17 th European Annual Conference on Human Décision Making and Manual Control

This paper describes a methodology to use Human Error Probabilities (HEP) and design to create robust complex Systems. If a Human Reliability Assessment (HRA) is performed, than this is done mostly after the detailed design phase is finished. The presented method helps to understand the influence of Human Error (HE) in an earlier design phase, e.g. at the functional analysis level. The method consists of several steps. First alternative configurations of functional control groups with different complexities are developed. For each configuration, a fault tree is developed to find the initiating events (fàilures of equipment) which lead to a chosen top event. This top event is an undesired event such as an overflowing tank. The initiating events are used to create event trees (ET) with special emphasis on operator actions, such as monitoring the process and fault diagnosis. A diagnosis diagram simulâtes the fault diagnosis process to identify the initiating failures. The probability of a top event due to human error can then be found, by using HEP-factors and by normalising the failure probabilities of the equipment. The methodology is demonstrated by two examples of functional control groups with two different levels of complexity. The results of the example configurations indicate that: The Basic Human Error Probability (BHEP) of a top event decreases with increasing task complexity (measure for task complexity: maximum number of consécutive alarm points in a configuration. In this paper, the configurations have maximal one to four alarm points). In a very simple system, too few recovery paths exist. This does not imply that the more alarm points the lower the BHEP. The HEP for missing consecutive alarms above 10 are not available. It is plausible that an adverse effect of the number of alarms on the BHEP can be seen for large alarm sequences. This suggests that there exist a minimum BHEP for a certain number of alarm points.