A practical method of common cause failure modelling

The need arose in a large team of safety assessors for a standard method of treating common cause failures to be defined so that a consistent approach could be followed across a range of projects. The requirements of the model were: acceptability, applicability, flexibility and consistency. It was found by combining the s-factor model with limiting values and allowing the value of β to reduce as the protection against CCF increases, it was possible to meet the objectives of the model. An example of the use of the model is included.