Bam-nuttall undertook the bridge deck replacement works for Arun River Bridge, which is a four span wrought iron girder bridge on the Arun Valley constructed in circa 1861, work comprised of removing the existing timber cross girders, longitudinal timber rail bearers, timber decking and metallic hand rail and replacing all of the existing timber rail bearers and deck with new prefabricated steel rail bearers and steel cross girders. With this proposal the bridge had a low impedance to earth, with minimal insulation between the continuous running rails and the metallic bridge.

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This caused the following concerns –

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• Stray Current – GL/RT/1254 dictates that the Infrastructure Controller has a responsibility to minimise, as low as is reasonably practicable, the export of stray current through the design of the electrification system design and specifically through the traction return system

• Touch Potential – There is a touch potential problem when a maintainer work on the running rail which will be on traction negative potential, because of the steel bridge construction;

• Fault Disconnection – DC Power supplies are unearthed, the probability of an undetected earth fault is very high as the conductor rail is supported on a steel girder.

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A proposed primary and secondary insulation method was developed in conjunction with Bam-nuttall and Pandrol to maintain a safe level of insulation to prevent stray DC corrosion. The primary method consisted of the use of the Pandrol VIPA base plates as they offered higher insulation between the running rails and the running rail supports.   The secondary method consisted of a HDPE/ GRP continuous sheet along the length of the longitudinal steel rail bearers at a width of 550mm, which provided an overlap of 10mm either side of the longitudinal steel rail bearers.  GRP handrails and GRP walkways were proposed as part of the designs to avoid touch potential.