In exceptional circumstances, new conductors can be installed under live-line conditions. It is not the preferred course of action, but where a critical element of the network needs upgrading and it is difficult to suspend supplies there may be no alternative.
This approach was adopted in Johannesburg, South Africa, by power distribution business City Power. It wanted to increase the power transfer of its 50-year old, 88kV network and ruled out as impractical installing new transmission lines.
Mott MacDonald teamed up with US contractor Quanta Services, a world leading specialist in the field, to develop a solution. We were ideally placed to help because our southern African power business grew out of Merz & McLellan, which had designed the original transmission infrastructure and retained the design drawings for the towers in its archive.
Faraday cages
The project started with a complete condition assessment of the lines, tower insulators and footings, access roads and facilities. This was followed by line design, conductor selection and general engineering studies. Most of the tower and footing re-conditioning took place before the line work commenced.
The project, the first of its type in Africa, involved replacing some 400km of conductor under energised conditions. The main principle of live-line working, says Jason Rowan, manager of Mott MacDonald’s South Africa-based power group, is to protect the engineers from the 88,000 volts passing through the line: “You do that by using what are called ‘live-line buckets’. The operatives are in suits that have stainless steel woven into the material, effectively providing each with their own Faraday cages.”
The bucket boom arm acts as an insulator up to 500kV. Operatives use a wand to bond to the live conductor before attaching a clamp from the bucket to the conductor. They are then at the same potential as the wire and can work on it safely. It is the same principle that prevents birds from frying when they land on power cables.