to accommodate (usually) a single optical fibre cable or unit. The fibre product can be installed by pulling, pushing or more, normally by air-blowing. The upper size limit is variable, but products of 20mm outer diameter and greater are normally regarded as ‘ducts’. The advantage of these sub 20mm units it that they are suitable for interconnection with inexpensive and convenient ‘push-fit’ connectors (rather than the compression fittings required for larger ducts).

Microducts are most effectively used when they form an interconnected full network. For that reason it’s wise to use microducts that conform to recognised specification. Individual microducts are described in detail in EN 50411-6-1. This document defines the range of sizes that are standardised in Europe and basic performance criteria. IEC 60794-5 and its associated specifications IEC 60794-5-1 (for blown cable) and IEC 60794-5-20 (for blown fibre units describe some typical microduct bundle types, although the types in use are continually advancing.

Each microduct requires a flexible, durable outer plastic material that is capable of withstanding the internal air pressures that may be applied (typically between 10 and 15 bar). The inner surface must have as low a coefficient of friction (CoF) as possible, either when used with an additional lubricant, or preferably by means of a lubricant incorporated into the material. The simplest way to achieve this would be to dose the polymer used with friction reducing additives. However, this can be fraught with problems. Firstly it means far more (costly) additive is used than is necessary. Secondly the additives will appear equally on the outer surface (where they are not needed) and on the inner surface. In fact, additives that cause ‘slippery’ properties on the outside of the microduct can cause handling problems.

So currently the most successful microducts consist of two layers, a thin, low friction, inner layer and a thicker outer layer consisting of unmodified material. A low (inner) surface CoF is a desirable property because the installable length is inversely proportional to this parameter. Frequently, both materials are the same or slightly different grades of high or medium density polyethylene. To achieve low friction properties, two additives are typically incorporated into the inner material: a ‘slip’ agent to reduce the coefficient of friction and an ‘anti-static’ agent to reduce static attraction between the microduct and the cable. In the case of very lightweight cables (for instance those weighing less than about 5g/m) antistatic agents can be vital in improving the performance of the product. For heavier cables, the effects of static are much less pronounced and so larger duct may not need this additive; this is because the forces involved are much higher than those due to static attraction.

For an air blown installation (the most common) there are several parameters that are involved:

• The air pressure used
• The diameters of the cable and microduct
• The cable weight, it’s stiffness and CoF

The trajectory of the route (bends and inclines). So for example a 5mm OD and 3.5mm ID microduct (commonly used for 1 to 2mm diameter fibre units) will facilitate install lengths of between 500 and 1500m for a benign route using 10 bar pressure. Longer install lengths are possible for larger microducts and larger cables, especially if the cable is relatively stiff since a significant degree of pushing can be used to supplement the blow.

Overview of Cable types/blowing distances

Where to use what

Multiple-microduct packages are provided by sophisticated solution providers. There are at least four fundamental types:

• Direct Buried (for installation below ground) –
• Duct/Direct Install (for insertion into existing main ducts)
• Aerial (for suspension between poles and towers)
• Indoor for use within buildings and where it’s common to have enhanced fire performance.

Direct Buried products are generally of heavier construction with thicker sheaths and so are able to withstand higher impact and crush loads. Depending on their make-up, they can be used in excavated trenches, mole-ploughed routes or inserted into slot cuts in the wearing surface of roadways.  (Get Emtelle´s free report on microtrenching here-> http://www.emtelle.com/landing_page/microtrenching/)

Direct Install products are by comparison of a lighter construction and are suitable for pulling, pushing or blowing into existing main ducts (typically with a 32 to 100mm internal diameter). Aerial routes are generally benign and with an obvious trajectory but the extreme ambient conditions (compared with steady below-ground temperatures) put a heavy demand on the lining material properties.

Microducts used within buildings may at first sight seem to be the most forgiving application. Whilst it’s true that a temperate, dry environment is present (and there is little danger of impacts from implements), a very different challenge is posed. Around the World, fire performance building codes means the fire hazard presented by different microducts and the cables within must be controlled by material and design changes. In Europe the Construction Products Regulations mandate the necessary fire properties and in the US the NEC and UL test methods set the standard.

Read Case story from a newly built Danish Hospital http://www.emtelle.com/experience/blown-fibre-network-dnu-hospital-denmark/

Geometry dictates that some microduct counts are more effective than others. A 7-way bundle close packs perfectly to give a round product, a 12- way product can be made acceptably round and a 24-way product with a centre microduct twice the diameter of the surrounding ones also gives a circular shape. To achieve near-roundness with other shapes, filler rods are frequently used. Other options for the packages include a bonded aluminium water vapour barrier, a trace wire, or – for non-metallic operation – a water-swell tape water ingress barrier. Water swell tapes include a material that absorbs many times its own weight in water.  The material choice for the outer microduct bundle is important; if the bundle itself is to be blown into an existing duct it must possess good blowing properties (low coefficient of friction), but if needs to resist biological attack, commonly from insects, it may need to be made from a tough polyamide such as nylon-12. Finally, as covered, above, a choice between halogen-free thermoplastic or fluorinated polymers may be required according to fire regulations.

So in conclusion it’s clear that different customer circumstances dictate a different microduct size and type, count, format and material content. Prospective customers are well advised to consult microduct experts before embarking on a build programme.  Contact our Emtelle Product Experts here

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