Quantification of Man-Machine System Reliability in Process Control

Application of reliability engineering to process plant design, operation and control has led to a demand for estimates of `human reliability,´ or `human error.´ However, it is desirable to treat the concept of `human error´ with caution and to avoid an approach in which the operator appears to be held solely responsible or even blamed; in reality the error arises out of a quite specific combination of conditions in the man-machine system and it is on the total system that attention should be centered. Problems such as performance of discrete tasks and vigilance tasks or behavior in emergencies have been widely studied. While some of this work appears applicable to process control, there remain significant gaps. A more comprehensive taxonomy of process operator error seems to be needed. Systems exist for reporting incidents in process plants, each with a particular objective such as improvement of personal safety, estimation of insurance rates, or reduction of maintenance costs. Data produced are specialized and often useless for other purposes. In particular, it is difficult to abstract data on malfunction involving the operator. A taxonomy suitable for process control is suggested and a scheme for collection of data on reliability in process plants is outlined. Attention is drawn to the relevance of the process and control system characteristics and to their variability. The importance of the process computer is emphasized, both in its impact on the control system and as a research tool.