That is the question.
By ISA-18.2 definition “an alarm is an audible and /or visible means of indicating to the operator an equipment malfunction process deviation, or abnormal condition requiring a response.” This means that alarms should only be used to indicate when something is wrong (not an expected event) and that if a (timely) operator response is not required (other than acknowledging it), then the point should not be an alarm.
How about “alarms” that don’t meet the definition for being an alarm? These are called “non-alarms.” The modern DCS makes it extremely easy to add alarms without significant effort or cost, so many systems are full of non-alarms. Presenting non-alarms to the operator, along with legitimate alarms leads to alarm overload, alarm floods, and increased operator stress. The alarm system must be rationalized in order to weed out the non-alarms from valid alarms.
Can non-alarm still deliver valuable information? Yes. So how can this information be delivered to the operator without compromising the performance of the alarm system? Enter the alert: “An audible and/or visible means of indicating to the operator an equipment or process condition that requires awareness, that is indicated separately from alarm indications, and which does not meet the criteria for an alarm.” An example of an alert may be a system diagnostic alarm that the operator should be aware of, but requires a response from maintenance.
Alerts and other advanced alarming concepts are described in the Technical Report ISA-TR18.2.4-2012, “Enhanced and Advanced Alarm Methods.” This TR, which is packed full of useful information, is in the final stages of editing prior to release.