Creating a safe and resilient earthing system for workers

As part of transformer replacement works at the Naseby substation, Transpower partnered with Beca to review its earthing. Designed to stabilise the earthing system and address its performance variability, this mitigation was vital for site safety.

The challenge was to develop a design that would withstand seasonal variations in the moisture content of the earth, which could affect the electrical resistivity of the soil.

The existing substation earthing system was made of a buried horizontal earth mesh in the switchyard and an extensive buried mesh outside the switchyard, extending to the edges of the Transpower property boundary.

Recurring changes in moisture levels and damage caused by tree roots was impacting the performance of the earthing system and had created a potential risk – particularly during the transformer replacement works.

Changes in soil quality over a 10-year period had resulted in a 50% increase in earth grid resistance and multiple ‘step and touch’ voltage hazards, if an earth potential rise (EPR) fault condition occurred.

Our team recommended the installation of six, 20 metre vertical earth electrodes, evenly spaced around the perimeter of the earth grid to mitigate the EPR hazards. This allowed site construction and post-construction works to continue safely, regardless of the upper soil layer’s moisture content and the condition of the buried earth mesh outside the switchyard.

The finished design provides ongoing protection to the site, allowing Transpower people and visiting contractors to undertake their work in a safe and resilient environment.