Situation: There are three operator consoles (positions) in the same control room. There is one general alarm horn that goes off whenever a new alarm comes in from any one of the three consoles. The horn draws the attention of each of the three operators.

Question: How are the ISA-18.2 KPIs for alarm system performance applied when there are multiple operators in a single control room?

Answer: The guidance on average alarm rate per operator console from ISA-18.2-2016, is shown below. 

“Analysis of alarm rate (i.e., annunciated alarm rate) is a good indicator of the overall health of the alarm system. Recommended targets for the average alarm rate per operator console (i.e., the span of control and alarm responsibility of a single operator) based upon one month of data are shown in Table 5. These rates are based upon the ability of an operator and the time necessary to detect an alarm, diagnose the situation, respond with corrective action(s), and monitor the condition to verify the abnormal condition has been corrected.”

“Sustained operation above the maximum manageable guidelines indicates an alarm system that is annunciating more alarms than an operator is likely able to handle, and the likelihood of missing alarms increases, even if the average for that interval is acceptable.”

Understanding Operator Workload

The idea of the alarm KPI targets is to put a cap on the “cognitive load” for how much time the operator spends responding to alarms.  One customer defined their targets as shown below. 

Their goal was to reduce the amount of time the operator spent on reactive, unscheduled activities (typically 75%) and increase the amount of time spent on proactive, scheduled activities (to 50-80%). An effective alarm system helps operators spend more time on proactive activities.

Analyzing Performance – What it Means

Evaluation of alarm performance in a multi-operator control room depends on how much time / effort each operator spends reviewing and processing the alarms that are their responsibility + alarms that are NOT their responsibility as shown in the three cases below:

1. If each operator has the same span of control, they would all receive the same set of alarms. In the worst case they could each the same amount of time detecting-diagnosing-responding-verifying each alarm independently and in parallel. In this case the cognitive load for alarm response for each operator could be estimated as 1X.
2. If each operator has a different span of control and received only alarms from their span of control, then a shared horn would draw their attention for each new alarm and they would need to spend time determining if the alarm was their responsibility.  The cognitive load for alarm response for each operator could be estimated as > 1/3 X and <1X .
3. if each individual operator console received only alarms from their span of control, and the annunciation of other areas did not draw their attention (horn has a different tone), then the cognitive load for each operator could be estimated as1/3 X.

One way to optimize this situation would be to implement different alarm horn tone sets for each operator. This way when an alarm occurs, the operator who “owns” the alarm would know it.  Each tone set would also have a unique tone to indicate the priority of the alarm.  A DeltaV system, for example,  comes with a set of special alarm horn sounds created to support multi-operator control rooms and to comply with human factors principles.

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