A review of quantified risk assessment applied to earthing systems of high voltage installations

Safety assessment of earthing systems of high voltage installations by national and international standards are under consideration by international technical working groups. Depending on the selected value of parameters, one standard may be either more or less conservative compared with another. IEC and CENELEC standards for design of earthing systems of ac substations are still based on the concept of satisfying safety under a defined set of worst-case conditions. Yet, in the U.K. and elsewhere, progress has been made to implement forms of risk assessment in earthing applications that provide a measure of individual risk. This paper provides a review of risk assessment processes applied to earthing system safety assessment with particular emphasis on the tolerability of risk framework. Here, a description of risk assessment, both quantitative and qualitative, is provided based on the UK health and safety approach. This framework describes good practice and the determination of control measures that must be put in place for addressing hazards and setting limits for tolerability. Attention is then given to the specific case of risk assessment in high voltage electrical substations and the probabilistic expression of the component parameters of interest, e.g. fault current magnitude and distribution of hazard potential, human being presence, human being susceptibility to electric shock, fault occurrence, etc., and the convolution of the component probability distributions to yield a final measure of individual risk. Through a simple illustrative case study on a generic high voltage electrical installation, the overall individual risk is examined and it is shown how the definition of regions susceptible to electrical shock affect the result considerably.