CT p38-40 Blown Fibre Nov08.qxp 24/10/2008 11:54 Page 40
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critical, as is the design of the propulsion blown fibre is that it reduces the number
connection from equipment. Certainly, a standard of optical fibre splices in a network. Thus,
the outer atmosphere via an compressor cannot be used. optical attenuation losses are
‘O’ ring seal. It is possible to install all Compressors are specially designed to significantly reduced.
the way from the high fibre density to the ensure that the correct pressure, air From an installer’s perspective, Blown
low fibre density without using splices. quality (in terms of humidity), and flow Fibre is essentially a staged installation,
Certain blown fibre systems allow rate can be maintained throughout which allows cash flow to be better
simple and quick termination (miller installation. managed throughout a project. In a new
strippers down to core and Blown fibre can be installed at a speed build for example, the protected
cladding), while the of around 40m / min over a typical microducts can be installed early on in a
active maximum distance of one kilometre. This project and the blown fibre introduced
distance can be extended by ‘onward once the building project has progressed.
blowing’, whereby the fibre is blown For the owner of a network or a
through the first section of microduct, commercial premise, the advantages of
equipment collected at the end, and then blown blown fibre are very tangible when the
employed is the same on through the second potential savings are considered against
as within any optical fibre network. section. This technique the whole lifecycle and associated costs
Management within a patch panel can be adds complexity to both of both premises cabling and
achieved without the need for extra fibre the network design and conventional sheathed cable systems.
protection. installation, while increasing the risk of Ros Neat, Consultant Engineer, joined
hidden damage. Brand-Rex in September 2003 bringing
Microduct matters experience of optical fibre and air blown
Available in various outer diameters An installation perspective fibre manufacture, test and international
ranging from 3-8mm, microducts can be For integrators, the clear advantage of standardisation.
deployed as a single protected tube or
bundled as a package of 4, 7, 12, 19 or
Figure 1.
24 microducts. These are encased in an
outer protective sheath that should have
the same mechanical and environmental
performance as traditional cabling – i.e.
fire performance for indoor use, and
durability for outdoor use.
In external fibre scenarios, ‘closures’
are used to protect external joints. In
premises cabling scenarios, ‘enclosures’
are employed to protect the junction
between microducts. These can also
incorporate gas blocking technology.
Having spare capacity and the ability to
future-proof a network is a further plus
for the blown fibre concept. When first
installed, it is likely that only a third of the
microducts will be populated with fibre.
So, if a user requires additional capacity,
more fibre can be simply blown in!
Blowing fibre
Viscous drag is the fluidics term
describing the process whereby the fibre
unit is pushed through the duct by a gas
– usually air but, in some cases dry
nitrogen. The forces at work are drag and
friction. The latter can slow the passage
of the fibre and even stop it. Hence the
properties of the physical interface
between the optical fibre and the tube is
40UIbackUIforwardCABLEtalk November 2008
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