Since they were first introduced in the 1980s, optical fiber cables have dramatically shrunk in size. A 96 fiber cable can now weigh 30kg/km (down from 300kg/km) and have a diameter of 7mm, compared to 20mm for first generation cables.
Similarly, 12 fiber drop cables used to connect individual FTTH customers now weigh less than 10kg/km and have a diameter of 1-3mm. These are normally installed into microducts, which typically range in outside diameter size from 3-18mm.
This leads to new challenges for installers when it comes to equipment. Previously cables would have been installed with heavy equipment, such as winches and capstans, or heavy compressors and blowing heads. However, this has four big disadvantages in the last drop:
1. People
It requires multiple operators, pushing up costs.
2. Disruption and mess
Customers don’t want bulky equipment in their buildings or apartments, particularly if it damages their homes.
3. Equipment cost
Operators need to invest in buying or hiring expensive machines to carry out installations.
4. Time
While the cable install itself may not take long, setting up (and dismantling machines) is time-consuming, limiting the number of installs that can be completed in a day.
What is a handheld pushing machine?
What is needed is a single operator, a self-contained installation head and low-cost, lightweight ancillary apparatus. For example, this handheld pushing machine caters for cables from 2 to 5.5m diameter and microducts from 5 to 12.7mm outer diameter.
It’s necessary to understand a little of the science behind "pushing" to see how it can be used. When pushing there is no cable tension, meaning that, unlike pulling, the challenge is not one of over-straining the cable, but in forcing the cable to buckle by over-pushing. Buckling the cable effectively locks it into the duct and can cause permanent damage.
This can be controlled by controlling and optimizing the cable stiffness and by reducing the coefficient of friction between the duct and cable. While low friction is always beneficial, stiffness is a compromise. It needs to be sufficient high to tolerate the push force exerted by the head, but low enough to enable to cable to flex around curves in the route.
Cable pushing machines typically exert around 40 to 50N of push force at the drive system, which can be a belt or driven wheels. Provided that the microduct’s internal diameter is a relatively close fit (for example no larger than a 6mm bore for a 3-4mm cable) there is no danger of the cable kinking at these force levels. This means that, for a low friction duct, a push distance of 100 to 200m is possible, depending on the degree of bend in the route.
In practice, this enables installers to deploy the majority of drop cable connections by using pushing machines.To meet these contrasting needs you need a cable designed specifically for pushing. The relatively hard polymer used as a jacket confers low friction properties when used with optimally lined microducts, providing the right level of stiffness to avoid the risk of buckling - while still having the flexibility to push around corners.
Of course, even a pushing machine requires a power source and in the interests of simplicity and cost many use a standard 10.8V Euro/12V Li-ion US unit.
Adding air assistance
Despite the optimization of pushable cables, there will always be instances where pushing alone is insufficient to deliver the required installation. For example, the route may curve unexpectedly or the actual install length may be longer than planned. In these situations air assistance will be required.
For this reason, most high quality pushing machines provide an optional inlet for compressed air sources of up to around 12-15 bar. Whereas pushing puts the cable into mild compression, the use of a high speed air flow brings a distributed force to bear on the cable, which eases it around any significant bends, meaning that the install length can be extended to over 1km. The good news is that the same ultra-low friction duct used for cable pushing provides the same excellent properties when used for air assisted deployments.
When adding air, it is important to use the right type of source. Large, wheeled and towed petrol compressors provide ample air but aren't necessarily appropriate quality for installation.
The user needs to determine the air pressure and volume needed, although pushing equipment manufacturers can advise on this. Additionally, they need to set the degree of filtering and contaminant removal. This is because it is vital to remove moisture from the air supply using an after-cooler and water filter and to take out any residual hydrocarbons, since both of these contaminants interfere with effective blowing. One way to achieve this is to use an air cylinder which contains clean air under high pressure (alternatively a compressed nitrogen tank will work).
However, for those users who want to avoid the logistical issues associated with obtaining and returning a large number of cylinders, a small compressor remains the best option. Historically 10 bar and 15 bar compressors have provided relatively large air outputs (measured in cubic feet per minute (CFM) or cubic meters per minute (m3/min)). A substantial compressor will generate in excess of 1 m3/min but will weigh around 100kg. Such a size rules out single operator working and necessitates specialized vehicles to transport the compressor.
Coping with leakage
In the past, one of the reasons that users tended to opt for these large machines was that older style blowing heads lost a large volume of air through leakage. This meant that a modest air supply wouldn’t let cables be pushed "hard" enough to ensure they reached their destination.
However, recent work has led to the availability of one-person portable compressors. These weigh around 25kg in weight, meaning that the pushing machine and its ancillary compressor can be handled and transported by a single person in a standard commercial vehicle. This further brings down staff costs and makes pushing available for a wider range of installs.
Pushing to the future
Installers have three options when it comes to last drop deployments – blowing, pulling or pushing cables. Given the relatively short runs and often complex routes of the last drop, pushing cables is becoming much more common as an option. Using a pushing machine extends the usefulness and range of this technique, helping to bring down last drop install costs and speed up deployments. Particularly when used in combination with cable and microduct designed for pushing, they deliver major budgetary and time benefits. This means they should now be a standard part of every operator’s toolbox when carrying out FTTH deployments.
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