How Many Alarms Can an Operator Handle?

Two recent studies provide insights

By David A. Strobhar, Beville Engineering, and Craig M. Harvey, Louisiana State University

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It may seem obvious that fewer alarms presented to an operator will result in quicker resolution of abnormal situations. However, you must consider the operator's whole workload in any assessment. After all, operators do more than just respond to alarms. Any published targets for alarm rates should specify what's assumed to be the workload for the other tasks; alarm rates in isolation have limited meaning.

However, setting the other tasks (monitoring, control, administrative, communication, etc.) to zero establishes the upper limit of console workload based upon alarm response only. This research is doing that. So for an upset, when dealing with alarms is likely the major component of workload, 30 alarms per ten minutes will result in low priority alarms backing up. Of course, making simple changes to the way alarms are displayed also impacts the operator's reaction time. Even within the alarm workload variable, you can make improvements that reduce overall operator workload.

The second study failed to yield the degree of improvement seen in the first one when the display grouped alarms by priority (category). While 80% of the professional operators and controllers preferred such grouping, the performance benefit from it wasn't as great as for the students. Perhaps this is because they are professionals. However, the two studies had different alarm priority distributions — low, medium and high evenly distributed in the first and the ISA/EEMUA distribution in the second. The professional operators faced fewer of the highest priority alarms at an alarm rate of 30 alarms in ten minutes than students saw at an alarm rate of ten alarms in ten minutes. So the effect of the categorical alarm display may not appear unless there are high rates of high priority alarms.

Something extra came from the second study by having the professionals repeat a part of the first study — handling ten and 20 alarms in ten minutes with the same simulation used by the students. The two groups performed about the same at the ten-alarms-per-ten-minutes rate. However, the professionals were about twice as fast as the students for the higher rate condition.

So what is the value of a trained operator? When it comes to alarms, an experienced person can perform twice as fast as a novice during upset conditions. However, that value/ability won't come out when things are calm — the veterans and rookies will look about equal.

This research raises two questions:

1. Are the ISA and EEMUA targets correct? From the research conducted by COP, you can view the ISA and EEMUA numbers as conservative. If a plant achieves them, operators should be able to manage the alarm workload.

2. Could an operator handle more alarms? The COP studies indicate that experienced operators probably could deal with an increased number of alarms; however, the studies don't consider all factors that affect an operator's workload. The research also seems to show there's a breakpoint where operators can't successfully handle the alarm workload safely. The actual breakpoint likely depends upon several variables, including: the rate of alarms, how alarms are displayed to operators, and additional operator workload.

COP's initial research begins to provide a scientific basis for establishing realistic alarm rates for operators. The center also will investigate other variables that can affect how operators handle alarms. Next on the agenda is to explore how alarm presentation impacts performance.

Stay tuned!

DAVID A. STROBHAR, PE, is principal human factors engineer for Beville Engineering, Inc., Dayton, Ohio, and is heavily involved in the activities of the
Center for Operator Performance. CRAIG M. HARVEY, Ph.D., PE, is an associate professor and interim chair of the construction management and industrial engineering dept. of Louisiana State University, Baton Rouge, La. E-mail them at and

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