Energy Queensland Limited (EQL) has recently rewritten its major substation earthing standard. To support the writing process, a thorough literature review was undertaken of the latest Australian and international practices being adopted by other network operators. This highlighted that despite risk-based earthing being prevalent in standards and industry guidelines for more than a decade, there was still a very wide range of approaches being practiced. This paper will describe the findings of the survey and show that some network operators may be inadvertently tolerating high hazard levels obfuscated by low likelihoods contact scenarios when purely focusing on risk as the design criteria. In addition to the industry survey, the latest legislation and legal approach to hazard management was also reviewed and will be presented.
EQL has taken a subtly different approach to managing substation earthing systems in its new standard. The new standard is hazard focused by setting the Probability of Fibrillation (Pfib) limits for new designs and existing substations under maintenance. This drives designers and maintainers to focus on lowering the hazard, not just accepting a particular risk level based on Pfib and the Probability of Coincidence (Pcoinc). If the Pfib limits have not been achieved by a design or as a result of a maintenance test, a mitigation assessment tool has been created which will then allow the consideration of coincidence and mitigation cost to assess what further mitigations must be applied to reduce the hazard “so far as is reasonably practicable”. The tool also demonstrates which mitigations are considered grossly disproportionate as defined under WH&S legislation. EQL considers this to be a method of encouraging designers to create defendable designs, addressing some of the limitations that may be experience when simply applying ALARP processes.
Several new projects have been assessed against the new Pfib limits with very minimal (if any) increase to the cost of the earthing system. For existing substations under maintenance testing regimes, the Pfib limit has been set based on financial analysis of brownfield earthing system rectifications, acknowledging that addressing a deficiency in an existing substation is far more expensive than addressing a deficiency at design stage.
The new method is expected to be more defendable should a design be subjected legal scrutiny and more thorough from an engineering standards perspective. The new standard is not anticipated to result in a substantial increase in cost to the business from a design or maintenance perspective.