Cable jetting
Cable jetting is a technique to install cables in ducts[1]. It is commonly used to install cables with optical fibers in underground polyethylene ducts and is an alternative to pulling.
Pulling
Traditionally fibre optic cables were pulled through ducts via a winch line. Every time a bend or undulation in the duct is passed the pulling force is multiplied by a friction dependent factor (which can be reduced by using lubricant[2]). This means that the higher the local pulling force is, the higher the friction will be which the cable is experiencing while being pulled against the internal duct wall. This "capstan effect"[3] leads to an exponential force build-up with pull distance, producing generally high pulling forces.
Jetting
With cable jetting the cable is blown through the duct while at the same time a pushing force is exerted on the cable. For blowing compressed air is supplied to the duct inlet. This compressed air flows through the duct and along the cable at high speed. Preferably no suction pig is used at the cable head. The high speed air propels the cable, due to drag forces and pressure drop. Now the friction of the cable is compensated locally by the distributed airflow and large forces that would generate high friction are avoided. Because of the expanding airflow the air propelling forces are relatively small at the cable inlet and large at the air exhaust end of the duct. To compensate for this, an additional pushing force is applied to the cable by the jetting equipment. The pushing force, acting mainly near the cable inlet, adds synergetically to the airflow propelling forces. In this way the maximum jetting distance is increased considerably. For cable jetting special lubricants have been developed [4].
Advantages of jetting compared to pulling
- Longer installation distances can be reached.
- Forces exerted on the cable are lower.
- The step of installing a winch rope is avoided.
- Equipment is needed only at one side of the duct route.
Practice
Today the cable jetting technique is used worldwide, from small optical telecom cables (1.8 mm diameter) in small microducts (3 mm internal diameter) [5] [6] up to large copper telecom cables (35 mm diameter) in large ducts (50 mm internal diameter). With this technique distances per blow of 3.5 km have been reached [7], while spliceless links of 12 km have been reached by placing jetting equipment in tandem. It is possible to install 12 km in one day with one small crew.
History
The technique of installing cables using compressed air was developed during the 1980´s by British Telecom. This early version of jetting did not use additional pushing. True cable jetting was invented by Willem Griffioen in the late 1980´s. At that time he was active at KPN, The Netherlands (currently he is employed at Draka Comteq Telecom, The Netherlands). The necessary equipment was developed in cooperation with Plumettaz, Switzerland.