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Practice·7 min read·

Alarms people act on rather than silence

An alarm system that cries wolf is worse than none, and the failure is almost always in the design rather than the operator.

Short answer

An alarm should require an operator action, and anything that does not should be an event or a log entry instead. The standard failure is alarm flood: hundreds of alarms from one root cause, which trains operators to acknowledge everything without reading. Deadbands, delays and suppression of consequential alarms fix most of it.

An alarm has four states, not twoNormalActiveUnackedActiveAckedClearedUnackedacknowledged and goneA condition that clears itself before anyone looks still needs recording.

The test for an alarm is whether an operator can and should do something about it. Almost every bad alarm system fails that test in the same way.

The three faults

  • Chatter. A value hovering at a threshold with no deadband annunciates every scan. Two percent of hysteresis fixes it.
  • Transients. A pressure dip during a valve change is not a fault. An on-delay of a few seconds removes most of these.
  • Flood. One root cause produces fifty alarms. The operator acknowledges all fifty without reading any, which is the behaviour the design taught them.

Suppression is not hiding

If a pump has tripped, its low-flow alarm is a consequence, not information. Suppressing consequential alarms while their cause is active is the single highest-value change in most systems.

The measure that matters is alarms per operator per hour during an upset. If it exceeds what one person can read, the system is not communicating, it is broadcasting.

Priority means something or nothing

Three levels, used honestly. If everything is high priority then nothing is, and operators learn to treat the colour as decoration.

Common questions

What makes a good alarm?
It requires an operator response, it arrives in time for that response to matter, and it identifies a root cause rather than a consequence. If an operator can do nothing about it, or if it always arrives with fifty others, it is not an alarm; it is noise wearing an alarm's colours.
What is alarm flood and how do you prevent it?
Alarm flood is many alarms arriving from one root cause, faster than an operator can read. Prevent it by suppressing consequential alarms when their cause is already alarming, adding on-delays so transients do not annunciate, and using deadbands so values hovering at a threshold do not chatter.

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