Alarm Handling
Introduction
This document will introduce you to some of the principles of good alarm design.
Alarms are signals that are given to the driver or control room operator typically by
one or more of the following:
• An audible sound
• Some form of visual indication
• The presentation of a message or some other identifier
Alarms are a very important method of attracting the user's attention to significant
changes that require assessment or action—alarms are not the same as status
indications. Effective alarm design must avoid overloading the user with irrelevant or
duplicated alarms and should present accurate, useful and timely messages to
facilitate safe operation of the railway. Alarm design and management is important
and should be defined using a clear philosophy.
Case study
The 1994 explosion and fires at the Texaco Milford Haven refinery injured 26 people
and caused damage of around £48 million. Key factors that emerged from the
investigation were:
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There were too many alarms and they were poorly prioritised.
The control room displays did not help the operators to understand what was
happening.
There had been inadequate training for dealing with a stressful and sustained
plant incident.
In the last eleven minutes before the explosion the two operators had to recognise,
acknowledge and act on 275 alarms.
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Issues
These are some of the key issues when either specifying a new alarm system or
reviewing an existing one.
What to alarm
Proposals for alarms come from a larger variety of sources, for example:
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Custom and practice (“There is one on our existing system….”)
Equipment manufacturer’s standard provision
Fault reports from the alarm processing hardware/software
Requirements of safety authorities, insurers, management, etc.
Software designers (“It seemed an easy thing to offer….”)
As an alternative to automation: cheaper than fitting a control system
Operating experience and feedback
Simulator studies
Task analysis
Safety reviews, e.g. HAZOPs
Qualitative or quantitative risk analysis
The majority of these are informally structured design processes and may lead to
excessive numbers of poorly justified alarms. Every alarm should be:
• Fully justified
• Properly engineered
• Consistent with the overall alarm philosophy
Alarm design
The presentation of alarms should be designed to take into account human
limitations and task demands. In doing so, every alarm system should:
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Alert, inform and guide the users
Only present the operator with useful and relevant alarms
Have a defined response to each alarm or pattern of alarms
Allow enough time for the operator to acknowledge and respond to the alarm
Be explicitly designed to take account of human limitations and task demands
The characteristics of a good alarm, or groups of alarms, are:
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Relevant - not spurious or of low operational value
Unique - not duplicating another alarm
Timely - not long before any response is needed or too late to do anything
Prioritised - indicating the importance that the operator deals with the problem
Understandable - having a message which is unambiguous and easy to
understand
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Diagnostic - identifying the problem that has occurred
Advisory - indicative of the action to be taken
Focusing - drawing attention to the most important issues
Visual messages
Many systems rely on the use of lists to summarise recent alarms to operators.
These lists must be designed such that repetition does not cause them to become
unusable. The typical content of an alarm list could include:
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The alarm state marker (e.g. unacknowledged, acknowledged, standing,
clear, reset)
The alarm priority marker
The alarm message (see recommendations below)
Time and date
Attributes of a good alarm list message are:
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Clearly identifies the condition that has occurred
Uses nomenclature the operator is familiar with
Uses consistent abbreviations from a standard directory of abbreviations
Has a consistent, hierarchical message structure
Does not rely on the learning of tag names or numbers
Has been checked for usability during simulated and actual system operation
Although alarm lists are the most common way of displaying alarms, the operator
interface can be significantly improved by also communicating alarm information
using schematic diagrams. In practice, it is generally impossible to have every active
alarm displayed to the operator via schematics, therefore the display needs to be
carefully thought out. Alarm information on schematic diagrams (such as overview
displays) is often associated with:
• Highly critical/important alarms
• Generalised status information to guide more detailed investigation
As a general rule, visual coding of alarms should be related to its importance, and
should be developed further in consultation with the end users and with due regard
to ergonomics best practice. In particular, conspicuity of alarms should reflect alarm
priority and state (unaccepted, accepted, actioned, etc.).
Audible alarms
Audible warnings are normally generated in conjunction with the visual display of
new alarms. However, control rooms are equipped with a range of devices and
systems that generate audible warnings, e.g. telephones, radios, etc. Therefore, it is
imperative that an integrated design is developed for all audible signals in the room.
Relevant considerations are:
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• So that they can be heard, acoustic signals should be set at a level
considerably higher than the ambient noise at the signal frequency, i.e. 1525dB higher. However, levels above this should not be used as it may startle
the operator and run the risk of being turned off or otherwise suppressed for
being too loud.
• Acoustic signals should be easily recognisable in terms of the situation they
are signalling.
• The same acoustic signal should not be used for more than one high-priority
function. However, low-priority messages may be grouped with the same tone.
Acoustic signals may be constant or variable frequency. Whichever form is used, a
higher pitched sound should be assigned to a higher priority message.
Further information
For more information on how to audit existing alarm systems and plan for new ones,
have a look at the following:
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HSE Information Sheet: Better alarm handling
Alarm systems: A guide to design, management and procurement - The
Engineering Equipment and Materials Users Association (EEMUA) - EEMUA
Publications, Publication No. 191- ISBN 0 85931 076 0
RSSB Human factors good practice guide to managing alarms and alerts,
RSSB Research & Development Project T326
RIAC Human Factors Working Group