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Fibre optic cable blowing isn’t a new technique for cable installation and, in fact, it has been in use since the late 1990’s. However, the cable blowing technique may be new to some installers. A range of CBS cable and fibre blowing machines  help get the job done with ease for both the seasoned installer or the novice. In this article, we discuss one of the key parameters to a successful installation, the fill ratio.

The Fill Ratio

The fill ratio can be calculated for a single fibre optic cable installed in an innerduct or microduct as: d2 ÷ D< 65%

or

d2  < 65%
D2

The lowercase “d” stands for the outer diameter of the cable, while the uppercase “D” stands for innerduct’s inside diameter.

This means that when the cable outer diameter squared is divided by the innerduct inside diameter squared, the outcome should be less than 65% (or .65) for the most effective cable blowing. Also, the fill ratio should not fall below 35%, as the cable may have more of a tendency to “kink” in the innerduct due to excessive turbulence. Likewise, when the fill ratio is greater than 80% it becomes increasingly likely that there is insufficient space around the cable for the viscous drag principle of the air to act on the cable jacket.

The fill ratio is one of the parameters determining how far a fibre cable can be blown. A larger fill ratio is preferred when the cables are relatively stiff and the duct run is straight. A smaller fill ratio is typically preferred when there are multiple bends in the duct run, allowing more room in the innerduct for the cable to navigate the route.

The installer doesn’t specify the innerduct or the cable, but the installer can easily calculate the fill ratio and determine if the ratio is in the “sweet spot” or not. A fill ratio out of range may explain why the cable installation didn’t proceed as planned.