B8.5.1 COMAH continued (2)


Low Tier Sites


Low Tier Sites must notify the Competent Authority and provide basic information, such as details of the site activities and the dangerous substances on site.

Prevent Accidents:

There is a general duty to prevent risks and for those that cannot be eliminated, mitigate these risks. The prevention of risks should be based on the ALARP principle for human risks, which is ‘As Low As Is Reasonably Practicable’; and for risks to the environment, BATNEEC should be used, which is ‘Best Available Technology Not Entailing Excessive Cost’.

Major Accident Prevention Policy (MAPP):

A MAPP is a document that sets out the policy for preventing major accidents. It is usually a short document that references the safety management system. The MAPP can refer to detailed information contained within other documents, such as plant operating procedures and training records. The general areas that should be included are:

  • Organisation and personnel.
  • Identification and evaluation of major hazards.
  • Operational control.
  • Planning for emergencies.
  • Monitoring, audit and review.


Top Tier Sites

Top Tier Sites must comply with the same requirements as the Low Tier Sites (as outlined above) and also comply with additional requirements.

Safety Report:

The site operator must prepare a safety report to demonstrate that the actions required for the prevention and mitigation of major accidents have been taken. According to the HSE, the safety report must include:

  • A policy on how to prevent and mitigate major accidents.
  • A management system for implementing that policy.
  • An effective method for identifying any major accidents that might occur.
  • Measures (such as safe plant and safe operating procedures) to prevent and mitigate major accidents.
  • Information on the safety precautions built into the plant and equipment when it was designed and constructed.
  • Details of measures (such as fire-fighting, relief systems and filters) to limit the consequences of any major accident that might occur.
  • Information about the emergency plan for the site, which is also used by the local authority in drawing up an off-site emergency plan.


It should be noted that the Safety Report is not in addition to the MAPP (as required by Low Tier Operators). All requirements of the MAPP will be included in the Safety Report and therefore Top Tier Sites will only produce one document.

The Safety Report must be reviewed (and updated is required) if the plant is modified, when new information becomes available or after a period of 5 years.

Site Emergency Plan:

Top Tier Sites must compile on emergency plan, which outlines how the consequences of a major accident would be dealt with.

Provide Information to Local Authorities:

The Local Authority requires information in order that it may prepare, review and test off-site emergency plans for dealing with the off-site consequences of a major accident. The exact requirements should be discussed with the relevant Local Authority.

Provide Information to the Public:

Anyone who might be affected by an accident at a COMAH site should be provided with information about the possible major accidents and their consequences and what to do in the event of an accident. This information should be provided without the effected party having to request it.


The Competent Authority is permitted to recover costs incurred as a result of COMAH, such as the recovery of costs for time spent in relation to COMAH activities on a site.



lanning (Control of Major-Accident Hazards) (Scotland) Regulations 2009 [SSI 378]

Amends 1993/323 to increase the range of dangerous substances that you need consent for to store or process; the definition of these substances; and the amounts allowed before consent must be obtained.

Planning (Control of Major-Accident Hazards) Regulations Northern Ireland 2009 [SR 399]

Increases the range of dangerous substances for which consent must be obtained for storage or processing; puts tighter controls on the amounts allowed before consent must be obtained.