EoW May 2013

News Technology

Tensile testing of overhead optical fibre cables

The LR30KPlus applies a tension load to the end pulley on its rails such that the load is transmitted evenly along the entire length under test and measures the elongation of the cable. Some testing involves applying load at pre-determined strain rates and holding the load at an elevated level as the cable stretches. IEC-60794-1-2 Method E1 sets out the pass/fail criteria for optical fibre tested in this way. • Under load, the fibre attenuation is not increased more than a predetermined value, typically 0.05dB over the fibre length measured • Under load, the fibre does not elongate by more than a pre-determined value over its initial length. The allowed elongation under installation load is typically 0.25% • A complete materials testing solution The tensile testing of optical fibre described here provides a perfect example of the versatility offered by the Lloyd Instruments range of single or twin column universal materials testing machines. The instrument range offers versions allowing precisely controlled tensile or compression forces to be applied to a maximum values ranging from 1kN (225lbf) to 150 kN (33,722lbf), meaning that the most appropriate testing machine can be chosen for the required application. Universal testing machines are so called because they can be adapted to make an extraordinary range of measurements by using a wide variety of grips and fixtures that accommodate a huge range of sample types, shapes and sizes so that the forces can be applied in different ways. NexygenPlus materials testing software ensures fast, reliable and powerful testing and data analysis. As described earlier, testing of large products places special demands on the instrument. In addition to the ‘pogo’ version described, extended column versions of the instruments are also available which increase the crosshead travel range to allow the testing of high elongation, ductile materials or longer than normal test specimens. For extra large or awkwardly shaped samples which do not fit within the standard working area of a test machine, purpose built compression cages can be supplied for most instruments in the range. The test machines are mounted on top of the compression cage, whose dimensions and size depends on the individual application and required specification. These are:

Overhead cable systems have become a key factor in telecommunications networks used by operators and power utilities. They are popular since no civil works are required to implement them and the rights of way have already been established, so it is possible to minimise costs and, importantly, the time required to get the network up and running. Optical cables are available in a variety of configurations: composite optical ground wire systems (OPGW) for installation on high voltage electric lines; self-supporting optical cable systems (ADSS) for all types of lines including electric lines, overhead distribution lines for railways, etc; optical cable systems lashed to the ground wire or the phase cable on the electric lines and composite optical phase cable systems (OPPC), for high voltage electric lines. Overhead cables are subject to a wide range of environmental conditions and factors such as wind, temperature and ice can result in elongation and/or compression of the cable which can lead to increased signal attenuation or even breakage. It is therefore essential that mechanical properties including tensile strength of the cables are measured as part of the manufacturing quality assurance procedure to allow the publication of accurate specifications for mechanical and environmental properties as well as optical performance. Tensile testing of optical cable is one of the more difficult mechanical tests to undertake due to the long cable length and potentially high forces required. However the International Electrotechnical Commission has published an international standard (IEC-60794-1-2 Method E1) to specify the requirements for this testing. This test simulates conditions that could arise during and after installation, and simultaneous measurements of attenuation and elongation strain are recorded versus tensile force. It is a non-destructive test where the tension applied is within the operational values for the cable. The image above shows an experimental arrangement for the tensile testing of optical fibre using a special ‘pogo-style’ configuration of the LR30KPlus twin column materials testing machine from Lloyd Instruments. The LR30KPlus is suitable for testing applications up to 30 kN (6744 lbf). The crosshead can be driven over a wide speed range of 0.001-508mm/min (0.00004 to 20"/min) and the instrument uses high accuracy interchangeable XLC Series load cells for tension, compression and cycling through zero force measurements. optical fibre

▲ ▲ Tensile testing assembly for optical fibres

It has a high stiffness frame which incorporates a crosshead guidance system to prevent side loading of the test sample. Normally, tensile tests are carried out on a sample mounted in a suitable test jig which has one part affixed to the load cell and the other attached securely to the base of the instrument. This is not practical for optical fibre testing due to the long lengths of fibre that must be tested, so in order to couple the LR30KPlus to the horizontal optical fibre pulley arrangement, a ‘pogo‘ arrangement of the LR30KPlus is used. The tester is mounted horizontally to a rigid frame with a rod fitted to the bottom of the load cell and this runs through a linear bearing in the lower crosshead and connects to the frame containing the end pulley which is mounted on rails. This allows a uniform tensile force to be applied to cable under test via the pulley. The tensile test is set up as a completely automated measurement. The cable is paid out around the pulleys so that a pre-determined length (between 50 and 80m) is available. The ends of the cable are firmly secured so that they are fixed when tension is applied. Individual fibres in the cable under test are attached to an instrument to measure signal attenuation during the test, typically an optical time-domain reflectometer.

Lloyd Instruments Ltd – UK Website : www.lloyd-instruments.com

▲ ▲ ‘Pogo’ arrangement for LR30KPlus twin column materials testing instrument

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May 2013