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Aerial Deployment Comparison: ADSS and Loose Tube Cable

(This article originally ran in the September 2013 issue of OSP Magazine)

Service providers planning aerial fiber optic cable deployments have a number of important decisions to make. When it comes to which fiber cable technology to use in aerial deployment in North America there are 2 strong options: loose tube cable, and all-dielectric, self-supporting (ADSS) cables.

For the most efficient and effective application, you must understand the strengths and weaknesses of both. So, let’s start by reviewing some definitions. Although a cross-sectional drawing of either cable type looks almost identical, there are some fundamental differences in the cables.

Loose Tube Cables are typically designed for underground deployment or aerial deployment when lashed to a messenger. They are not designed to withstand large tensile loads, such as those resulting from wind and ice. Usually, the highest tension they’ll experience is during installation, and they are often limited to 600-pound short-term loads.

ADSS Cables are designed primarily for aerial deployments, but can be used in duct applications, especially those requiring higher pulling tensions. ADSS cables are designed to support their own weight as well as any weather-related loads such as ice and wind. The lowest tension an ADSS cable experiences is during installation. And during its lifetime, can withstand loads up to thousands of pounds, depending on the design of the cable.

Deployment Considerations

There are subtle differences in the design of the 2 cable types pertaining to the lay length of buffer tubes, as well as the amount of aramid yarn and other strength elements used.

There are several similarities between ADSS and loose tube cables. These include:

Both products can either be installed with the moving reel or stationary reel installation method. When starting with bare poles, the ADSS install is usually faster, since only 1 cable is installed, versus 2 for the lashed method.

Either loose tube cables or ADSS cables can be placed in conduit. Dielectric loose tube cables or ADSS cables should not be direct-buried, although armored loose tube cables can be direct-buried.

Both ADSS and loose tube cables can be used in networks that require mid-span access, with one important difference: In the lashed loose tube cable scenario, the snowshoes used for cable storage are stored in the middle of the span. In the ADSS scenario shown in Figure 1, the snowshoes are located on either side of the pole. That’s because ADSS is not designed to handle the complete load of all of the spare cable and additional ice/wind loading accompanying it, contrary to the lashed network, where the strand absorbs the tension.

Figure 1. Snowshoes store spare cable on either side of the pole in an ADSS network.

For both products, it is very important that the installer follow the recommended installation procedures, especially when pulling the cable around large angles. If the pulling tension is too high and the pulley size is too low, the cable can get crushed during the installation process. This is easily avoided by following the recommendations.

There are also differences between the 2 options. Some of these include:

ADSS is all-dielectric, so typically there is no cable bonding, grounding, or maintenance needed. By comparison, lashed cables sometimes need periodic tightening or repair of the lashing wire if it has broken or loosened.

ADSS uses different hardware to attach to the poles. This hardware is specially designed to support the cable without crushing its buffer tubes and the fibers inside.

When working with a power utility, ADSS can be placed on transmission lines or in the supply zone, which can help avoid make-ready charges. ADSS can also be used in the communications zone on a joint-use distribution pole.

Other aspects of the installation processes can also be a bit different. With lashed cables, tension is placed on the strand, and the fiber optic cable is lashed onto the strand at very low tension. With ADSS, tension is placed on the ADSS cable during installation.

Restoration of ADSS cable is typically faster since only 1 cable is being restored instead of 2 (the strand and the fiber cable). ADSS cable can potentially allow restoration loops to be placed farther apart, decreasing the overall number of loops. See the new similarity in the previous section.

Don’t Be Up In The Air

To determine the most cost-effective option for your aerial fiber deployment project, you must take several factors into consideration when examining the Total Cost of Ownership (TCO).

In general, ADSS cable is more expensive than loose tube cable. However, the total material and labor costs related to ADSS can be lower, making the overall price per foot comparable. Similarly, ADSS hardware on a cost/pole basis is typically more expensive, but the higher price/pole is offset by the fact that no strand or lashing wire is needed.

Let’s examine the scenarios in which ADSS or loose tube cable is better suited:

New fiber build to an area without a strand already in place: ADSS may be the better choice. In this scenario, the total installed cost of the ADSS is almost always lower. Since only 1 cable is installed (as compared to lashed cable, which requires a strand, cable, and lashing wire), ADSS can be installed quickly, resulting in both lower labor costs and a quicker path to revenue.

New fiber build to an area where a strand is already in place: Lashed loose tube cable will be the least-expensive option for the vast majority of these situations. The only exception is when the strand is already overloaded with multiple cables. Lashed loose tube is also a good choice if the service provider knows in advance that multiple cables will need to be located along the same right-of-way.

The key to choosing the right aerial cable is to know exactly how you plan to use it. For aerial installations on existing strand, or in situations where multiple cables will need to be deployed over time, lashed cables often make the most sense from a cost and performance perspective. However, for most other scenarios, ADSS cable has many advantages and is worth investigating.

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