Optical attached cable

Wrapped cable systems were developed independently in the UK (SkyWrap) and Japan (GWWOP) during the 1980s and have been widely used,[1][2][3] with installations in every continent except Antarctica.

Through licensing and through independent development, wrapped cable systems have also been supplied by French, Italian, German and Russian companies.

However, a radio controlled power unit using batteries or a petrol engine is normally required when the host conductor is on a high voltage transmission line.

In the UK, Raychem Ltd had a background in polymeric materials with resistance to high voltage environments; used for example in heat-shrinkable 33kV cable terminations and in polymer insulators.

A subsidiary of Cookson Group was FOCAS Limited who completed the development work and commercialised the product as SkyWrap, supplying more than 16,000 km to customers in over 30 countries.

FOCAS was acquired by AFL in 2000[8] In Japan, Furukawa Electric Company developed a product explicitly for installation on the earth-wire of transmission lines, and this was reflected in the name: Ground-Wire Wrapped Optical cable.

The first installation was carried out in 1985 on a 275kV transmission line owned by Chubu Electric Power and included a remote controlled pulling device and a self-compensating counterbalance on the wrapping machine.

BICC also had a technology licence from Furukawa, but carried out extensive development work to make the product suitable for the European market: BICC re-designed the cable to use loose-tube technology and so provide a zero-strain environment for the optical fibres at all operating conditions of the overhead line; the tug and wrapping machine were re-designed to reduce the overall load on the conductor during the installation process.

A second Russian company, Scientific Innovations, introduced a more conventional design of wrapping machine in the mid-2000s, with a single drum of fibre-optic cable and a counterbalance arm.

Tight-buffer cable designs do not provide sufficient strain margin and the optical fibre transmission performance is compromised under strong winds, heavy ice accretions and at high temperatures.

This uneven distribution of excess fibre length compromises the optical performance of the cable with increased optical attenuation at low temperatures in the relatively crowded sections of tube in the middle of spans and at high temperatures where the fibres are under strain at the high sections close to the towers.

Conductors on overhead lines are subject to two different sources of severe temperature excursions: lightning strike and fault current.

The amount of energy converted to heat in a lightning strike can be sufficient to melt several strands in a multi-stranded conductor.

In order to cope with these temperatures, the sheath of a wrapped fibre-optic cable must be made from a high-temperature material or it must be crosslinked to prevent melting.

Suppliers will carry testing of their cable to demonstrate that it can survive a number of lightning strikes or fault current episodes.

Counterweights are required for two separate purposes: to counter the turning moment of the wrapping machine and to balance the cable drum payload.

This motion is achieved by means of a gearbox that converts the linear travel of the wrapping machine along the conductor into a rotating movement of the drum carrier.

The more sophisticated designs of wrapping machines have automatically adjusting counterweights that maintain balance throughout the span as the cable drum becomes progressively lighter.

Ideally, the centre of gravity of the rotating part of the wrapping machine should lie on the axis of the host conductor at all times.

[15] The AFL (UK/USA) machine can be configured to wrap 2 cables simultaneously from a pair of drums, in order to achieve double the normal fibre-count.

There are two variants of wrapped optical fibre cable systems for installation on overhead electricity power lines: they differ in the nature of the host conductor.

This means that the fibre-optic cable and joint closures on the grounded side of the PTG can be accessed and worked on safely, even if the overhead line is energised to full system voltage.

This is an attractive concept for many power utilities because it means that the communications network is under their own control and can be tailored to meet their particular requirements with suitable attributes such as redundancy, latency and bandwidth.

Once built, the network is relatively inexpensive to operate compared to rental charges previously paid to phone companies.