 a cap. The installation of optical fibres on high voltage power lines. Although cable is sheathed in a track-resistant material to an overall diameter the communication needs of an electricity authority are relatively of only 13mm. In temperate climates, subject to simultaneous ice modest, the high voltage power transmission and distribution net- and wind loading the maximum span between towers is 550m and work potentially provides an alternative communication route for 720m over river crossings. Without ice loading, spans can be signi- use by common carriers. In an earlier development, optical fibres ficantly larger. have been incorporated into overhead line conductors, but applica- The optical loss of the fibres is designed to remain within tion is limited to new or reconstructed routes. 0.5dB/km at 1300nm during all conditions of service. This includes The cable shown in Figure 15.10 (Rowland, 1987) has been temperature excursions from -40°C to +70°C and a 25 year service developed by STC Telecommunications for economic retrospective life. The cable is supplied in 4km lengths and supported on the installation on live power transmission lines operating at voltages bottom crossarms. The installation fittings and procedures are com- up to 150kV. patible with safe working practices on live routes. Copper clad steel kingwire with optical fibres Optical fibres Water blocking compound Copper hydrogen barrier Strength member Strength member Polyethylene insulation Inner serving: polypropylene bedding for armour wires Armour High density polyethylene sheath Figure 15.11 Submarine optical fibre cable 15/16 Telecommunication cables Ongoing research is expected to extend the area of application of to allow surplus fibre for a splice remake and this too must be this self supporting all dielectric communication cable to include stowed within the constraints of the minimum bending radius spe- 400kV power lines. cified. The complete splice housing is finally rendered integral with the cables by polyethylene injection welding