The alarms should be processed in such a manner as to avoid operator overload at all times (alarm floods). The presentation of alarms should not exceed that which the operator is capable of acting upon, or alternatively the alarms should be prioritized and presented in such a way that the operator may deal with the most important alarms without distraction of the others.
Applicable alarm processing techniques include grouping and first-up alarms, eclipsing of lower grade alarms (e.g. suppression high alarm when the high-high activates) suppression of out of service plant alarms, suppression of selected alarms during certain operating modes, automatic alarm load shedding and shelving.
The alarm processing should ensure that fleeting or repeating alarms do not result in operator overload even under the most severe conditions. A number of alarm processing techniques (e.g. filtering, deadband, debounce timers, shelving) are available in the Standards.
One study showed that operators having to make more than 14 control movers per hour and answering 20 or more alarms per hour (one alarm every 3 minutes) are excessively loaded.
Care should be taken in the use of shelving or suppression to ensure that controls exist to ensure that alarms are returned to an active state when they are relevant to plant operation.
Alarm System Management Procedures
Management systems should be in place to ensure that the alarm system is operated, maintained and modified in a controlled manner. Alarm response procedures should be available, and alarm parameters should be documented.
The alarm selection process is best handled in a group setting. During the alarm assignment process, the group should review the consequences of missing an alarm before a priority is assigned. This is an important step in that it helps maintain a consistency between consequences and priorities.
The performance of the alarms system should be assessed and monitored to ensure that it is effective during normal and abnormal plant conditions.
The monitoring should include evaluation of the alarm presentation rate, operator acceptance and response times, operator workload, standing alarm count and duration, repeat or nuisance alarms, and operator views of operability of the system. Monitoring may be achieved by regular and systematic auditing.
Matters which are not worthy of operator attention should not be alarmed.
Logging may be a suitable alternative for engineering or discrepancy events to prevent unnecessary standing alarms. A system for assessing the significance of such logged events to ensure timely intervention by maintenance personnel may be required.