Probabilistic safety analysis and human errors

Propabilsitic safety analysis (PSA) has been used as a tool for identifying potential safety threats mostly in nuclear power plant but also in process plant operation. The construction of failure models and the use of failure data have for technical systems provided rather accurate predictions of observed failure rates. Actual accidents and disturbances have however shown that also human errors contribute to the risks of operation. The paper discusses on a general level some of the problems associated with the inclusion of human errors in a safety analysis. The construction of fault sequences involving human errors are not straightforward due to the very large spectrum of human actions possible. The quantification of different possibilities for human errors has also to rely on subjective judgement. In spite of such difficulties it seems however evident that some quantification has to be made in order to priorize between different improvements proposed in the system design, procedures or staff training