Telecom and Data

7 tips to blow micro-cable far away

Julien Loterie Mar 11, 2020

Installer on site blowing micro-cable

Cable air-jetting is the process of pushing a cable into a duct while simultaneously injecting compressed air. The airflow carries the cable, which helps the cable progression in the duct. How far can a cable be blown? It is very difficult to answer this question as it depends so much on several different criteria: the percentage of occupation into duct, the equipment and its quality, the subduct installation and route, the weather condition during installation and, of course, the experience of the installer.

To help you maximise the performances, whether it is on speed or distance, here are 7 key points to keep in mind.

1. Safety first

You cannot improvise as a fibre blowing technician! All involved people must be properly trained for preparation procedures and cable blowing. Quick reminder of main rules to keep you and your team safe:

  • Wear your personal protective equipment: eye protection, ear muffs, helmet and protective suit. It is subject to local regulations and company safety rules, but we strongly recommend using them to avoid injuries.
  • Before starting the machines, test the communication between the different working stations: safety messages such as “STOP” or “RUN” must be perfectly understood.
  • Warn everyone when the duct is put under pressure
  • Shut down the air pressure between the compressor and the blowing machine prior to any operation on the connections

2. Use the right cable depending on the duct size

The cable and duct sizes must be compatible.To ensure your cable is compatible with the duct, the percentage of occupation should be between 30% and 50%. Below and above those limits, the performances are impacted progressively. 

Percentage of occupation = (cable OD)² / (duct ID)²
The easy calculation is:   0.5*(duct ID)  < (cable OD) < 0.7*(duct ID)

”cable OD” stands for “Cable Outer Diameter”
“duct ID” stands for “Duct Inner Diameter”

3. Take care of your cable

Place the cable drum on a pay-off and make sure the cable will be protected against water, sun and dust while blowing.

Mount a cap on the head of the cable. This cap prevents air penetration into the cable at high pressure and ensure there will be no rough contact between the cable head and the tube or the connectors.

4. Check your equipment

The equipment must be adapted to the cable and the duct. Here is a list of the main questions you should ask yourself:

  • Are the blowing machine, accessories and compressor suitable for the current cable and duct?
  • Is it possible to reach the right level of air flow and pressure? The use of a flow-meter is recommended.
  • Do I have connections of the right type for this installation job?
  • Is my stand (pay-off) suitable for the drum size?

Note that the best results are achieved when a dryer and an air cooler are installed between the compressor and the blowing machine.

5. Check the duct condition

Duct (and its accessories) quality and installation are key to secure a smooth and easy route without wavy effects: a perfect installation is supposed to be made without any bend to achieve the maximal blowing distance. On the field, it is not so simple and all additional bends along the route will impact the blowing distance. Be careful with the connectors too, as they are not designed to plug ducts in a bend.

To check the proper condition of the duct, start by making sure that the far end of the duct is secured with a trap. Then, use a gauge to check that the duct has not been damaged during the installation. This gauge should have a diameter slightly smaller than the internal diameter of the duct. A ball chain is efficient for this operation. It should take less than 15 minutes per km of duct at 8 bars maximum to finalise this operation. Otherwise, problems with the ducting may be suspected.

When the duct has passed the gauge test, clean it to remove moisture and dirt. The best way to do so is to use sponges made for this purpose. If it is not possible to get a clean and dry duct even by repeating this operation, the duct shall be considered as inappropriate for blowing.

6. Use lubricant

Special lubricants have been developed for cable jetting to further reduce friction. Even if lubrication is time consuming, it is also a warranty of good performance and eventually of efficiency. When distance is short, lubrication can be skipped. Use the lubricant recommended by the duct manufacturer.

The quantity of lubricant must be adapted to the duct size and length, as for the provider’s recommendation. Spread the lubricant by blowing a foam carrier through the duct.

7. Follow the blowing procedure

We recommend to start pushing the cable at a speed between 40 and 60 m/min, without air pressure in the duct. When the speed begins to decrease, open the air inlet valve of the blowing machine to reach a pressure between 1 and 2 bars in the duct. Speed goes up to the previous value. If cable does not speed up, increase the pressure by steps of 1 bar.

All along the blowing operation, adjust the cable pushing force to regulate the rate of progression of the cable in the duct.

What to do when you cannot progress anymore, and if the target is almost achieved?

  • Stop the machine and release the air pressure in the tube.
  • After 5 minutes, open the air flow until the maximum air pressure is reached.
  • After a few minutes, restart the machine to push the last meters.
  • Repeat if necessary.

About the author

Julien Loterie

Julien Loterie joined Nexans in 2000 as Quality Manager. In 2005, he switched from power cables to fibre cables. Along the years, its knowledge never stopped growing, and he is now Product and Solutions Manager. Julien is also one of Nexans Technical experts on fibre optic cables and fibre optic cable installation processes.

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