Similar to the thought experiment “If a tree falls in a forest and no one is around to hear it, does it make a sound?”, we ask - If an alarm is generated, and the operator fails to acknowledge it, was it really an alarm? A prevalence of unacknowledged alarms is often a symptom associated with poorly performing alarm systems and often seen in post-incident analysis. In many control systems, the need for a notification (such as an alarm) to be acknowledged is a configurable behavior. Let’s review the considerations for alarm acknowledgement.

The operator response to an alarm consists of the three steps shown below, where acknowledgement can be considered part of the “Detect” step. Per ISA-18.2 / IEC 62682 alarm acknowledgement is an “operator action that confirms recognition of an alarm”.  Since recognition / detection is the first step in an operator’s response, it can be assumed that if the notification is important and requires action (which is why it is an alarm), then it should also require acknowledgement. 

Figure. Feedback Model of Operator Response to Alarm 

If a notification is configured as “No Ack required” in the control system, then it is possible that the operator may not notice it. In most control systems generation of a new alarm will cause blinking within the HMI (alarm summary list and process graphic) and cause an audible horn to go off (to help draw the operator’s attention). An alarm that is configured as “No Ack required” will not blink and will not trigger the horn; this increases the likelihood that the operator will not detect it (or not right away). For a true alarm, failure to detect and respond will lead to an undesired consequence; therefore, alarms should require acknowledgement. 

In one popular control system, the default setting of Low priority alarms was changed from “No Ack required” to “Ack required” in a recent release. In another system, alarm acknowledgement is used as a latching function to prevent equipment from being energized until the alarm is acknowledged. Most control systems provide the ability to acknowledge one alarm at a time, groups of alarms, and sometimes an “Ack All” button. If acknowledgement is meant to be a signal that the operator confirms recognition of an alarm, can this recognition step occur effectively when multiple alarms are acknowledged at once? Probably not..

Requiring alarm acknowledgement does more than just draw the operator’s attention. In process plants striving for operational discipline (doing the same thing, the same way, every time) alarm acknowledgement indicates that the person accepts responsibility for the alarm until it is resolved or until it is transitioned to another responsible party. 

This all means that alarm acknowledgement is an integral part of the operator’s response to an alarm.