The definition of an alarm is:
Alarm: An audible and/or visible means of indicating to the operator an equipment malfunction,
process deviation, or abnormal condition requiring a response.1
I have seen other definitions specify that that alarm requires a timely response by the operator. So to summarise in slightly different language, an alarm should say:
SOMETHING IS WRONG AND YOU NEED TO DO SOMETHING ABOUT IT!
Note that I wrote that in capitals.
That is because the control system is effectively shouting at the operator. An alarm generally will make a continuous noise (and possibly flash a light) at the operator until they stop what they are doing and, at minimum, press the silence button (obviously the should then start looking at the alarm summary and work out what to do about the problem).
These are very useful to bring problems to the operators attention. The system is saying "I am no longer in control, please help".
The problem with alarms is when they are not used for this purpose.
Perhaps they are used to inform the operator that a batch process has ended, or that the control system has done its job and started or stopped a pump when a tank reaches a desired level.
These are good things to tell the operator, but they should not be shouted at the operator, forcing them to stop what they are doing. In this case, the system should generate a message or an alert to the operator. Unfortunately these are more modern concepts and old control systems are very easy to generate alarms, but more difficult to generate an alert or message.
- ANSI/ISA ISA18.00.02-2009 “Management of Alarm Systems for the Process Industries”. ↩