Advanced alarm systems: Display and processing issues Page: 4 of 9
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presented, which is alarm display). Three techniques have
been used (note that the definitions of these terms are the
author's; the terms "filtering" and "suppression" are often
used interchangeably: filtering (alarms determined by
processing techniques to be less important, irrelevant, or
otherwise unnecessary are eliminated and are not available
to the operators); suppression (alarms determined by
processing techniques to be less important, irrelevant, or
otherwise unnecessary are suppressed and not presented to
the operators, but suppressed alarms can be accessed by
operators upon request or by the alarm system based upon
changing plant conditions); and prioritization (all alarms
are presented to operators based on prioritization schemes).
There are clear tradeoffs between these approaches;
thus an issue remains about what contexts the various
options should be exercised. Filtering eliminates the
possibility that unimportant alarms will distract the
operators. However, the designer may be removing
information used for other purposes. In addition, the
designer must be certain that the processing method is
adequately validated and will function appropriately in all
plant conditions. Suppression also removes potential
distracting alarms. However, since they are accessible on
auxiliary displays, additional workload may be imposed by
requiring operator action to retrieve them. Prioritization,
on the other hand, does not conceal any information from
operators. However, the operator is required to
perceptually "filter" alarms, e.g., to scan for red, high-
priority alarms from the other alarms. Thus, there is a
potential of distraction due to the presence of less important
B. Related Research
Several studies have examined the effects of alarm
processing techniques on operator performance. The
HALO (Handling Alarms with Logic) alarm system was
developed by the Halden Reactor Project. In an initial
study, inexperienced students were trained with the system
and were asked to identify disturbances in a simulated
pressurized water reactor.' Alarm information was
presented as (1) unfiltered message lists, (2) filtered
message lists, or (3) filtered message lists with an overview
display. Alarm information was presented in static displays
rather than dynamic simulation. Diagnosis time and
accuracy were the primary dependent variables. The
results indicated that accuracy was improved with filtering,
but the benefit was specific with respect to the plant
transient. No significant difference was found for operator
response times. Also no differences were observed
between the filtered message list used alone and the filtered
list used with the overview display. Comparisons of
performance with and without filtering during simulated
transients were made in subsequent studies." The filtering
system reduced the alarms by approximately 50 percent and
the filtered alarms were not available to the operator. The
performance measures were detection time and percentage,
diagnosis time and percentage, percentage of checks, and
percentage action. Process variables and subjective
evaluations were also measured. Seven crews of two
operators each used the three systems is 12 simulated
scenarios. Filtering of alarms had little effect on observed
performance. It was observed that the detection of events
decreased from 81 percent to 51 percent when the event
occurred late in a scenario rather than early in a scenario.
None of the systems tested helped to mitigate the problem.
One potential problem with interpreting the results of this
study is that the display type and use of alarm filtering
were experimentally confounded. Thus, no conclusions
with respect to the independent effects of display mode or
filtering can be made.
Although the operators expressed support for the alarm
filtering system in another study using a verbal protocol
analysis taken in real time from three operators during
simulated malfunctions, no evidence was found that it had
a positive effect on their performance.'
In a test of the Dynamic Priorities Alarm System
(DPAS), the number of high-priority alarms was reduced
through mode, multi-setpoint, and cause-consequence alarm
processing."" Alarms were displayed on a combination of
tiles and VDUs. The color was used to support operators
in distinguishing between status and alarm information.
Performance with and without the new system was
compared. Nine crews of three experienced operators used
the systems during simulated scenarios involving single and
multiple failure events. Operator performance measures
included time to identify initiating event, time to identify
second malfunction, time to take control action, and alarm
utilization frequency. No difference between the two
systems was found for initiating event identification;
however, detection time for second malfunctions was
significantly reduced in three of the four scenarios when
the alarm handling system was available. DPAS
significantly reduced the time required to take a control
action in two of the four test scenarios. The finding that
second malfunction detection time was reduced with the
alarm system is not consistent with the findings from the
HALO research reported earlier where secondary event
detection was not enhanced.
The Electric Power Research Institute (EPRI)
conducted a study comparing tile and VDU-based alarm
presentations.12 One of the experimental conditions included
a VDU presentation of alarms where the typical alarms
associated with reactor and turbine trip were suppressed.
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O`Hara, J.M.; Wachtel, J. & Perensky, J. Advanced alarm systems: Display and processing issues, article, May 1, 1995; Upton, New York. (digital.library.unt.edu/ark:/67531/metadc787661/m1/4/: accessed October 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.