- An alarm requires an operator response. If there is no response, it is not an alarm.
- EEMUA 191 puts the long-term average at roughly one alarm per operator per ten minutes.
- Rationalisation is a documented decision per alarm, captured in a master alarm database.
Alarm floods are not an operator problem. They are a design debt that comes due at the worst possible moment. The Texaco Milford Haven explosion in 1994 is the canonical case: in the last eleven minutes before the blast, two operators had to act on 275 alarms, the overwhelming majority of which were not relevant to the developing upset.[1]
The definition does the work
ANSI/ISA-18.2, adopted internationally as IEC 62682, defines an alarm as an audible or visible means of indicating an equipment malfunction, process deviation or abnormal condition requiring a timely response.[2][3] Every word is load-bearing. If there is no defined operator response, and no time in which the response would help, the item is information — and belongs on a display, not in the alarm list.
Performance targets you can measure against
| Metric | Guideline | Meaning |
|---|---|---|
| Average alarm rate | ~6 per hour (1 per 10 min) | Manageable long-term load[4] |
| Peak burst | ≤10 in any 10-minute window | Above this, response quality degrades[4] |
| Standing alarms | Under ~10 at any time | Chronic standing alarms train operators to ignore the list[2] |
| Priority distribution | ≈80% low, 15% medium, 5% high | Priority only means something if it is scarce[4] |
| Stale alarms | None older than 24 h | A stale alarm is an unclosed decision[2] |
The rationalisation session
Per alarm, in a room with an operator present
What physical condition produces this alarm? If the answer is 'a threshold', keep digging.
In most plants a handful of tags produce the majority of activations. Chattering and fleeting alarms are typically fixed with deadband, on-delay or a repair — not with a priority change. ASM Consortium practice puts bad-actor resolution ahead of full rationalisation because it buys the operator immediate relief.[5]
Suppression is legitimate when it is designed: state-based suppression, shelving with an automatic un-shelve, and mode-dependent alarms are all sanctioned by ISA-18.2 provided each is documented and auditable.[2] Suppression that happens because someone silenced an annunciator is not. Presentation matters too — colour, layout and consistency are governed by ISA-101, and an alarm that cannot be distinguished from decoration on the HMI has already failed.[6]
Rationalisation readiness
Common questions
What qualifies as an alarm?
An alarm requires an operator response. If there is no response the operator can take, it is information or a log entry — annunciating it as an alarm only spends the operator's attention on something they cannot act on.
What is an acceptable alarm rate?
EEMUA 191 puts the long-term average at roughly one alarm per operator per ten minutes, with flood conditions handled separately. Rates well above that are a design finding, not an operations one.
What is a master alarm database?
The record of the documented decision behind each retained alarm: its cause, the consequence of inaction, the operator response, the time available to respond, and the setpoint that follows from those.
How do you start reducing nuisance alarms?
Measure first — rate, top contributors, chattering and stale alarms — then rationalise the worst offenders against the response test. Suppression applied before measurement hides the evidence you need.
Sources for every claim above
Each footnote in the article links here. Standards are cited by designation so you can verify the current edition with the issuing body.
- [1]The explosion and fires at the Texaco Refinery, Milford Haven, 24 July 1994 — investigation reportHealth and Safety Executive · HSE Books, United Kingdom · 1997View source
- [2]ANSI/ISA-18.2-2016 — Management of Alarm Systems for the Process IndustriesISA · International Society of Automation · 2016View source
- [3]IEC 62682:2022 — Management of alarm systems for the process industriesIEC · International Electrotechnical Commission · 2022View source
- [4]EEMUA Publication 191 — Alarm systems: a guide to design, management and procurementEEMUA · Engineering Equipment and Materials Users' Association · 3rd edition, 2013View source
- [5]Effective Alarm Management Practices — ASM Consortium GuidelinesASM Consortium · Abnormal Situation Management Consortium · 2009View source
- [6]ANSI/ISA-101.01-2015 — Human Machine Interfaces for Process Automation SystemsISA · International Society of Automation · 2015View source