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When performing work on transmission towers with de-energised circuits adjacent to live circuits, consideration and control of induced voltages are critical to ensuring the safety of workers.
Tragically, in the early 1990s, a Tasmanian transmission worker was fatally injured by an electric shock incurred while opening loops adjacent to a live 220kV circuit. The investigation found that voltage induced in the de-energised conductor had not been dissipated to a safe level by the earthing equipment used for the work. The transmission tower where the incident occurred was constructed from Weather Resistant Steel (WRS), a unique type of steel which naturally forms a thin oxide layer on the surface of the steel. Towers constructed from WRS required specific processes for earthing that differed from the common work practices used by the linesman.
After tower commissioning in the 1970s, it was identified that the oxide layer formed on WRS towers was a poor conductor of electricity. As such a series of dedicated earthing members were installed throughout the structure which were intended to be the ‘earth points’ for portable earthing equipment used during transmission line work. Furthermore, equipotential bonds were fitted to structural members at major joints to create continuity between all tower sections. In doing so, the structure and any conductors being worked on will be at the earth potential provided by the tower legs. In the case of the incident, the dedicated earthing members were not utilised.
Recently, TasNetworks engineering teams responded to safety concerns raised by field workers about the deteriorated condition of the equipotential bonds in these WRS towers. The design of the bonds, attachment method to the weather-resistant steel and age of the towers had resulted in a situation where it was unknown if the bonds were in adequate condition to perform their intended function.
This case study presents the risk analysis and operational controls that have been implemented to assess and manage the concern of potential differences generated across structural members, to prevent a repeat of a past significant event. Work completed includes induction modelling, tower resistance testing, asset management strategy review, and review of bond necessity and type.