Electrical cable for a cantilever bridge

Olex cable was installed as part of an upgrade to ensure the continuing operation of the four-lane Spit Bridge at Seaforth, NSW.

The cantilever bridge, which regularly opens to permit marine traffic to pass under, is part of the busy Manly to Sydney arterial road and caters to 70,000 vehicles daily. The bridge is scheduled to be widened to six lanes in the future and the increase in cantilever bridge weight will therefore demand a corresponding increase in electrical power for the bridge-lifting drive mechanism.

The cable installation, carried out by electrical field staff from EnergyAustralia's Chatswood Depot, posed a unique challenge due to the very steep and at times vertical gradients encountered in the 150 m cable run.

Olex's technical department played a pivotal role in determining the calculations for the installation. This enabled preventative action to be taken to ensure that the bunched 4-core 150 mm² copper low-voltage cable did not get away and was not damaged by self weight or the pulling and breaking forces experienced during installation as it was being lowered down a vacant 100 mm diameter, 10-year-old buried conduit.

For the installation, four Olex cable drums were set up on stands and a pulling sock was attached to the four cables along with a nylon support (lowering) rope. The assembled cable, restrained by the nylon rope, was then hand fed with the aid of a winch and was gently guided into the conduit.

At the bottom of the hill, a team in radio contact with out-of-sight operations at the top hand-pulled and guided the cable along its path.

As a result of Olex's accurate calculations prior to the installation, the cable did not at any stage break away or need any excessive pulling force during its journey from top to the bottom of the incline.

EnergyAustralia project engineer Darko Grcev was pleased with the assistance Olex provided for this installation and said: "The team of technical experts from Olex were able to provide accurate calculations to assist EnergyAustralia make a decision and achieve smooth installation without damage to the cable in this atypical project."

The installed cables were attached by the nylon rope and, as an additional safety precaution, stainless steel pulling socks were attached to a large concrete block buried a couple of metres from the cable entry point.