Human Error

So far only the failures of technical components have been considered. The technical systems, however advanced their automation, still rely on human intervention in some respects. A fault tree analysis would be incomplete if this aspect were neglected. In modern process plants direct operator control is unusual. Automatic controllers generally ensure that process parameters are maintained close to nominal levels, except perhaps for start-up and shut-down, when an increase degree of human intervention is normally required.

In fault tree analysis, human error should be introduced as basic events to be quantified. A quantification is at present, only possible for the failure of an operator to carry out a planned intervention, e.g. opening or closing a valve. Meanwhile, an unplanned act (e.g. playing around with buttons or changing positions of valves because of absent-mindedness or with the intention of causing harm) cannot be quantified [157]. The human error quantification still remains less exact than the quantification of the failure of technical components.

Human error is defined as an act outside tolerance limits. It is evident that the permissible interval of tolerance depends on the type of human act in question and on the circumstances under which it is carried out. Hauptmanns [157] distinguished human error as follow:

a. Error or omission: failure to perform a task or part of a task b. Error of commission: performing a task or step incorrectly c. Extraneous act: introducing some task or step which should not have been performed d. Sequential error: performing some task or step out of sequence e. Timing error: failure to perform a task or step within the allowed time oer performing them too early or too late.

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