This paper presents a method to analyse the consequences of human unreliability on system safety. System functions have to be identified and human's role to achieve them is defined in terms of procedures, i.e. lists of tasks to be performed in work contexts during normal and abnormal system functioning. A failed task can be caused by three behavioural dysfunction factors: an acquisition related failure, a problem solving related failure and/or an action related failure. A consequence analysis consists of identifying scenarios of human unreliability. This method is then applied to the rail system.