Home > Tech Talk: ADSS Vs Strand and Lash Fibre Cables in Aerial Electric Utilities

Tech Talk: ADSS Vs Strand and Lash Fibre Cables in Aerial Electric Utilities

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Electric utility companies are increasingly employing their existing infrastructure to build and expand fibre optic networks. These fibre networks can be for internal supervisory control and data acquisition (SCADA) or revenue generating fibre-to-the-x (FTTx) solutions. There are two primary alternatives for aerial fibre cables: all-dielectric self-supporting (ADSS) fibre optic cable, and steel messenger wire supported (strand and lash) loose tube fibre optic cable.

The decision to go with either solution will generally come down to cost and convenience. In scenarios where a strand and lash loose tube cable can be overlashed to an existing and available messenger wire, this can be the most economical option. However ADSS cable has other advantages that can see up to 20% savings in many cases.

Pole Placement Flexibility

ADSS cable is all-dielectric, which means that it does not contain any metallic components. The National Electric Safety Code (NESC), the authoritative body that defines the aerial distribution pole line design and application rules, made significant changes related to the use of ADSS cables in the mid-1990s. At that time, new definitions of fibre optic cables and clearance requirements allowed the use of ADSS cables in the supply space of an aerial distribution pole. NESC section 235 defines that there is no clearance specified between ADSS cables and conductors. Installing an ADSS cable in the supply space offers a higher level of physical protection than a strand and lash cable installed in the communications space of an aerial distribution pole.

‘Make ready’ costs can vary, but are typically much higher for strand and lash applications due to the need for guying existing structures, pole change-outs to maintain ground clearance, moving existing telecommunications cables, among many other potential changes. ‘Make ready’ savings from using ADSS in the supply space can generally be 15-20% of the total construction cost, although in some cases the savings can be even higher.

Installation Comparison

ADSS cables can span pole to pole without the need of a messenger support, while the strand and lash methodology lashes a fibre cable to a messenger wire for support between poles. Thus, ADSS cables are installed in a single pass, cutting the time and associated costs of installation significantly.

Compare the installation steps of each method:

ADSS Cable

  • Pole ‘make ready’ (pulley placement)
  • Cable Installation
  • Cable Sagging
  • Hardware Attachment

Strand and Lash Cable

  • Pole ‘make ready’ (pole change out, guying, moving cables, etc.)
  • Messenger Placement
  • Messenger Pre-tensioning (creep)
  • Messenger Hardware Attachment
  • Cable Placement
  • Lashing Wire Placement and Termination
  • Bonding and Grounding

The need for two-pass installation for strand and lash cables, and additional steps for bonding and grounding of the steel messenger, mean that ADSS cables can be installed in half the time. This is such a significant factor that even when considering higher skilled labour costs for qualified personnel (working in supply space), total installation costs will generally be lower for ADSS.

Total hardware costs are also typically higher for strand and lash networks, due to an increase in the number of parts needed to support the cable. ADSS hardware is essentially just tangents and deadends. Some ADSS tangent hardware can even be used in place of pulleys (sheaves) for installation, further increasing installation speeds and convenience.

Maintenance Comparison

Long term maintenance costs of ADSS cables are minimal. Annual route inspections and intermittent slack storage during the initial installation can further minimise long term maintenance costs from weather damage, road moves, and other factors. ADSS cables are designed for a 25-year life based on environmental conditions such as wind and ice loading, and UV exposure. ADSS cables should be designed for the span length and loading conditions of the application, which allows low strain on the fibre at the maximum operating tension.

A strand and lash loose tube cable relies on a steel messenger for support during its design life, which is typically defined as 20 years. Strand and lash cables generally require more maintenance in their lifetime. Lashing wire needs to be repaired periodically, and bonding and grounds will require repair and upgrading over time. Cable damage requires de-lashing to move spare cable to the damage point.

In colder areas, ADSS has another advantage. With a smaller overall diameter, and a circular cross section, ice load on ADSS cable is less and more uniform, compared to strand and lash cables. The non-uniform ice loading of a strand and lash cable can make the cable more susceptible to wind loading issues and galloping on longer span lengths, especially in heavy ice loading areas.

AFL manufactures both ADSS and loose tube cables for all kinds of environments. AFL can provide ADSS cables that are capable of span lengths over 1800 metres, and engineered to meet the strength requirements, environmental loading (ice/wind), and sag conditions to ensure that the cable does not fail, clash with other cables, or violate clearance requirements.

Acknowledgement: Carson Joye, Application Engineer, AFL, “ADSS Advantages to Strand and Lash Fibre Cables in Aerial Electric Utility Applications”, White Paper, 2018.

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