Case study: Optical fibre cable test solution

When a leading manufacturer of overhead optical fibre cables needed to conduct tensile testing as part of its quality control procedures, it was after a specific solution.

The company sought to measure the attenuation of signal in the cables as a function of tensile loading during manufacturing. Since overhead cables are subjected to a range of environmental influences including wind, temperature and ice, which can cause elongation and/or compression of the cable resulting in increased signal attenuation or even breakage, accurate testing is needed to ensure continued operational efficiency.

The Lloyd Instruments LR30KPlus test machine, supplied by Bestech Australia, was considered the best solution.

Challenge

Tensile testing of optical cable is one of the more difficult mechanical tests to undertake due to the long cable lengths and potentially high forces required. Traditional tensile testing is conducted on a comparatively small sample of the bulk material; however, for optical cable the International Electrotechnical Commission has published the IEC-60794-1-2 Method E1 standard, which specifies that tens of metres of the cable must be tested. A tensile load must be accurately applied and the elongation measured while simultaneously gauging signal attenuation in the cable.

Process

A pulley system was built to allow lengths of cable between 50 and 80 m to be tested and a specially configured LR30KPlus twin-column material testing machine was used for the elongation measurements. The LR30KPlus, suitable for testing applications up to 30 kN (6744 lbf), was mounted horizontally onto a rigid frame in a ‘pogo’ configuration. A rod was fitted to the bottom of the load cell running through a linear bearing in the lower crosshead. This connected to the frame containing the end pulley, which was mounted on rails to enable the movement required, allowing a uniform tensile force to be applied to the cable under test via the pulley.

The testing procedure

• The cable is played out around the pulleys so that a predetermined length (between 50 and 80 m) is available for test.
• 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 a ‘fibre analysis’ machine to measure the individual fibre signal transmission ability under tension load.
• A tension load is applied to the end pulley on rails so that load is transmitted evenly along the entire length under test.

Some testing involves applying a load at predetermined strain rates and holding the load at an elevated level as the cable stretches. However, the test is non-destructive since the tension applied is within the operational values for the cable. The tests are also fully automated and have a pass/fail criteria based on a maximum acceptable increase in signal attenuation (typically 0.05 dB) and a maximum elongation under installation load of around 0.25% over its initial length.

The benefits for the manufacturer included:

• Non-destructive testing
• Accurate measurements over tens of metres of cable
• Simultaneous elongation and signal attenuation measurement
• Conforming to IEC-60794-1-2 Method E1.