Measurement of the max. allowable push force for cables to be installed by blowing

Installation of fiber optic cables by blowing technology is common praxis since many years. However, since the diameter of cables is getting smaller and smaller (cable outer diameter less than 3 mm) installers observe difficulties during installation of these low diameter cables. It was observed that cables were often pushed "too hard" at the beginning of the blowing process which resulted in a severe cable damage. The analysis of the process showed, that the forces which were applied by the pushing caterpillar, or traction wheels was too high thus the cable buckled inside the duct which finally led to a cable damage or even to fiber breaks. To prevent that failure mode, installers want to get the max. allowable push force of the cable specified. This paper presents a method which enables the determination of the allowable push force with a test set-up easily to be implemented in the cable manufacturers laboratories. The test device is based on a standard lab scale tensile test equipment (e.g. Zwick, Instron, MTS). A short piece of duct (approx. 300 mm) is fixed in a frame attached to the tensile test equipment. The cable is pushed into the duct and the push force is recorded. The failure mode can either be detected by a drop of the push force or by an increase of the optical attenuation of the fibers. Measurements of the push force in dependence on the cable diameter, duct diameter and temperature will be presented. The correlation to real blowing trials will be discussed.