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

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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