Thirty years of learning about distribution earthing design in Australia

Thirty years ago, Australia entered a period of rapid development of earthing philosophy and approach. This was built on decades of international research on the health effects of electricity on the human body, as well as advanced statistical methods. The result was a gradual shift in national standards away from prescriptive targets and towards risk-based assessments. At the same time an increasing population and higher population density has led to much closer interactions between the public and electrical distribution networks. This hasn’t been an entirely easy change, and there has been a lot of learning from it. Risk based targets result in different earthing requirements for different installation conditions. For large projects such as transmission substations it is not uncommon for significant savings to be achieved because the targets are risk-based rather than prescriptive – these savings can be more than enough to cover the increased design costs, and in the case of increased total cost there is a strong justification for it on the basis of safety. For smaller scale projects however, for example distribution transformers, the additional design requirements and costs can significantly increase overall project costs. In order to limit cost increases there have been many distinct approaches. These have included the development of quasi-risk-based designs with a regional design approach, the development of worksheets and quick calculation devices to give an approximation, and, with increasing frequency, the adoption of optimised design software. This paper considers those developments and the associated implications, using case studies from diverse system and installation configurations to demonstrate pitfalls and opportunities.