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Physical fatigue and exhaustion: nearly everyone has experienced them at one time or another. Too much physical exertion without an acceptable rest period will cause a person's body to become incapable of performing further activity. The topic of physical exertion has been well studied and documented over the years. There are even OSHA guidelines on the limits of dynamic work and recommended rest periods for physical work. However, what about psychological fatigue and exhaustion: shouldn't there be guidelines for psychological work similar to those for physical work? Examples of psychological fatigue are nearly as common as those of physical fatigue, however, little is known about psychological fatigue. Having a process control operator with too much psychological fatigue can be catastrophic.

Control board operator loading is a fine balance between two extremes. On one hand, it is important that control operators have enough recovery time between their activities to avoid becoming mentally fatigued and exhausted. On the other hand, without enough stimulation, board operators can become bored and attain a low arousal level to the point that their performance is seriously degraded. What is needed is a methodology by which the psychological rest and recovery period for control board operators can be measured. Acceptable ranges of psychological rest and recovery time for control board operators also need to be defined.

Beville Engineering has been studying control operator workload for over ten years and has developed a methodology that is useful in gauging rest and recovery time for control board operators. The method is based upon measuring the average, or mean time, between job related tasks over a defined time period. Beville refers to the method as Mean Time Between Tasks, or MTBT. Mean Time Between Tasks is found by observing operator activities over a specified time period, deriving the amount of time the operators were not engaged in observable job related activities (referred to by Beville as Indirect time), and averaging the number of tasks completed per hour. For a control board operator, tasks are events such as making control moves, answering alarms, communicating to others, filling out paperwork, etc. Indirect time is then divided by the number of tasks to find the Mean Time Between Tasks.

MTBT is a valuable metric because it is composed of two different dimensions that contribute to loading, namely, the number of different tasks performed per hour and amount of time spent completing job related tasks. The coupling of the two dimensions produces a more accurate workload estimator than the two individually.

MTBT can be plotted to to create a simple yet powerful representation of operator loading. The following figure illustrates a MTBT comparison:

In the figure, the lower left hand corner indicates a longer MTBT (indicating a job that contains longer stretches between activities) while the upper right hand corner of the chart indicates less MTBT (indicating a job with shorter breaks between activities and a busier job). For control board operators, Beville has found an industry average MTBT to be 1.1 for steady state operations. Beville has found that a good range of loading for control board operators is between 1.0 and 0.5. Control board jobs with MTBT greater than 1.0 minutes between tasks are considered to be slow positions with long stretches between activities. A danger of positions that fall into this category is that operators have to sustain boredom for long stretches, which can cause arousal levels to drop and error rates to increase. These positions are generally viewed as being underloaded. At the other end of the spectrum, positions that have MTBT's of less than 0.5 minutes indicate positions where possible overloading is occurring. These positions are characterized by having high volumes of communications, high alarm actuation rates, and high numbers of control moves per hour. Positions in this region run the risk of fatiguing the operator with the high activity level without sufficient rest and recovery periods.

The above figure represents three positions with two samples per position, one AM and one PM sample and a fourth position that also included a resample. Position B is in the preferred region of loading, between 0.5 and 1.0 minutes between tasks. Position C indicates a job that is underloaded and at risk of making the operator sustain long stretches of boredom. Position A is a position that is overloaded in the AM and underloaded in the PM. This position warrants further analysis to determine the cause of the overloading and create a better balancing of the workload. In position D, three samples were taken, two that had normal activity and one in which a process upset occurred. All three position D samples are characterized by an unusually high number of tasks per hour when compared to other samples and the database average. In the upset sample, the unusually high task per hour number is even more pronounced. Often, high task numbers are indicative of control system problems where the operator is making a large number of control changes and/or answering a deluge of alarms. Further analysis is needed to pinpoint the exact cause of the high number of tasks.

To date, Beville's MTBT database contains over 150 control board operator jobs across a wide variety of process units. The samples are primarily oil refinery operators and a few chemical plant operators. Beville has also compiled MTBT data for other positions including outside operators, hourly foremen, and salaried supervisors.

MTBT has become a powerful tool in defining the many facets of control board operator loading, including psychological fatigue. MTBT has helped many managers understand how their control board positions compare to others in the industry.

Copyright © 1996, Revised 1998, Beville Engineering, Inc. , All Rights Reserved


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How to Build a Better Operator - ABB Automation & Power World Control


This year's Fall meeting for the Center for Operator Performance will be October 7-10 in Houston, TX. For more information, please contact Lisa Via. Guests are always welcome!


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David Strobhar's book, "Human Factors in Process Plant Operation," is now available in both hardcover and Kindle e-book.

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