Blown fibre -David Stockton brings us a recap of the blown fibre situation

Background

Since its original invention in the mid-1980s some degree of confusion has surrounded fibre or cable blowing and even the technique of cable pushing. In this article we attempt to clarify the different variations on the process, to differentiate between blowing fibre (units) and blowing cable, to look at the effects of pushing with blowing and to examine how the process can be optimised.

Researchers at British Telecom Research Laboratories in the UK during the early 1980s  discovered the phenomenon of ‘viscous drag’, the distributed force which creates minimal strain or compression since it acts along the whole length of the cable when a high speed, turbulent air flow is applied.  Previous installation methods, which push or pull the cable from one end lead to ‘locking forces’ around bends that can defeat conventional installations, especially where the routes are long or tortuous.

As the inventor, British Telecommunications, (BT), held the original patents on fibre blowing. BT’s ‘process’ patent used examples that were mainly very small cables, typically up to 2mm in diameter. However, the patent claims were not limited to this class of cable, and their patents were subsequently determined by the courts to cover the full range of cable blowing techniques where a non-parachute system is used. BT has actively asserted its patent rights and developed relationships with many industrial partners, however, with the master patents expiring by around 2004, blowing per se was opened up to all. Of course companies (including BT) have taken out product and improvement patents. Currently it is understood that a BT licence is only needed if their specific product and refinement patents are exploited and/or the licensee wants a know-how transfer.

Product Types

The original ‘blown products’ would today be classified as ‘fibre units’, not as cables as they did not contain any additional strength elements. The distinction between optical cables suitable for blowing (so called microduct optical fibre cables) and optical fibre units has been formalized by IEC and described in their publications IEC 60794-5-10 covering ‘Outdoor microduct optical fibre cable, microducts and protected microducts for installation by blowing’ and IEC 60794-5-20 covering ‘Outdoor microduct fibre unit, microducts and protected microducts for installation by blowing’.

Note that it is outdoor products that have been globally standardized, indoor ones have not due to the differing international fire standards. In practical terms, fibre units have to be kept in their covering microducts or in their relevant joints whilst microduct optical fibre cable can be stored outside of a closure or outside the microduct.  On the flip-side, MOFC’s are larger and heavier than fibre units so the user has a decision to make on functionality versus ease of use.

Get the scoop on the differences between Protected and uprotected microduct here

How to get the best from Blowing

The science of blowing is now well understood but the challenge has been to get cable providers, equipment makers and users to act on it! Let’s start with the more obvious points. So consider air pressure. What’s matters is not simply the air pressure at the start of the route, rather it’s the change in pressure with distance along the route; this is known as the ‘pressure gradient’. It can be compared with the voltage in an electric circuit; the voltage or potential difference drives the circuit in the same way that the pressure gradient drives a blown cable installation. We have a pressure gradient as high pressure (in the blowing head) decays to atmospheric pressure at the route end. So the magnitude of the pressure gradient depends on the pressure at the start of the route and the route length.

Sometimes users ask if a compressor’s air volume output is important. The answer is ‘yes’ and ‘no’. This is because if the required blowing head pressure can be maintained, there is one and only one air flow rate that exists for that duct and route. Again this situation is similar to electric circuits; voltage and circuit resistance give a single value of current; in blowing a pressure gradient and the duct/route give a single volume air flow. So users need to determine what head pressure they need to install their cables and the kind of ducts and routes they want to use – this will tell them what compressor air volume output they need.

“Now to the more interesting points. Do larger or smaller cables blow better, especially if they have the same weight – and let’s assume they are both equally stiff? The answer is that, everything else being equal, it’s the larger cable that installs better – at least up to a certain point where the cable diameter reaches around 80% of the microduct diameter. The upper limit is a practical value because beyond this point, bends, and microduct and cable tolerances can cause the cable to seize in the duct”.

So now we need to think about factors that are obstructing the cable installation. This is principally the friction between the cable and duct and the weight of the cable. Multiplying these two parameters together gives the force acting against the installation. So reducing both to the minimum assists blowing. Duct producers have led the way here but some cables, especially indoor ones still show high friction levels.

Finally, there is one factor that both help and hinder cable blowing. This is the issue of cable stiffness which arises because all ‘blown’ cables require some degree of pushing to get them to the point where the blowing force is high enough to take over. In a relatively straight route a high stiffness cable works better than a flexible cable, but in a route with many tight bends a more flexible cable works better. In the real world it’s not realistic for users to specify the stiffness they want depending on the routes that have to install. However cable providers could assist users by stating their cable’s stiffness so that predicted install capability can be predicted using blowing models.

What does the Future have in store?

Although cable blowing is now the predominant fibre installation method in many parts of the World there are still issues that can be improved. Firstly, cable jackets can be modified to increase the positive effects of viscous drag; this is especially true for indoor cables where the materials used to meet fire standards generally have high friction values. Secondly, there is a growing trend to make cables smaller – which as we’ve seen counts against installation ease – but if coupled with a significant weight reduction gives an overall improvement

Then of course there is the whole issue of blowing equipment that has the potential for automation and ease of use optimisation, but that is another story!

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