Warning signs

This Technical Measure Document refers to issues surrounding physical and electronic warning signs and how they can be used to minimise the foreseeable risks of a major accident and hazard.

The relevant Level 2 Criterion is

General principles

"For warning signs and alarm indications, the first requirement is to alert the operator to the situation, and then to aid his/her accurate and prompt diagnosis". Ball, P.W.

The following aspects should be considered with respect to Warning Signs:

  • Human factors;
  • Unconscious and conscious incompetence;
  • Ergonomic design;
  • Inadequate/lack of warning signs;
  • Unidentifiable warning signs;
  • Misinterpretation of warning signs; and,
  • Wrong warning signs used.

General issues

  • Appropriate management systems should be in place to ensure that areas of plant and plant items (valves, pipes, etc.) are identified which require warning or instructional signs and that signs are provided as appropriate. This also includes temporary works, such as restricted areas e.g. for tanker offloading, lifting operations, hot works, confined space etc.
  • Appropriate risk assessments should be conducted to determine hazardous areas/zones on site.
  • All site staff (including contractors) should be informed, instructed, trained and supervised as appropriate to minimise a potential for human error when recognising the meaning of warning signs.
  • The system of housekeeping should ensure that all damaged or missing labels, signs, etc are replaced swiftly
  • The maintenance and calibration of electronic warning signs need to be considered (noise/visual warning systems).
  • A warning sign should be compelling but not startling.

Visual warning signs

  • Where possible, accepted warning signs should be used so that they conform to the reader's assumptions.
  • The types of warning signs required to be in place within the designated zones should be appropriate for the hazard, ie mandatory, warning, caution, electronic, physical, intrinsically safe, chemical/heat resistant.
  • The long-term visibility of the warning signs, ie, lighting, degradation due to exposure to UV, corrosion, size, positioning, orientation should be considered.
  • Improvised signs, that are laminated to protect them, are susceptible to veiling reflections. In certain positions this can mean that they are unreadable.
  • Colour should not be used as the sole means of coding. It should always be used redundantly. For example, apart the issue of colour blindness, red is extremely difficult to detect under sodium lighting.
  • New designs or icons or pictograms should conform to accepted codes and widely used systems and should be user tested prior to being put into use to ensure that the designer's mental model of what the icon or pictogram means is compatible with that of the user.
  • The variability in human dimensions should be considered when placing warning signs. For example, a warning sign that is clearly visible to someone who is 5 ft 5 might not be visible to someone who is 6 ft 5, or vice versa. Signs should be placed so that all the people who need to, can see them.
  • Warning signs should contain no more information than is necessary to inform the reader of the its meaning.
  • Dyslexic or illiterate employees should be considered when considering the use of purely text based warning signs.
  • Colour warning signs and labels are perceived as representing a greater hazard than achromatic labels.
  • If signs are used to indicate direction, there should be no ambiguity as to the route the sign indicates. Wherever it is possible to take the wrong route a sign should be positioned to reduce the likelihood of this happening.
  • The typeface used for text on warning signs should be a sans-serif type. For example Ariel.
  • All types of viewing conditions should be considered when deciding on what types of warning sign to use.
  • If the warning message is more than a couple of words long do not use all capitals. This is because it slows down the reading time of the message.
  • The minimum size of letters within warning signs should be based on the following:

For Non VDU applications use this table:

Viewing distance (mm) Height in mm
501-900 5
901-1800 9
1801-3600 18
3601-6000 30

Or if the viewing distance exceeds in 6000 mm use the formula: Height in mm = Viewing distance in mm/200

For VDU applications use the following table:

Viewing distance (mm) Height in mm
500 3
700 4.3
1000 4.8

The preferred colour contrasts on VDUs are presented in the table below:

Character Background
Black White
Yellow Dark Blue
White Green
Black Light Grey
White Dark Grey
White Red

Non-Verbal Auditory warnings

  • All employees and contractors on site should know what each alarm means and what the required response is, if the cause of the alarm has the potential to affect them.
  • When an alarm triggers it should provide enough time to effect recovery where applicable.
  • Alarms should prioritised, where appropriate.
  • An alarm should reset automatically if the fault that generated it is rectified
  • Following an alarm, the response required by the operator should be clear.
  • Alarm signals should be at least 10 dB(A) over the background noise.
  • Do not use alarms that have a frequency of 1 kHz if the source of the sound needs to be detected.
  • Alarms should not prevent effective communication across the site.
  • The design of the alarm system should prevent masking and flooding of alarms. Masking is where one alarm noise masks a similar sounding alarm preventing the operator from detecting the signal. Flooding happens when a system alarms which has a 'knock on' effect on other related systems, the result of which is the triggering of myriad other alarms, flooding the site with sound.
  • There should be a noticeable difference between alarm sounds used to alert, than for routine signals.

Verbal Auditory warnings

Consider using verbal auditory warnings in combination with visual warnings. Use of both methods has been found to improve compliance with the warning message.

Verbal warnings can be more effective in crowd situations when signs can become obscured.

Major hazards

The safety report should address the following points:

  • Adequacy of management systems to identify when/where warnings signs are required;
  • Adequacy of management systems to deal with human failings to obey warning signs;
  • Adequacy of risk assessments programmes, which may identify the requirement for warning signs;
  • Adequacy of warning signs for emergency response, particularly for local fire brigades/police;
  • Adequacy of warning signs for visitors or intruders to site;
  • Suitability of warning signs for the area in which they are located e.g. use of non-flameproof electric/electronic signs in flameproof areas;
  • Maintenance/review of warning signs; and
  • Misinformation included on warning signs.

Codes of Practice relating to warning signs

The following HSE publications can be used as guidance material relating to safety issues surrounding warning signs:

  • HS(G)51 Storage of flammable liquids in containers, HSE, 1998.
    Paragraph 41 refers to the Health and Safety (Safety Signs and Signals) Regulations 1996 and the requirement of appropriate warning signs in storage areas containing over 25 tonnes of material.
  • HS(G)64 Assessment of fire hazards from solid materials and the precautions required for their safe storage and use, HSE, 1991.
    Paragraph 67 refers to electrical alarm signals and the importance of them having a common warning signal perceptible throughout a building.
  • HS(G)178 The spraying of flammable liquids, HSE, 1998.
    Paragraph 85 refers to The Health and Safety (Safety Signs and Signals) Regulations 1996 and also the need for hazard diamond warning signs to be used for flammable liquids.
    Paragraph 86 refers to The Chemicals (Hazard Information and Packaging for Supply) Regulations 1994 and their amended versions. These regulations apply specifically to how hazardous substances should be labelled.
  • HS(R)29, 'Notification and Marking of Sites : The Dangerous Substances (Notification and Marking of Sites) Regulations 1990 : Guidance on Regulations', HSE.
    Paragraph 2 – 3 refers to the need for Operators to inform the Fire Authority and Enforcing Authorities of all hazardous substances and access points by way of suitable warning signs as stipulated by The Notification and Marking of Sites Regulations 1990.
  • 'The Carriage of Dangerous Goods (Classification, Packaging and Labelling Receptacles Regulations 1996', HSE, ISBN 0 11 062923 X.
    This publication highlights explicitly the type of warning signs required for hazardous substances.
  • IND(G)184, 'Signpost To The Health & Safety (Safety Signs & Signals) Regulations 1996', HSE.
    This publication highlights the types of new warning signs required as from 24th December 1999. It also, emphasises the need for pipe-work carrying dangerous substances to be labelled with warning signs.
  • IND(G)186, 'Read the Label : How to find out if chemicals are dangerous', HSE.
    This publication is an introduction to the legal requirements and importance of warning signs for chemicals. It is a useful guide for those unfamiliar with chemical labelling and highlights the importance of Safety Data Sheets.

Further reading material

Judy Edworthy and Austin Adams, 'Warning Signs: A Research Prospective', Published by Taylor and Francis, 1996.

This publication highlights the human behaviour towards the perception of warning signs and the importance of their design, appearance, visibility and recognition.

Case Studies illustrating the importance of warning signs

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