Sensornet’s unique fibre optic technology has proved its ability to run accurate health checks on the condition of telecommunications cables integrated into overhead electric power lines. Energis, a major British telecommunications company, called in Sensornet to measure strain on a selection of fibres which form the backbone of its high-speed optical telecommunications network infrastructure.
Originally installed up to 10 years ago, the fibres are either wrapped around existing ground wires on overhead power cables, or use a more recent design in which the optical fibre forms the core of the wire itself. The utility company wanted an accurate assessment of the health of the cables following a number of years¡¦ exposure to wind and weather ¡V and to ascertain whether any of the cables were approaching the industry’s guideline figure of 2,000με (0.2%) as the limit for long-term strain on installed fibre.
Sensornet¡’s Distributed Temperature and Strain Sensor (DTSS) equipment successfully measured the strain on a total of 210km of telecommunications fibre and proved that 94% of all fibre sections measured had an average strain of less than 1,000με, and no section had a strain greater than 1,750με. This gave Energis confirmation of the integrity of its strategic fibre optic network.
A unique feature of Sensornet DTSS equipment is its ability to measure strain and temperature simultaneously and independently without the cross-sensitivity experienced by other instruments. The system measures strain and temperature every metre along the entire length of an optical fibre, rather than at a series of discrete points. The Sensornet DTSS used to perform the health check on these telecommunications cables has been designed to measure the full Brillouin spectrum of light at every metre along the fibre, with real- time analysis enabling both the strain and temperature to be calculated independently at all points.
During the health check, the system provided a 100με strain resolution over a distance of 5km, and produced accurate results within a measurement period of less than 20 minutes. A finer resolution ¡V below 10με¡V is now possible. The system is housed in a rugged field-transportable rack-mountable box with inbuilt PC, which includes a network connection, flip-up monitor, keyboard and an uninterruptible power supply.
Each telecommunications cable tested consisted of 24 single-mode fibres in groups of six, contained within four loose tubes packed with grease to reduce the transfer of strain from the outside of the cable. The four tubes spiral around each other, surrounded by more grease as well as a strong outer jacket. During the health check, Sensornet surveyed a sample of fibres at various points in the telecoms network and confirmed that none were experiencing excessive strain.
The first part of each test consisted of a series of measurements taken by the high-resolution optical time domain reflectometer (OTDR) built into the Sensornet DTSS system. This provided a trace of the reflected and backscattered light along the length of the fibre, and was followed by the measurement of the Brillouin spectrum ¡V the most important part of the test.
A detailed analysis of data from the 210km of optical fibre measured by the Sensornet DTSS showed that levels of strain in all the fibre tested were well below the industry¡’s guideline figure of 2,000με for fibre intended for a 40-year installed life. The Sensornet health check reassured the utility company over the condition of fibre within its core telecoms network ¡V and provided a testament to the excellent installation of the network ten years earlier. Energis is delighted with the results from the first 210km of measurements and has recently commissioned Sensornet to determine the health along other sections of its network.
Advances in Sensornet’s technology mean that this impressive performance is about to get even better. A new Brillouin measurement technique under development at Sensornet will offer improved acquisition speeds at finer resolutions ¡V a real-time, once per second measurement and resolution of 10με. Sensornet’s unique distributed temperature and strain sensing technology is being actively applied in many other industries, including oil and gas ¡V both on and offshore ¡V as well as in the civil engineering, tunnelling and aerospace sectors.