Alarm shelving allows an operator to temporarily suppress an alarm to prevent being distracted from more important alarms . Shelved alarms will reappear after a fixed period of time, ensuring they aren't forgotten and can be addressed after more critical alarms have been handled.
What it means. Alarm shelving should be used with care. Develop procedures to define who has authorization to shelve an alarm, for reviewing the list of shelved alarms and for determining if interim alarms are necessary.
Maintenance. You may have to take an alarm out of service for repair, replacement or testing. This section of the standard provides recommendations for periodic testing and for handling alarms that will be non-operational for an extended duration. For out-of-service alarms it's important to document the approver, the reason the alarm was removed from service and any details concerning interim alarms or special handling procedures needed. The system should provide a list of out-of-service alarms that can be viewed on demand, such as before starting up a piece of equipment.
What it means. Use only approved methods when taking an alarm out of service and provide documentation to remind people to return the alarm to service at a later time. Too many out-of- service alarms can cause the same incidents as too many alarms — because either way the operator fails to take action when needed.
Monitoring & Assessment. You must verify alarm system performance against objectives defined in the alarm philosophy (developed based on ISA-18.2). The number of alarms presented to a single operator is a key performance metric. Studies have shown that an operator receiving on the order of 150 alarms per day (i.e., approximately one every 10 minutes) should be able to respond effectively. In practice, many control rooms run at 10 times this level, forcing operators to deal with one alarm every minute — which makes it difficult for them to respond correctly. (Investigation of several industrial incidents has found operators were confronted during an upset condition with too many alarms to respond effectively.) Other key metrics are the number of stale alarms (those that linger on the console even when nothing is wrong) and how many alarms an operator faces during an alarm flood (Table 2) .
What it means. An unmonitored alarm system is almost always broken. Measuring the performance of your alarm system against established key performance metrics can serve as the first step in improving an existing system and is important in establishing the system is adequately performing. Monitoring and assessment must occur periodically.
Management of Change. Modifications to the alarm system must be reviewed and approved prior to implementation. In this case a "change" could refer to altering an alarm limit, adjusting its priority, adding a new alarm point or implementing an advanced alarming technique. Modifications shouldn't be made without proper analysis and justification. Once the change is approved, update the master alarm database to keep it current.
What it means. Even the most-well-designed alarm system can run into problems if there's no control over who can change it. Put policies and procedures in place to ensure who can change what is well understood and enforced. A good practice is to periodically review the actual running alarm system configuration versus the master alarm database to check that no unauthorized configuration changes have been made.