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english
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.
4. Installation
4.1 Aerial air-blown installation
Generally, aerial installed cables are the
cable type subjected to the toughest
conditions after installation.
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.
Figure 12
:
Micro cable blowing
▲
Figure 9
: A 24-fibre micro cable
▼
Figure 10
: Installation of aerial multi-duct assembly
▼
Figure 11
: Temporary connection tube attached to
duct ends in fibre joint closure
▼