COMAH (the Control of Major Accident Hazards Regulations) is the UK’s safety regime for sites that store or process dangerous substances above defined threshold quantities. It is the UK’s implementation of the EU Seveso Directive, so if you do process safety in Britain, COMAH is the law sitting over your major-hazard duties. It is a regulatory regime, not part of IEC 61511, and it is risk-based rather than prescriptive: operators must take all measures necessary to prevent major accidents and limit their consequences, and demonstrate that the risks have been reduced to ALARP.
Sites are classed as lower-tier or upper-tier based on the inventory they hold, and upper-tier operators must submit a safety report, a safety-case-style document showing that the major-accident hazards are understood and adequately controlled. This is where functional safety earns its place. A safety instrumented system (SIS) designed and verified to IEC 61511 is one of the protection layers an operator points to when arguing that a risk is ALARP. Enforcement sits with the Competent Authority, the HSE working jointly with the relevant environmental regulator for England, Scotland or Wales.
Key Points
- COMAH is the UK’s implementation of the EU Seveso Directive, applying to establishments that hold dangerous substances above threshold quantities.
- It is risk-based: operators must prevent major accidents, limit consequences, and demonstrate risk is reduced to ALARP, with upper-tier sites preparing a safety report.
- It is enforced by the Competent Authority (the HSE with the relevant environmental regulator), not through a single prescriptive chemical list as in US PSM.
Example
A UK upper-tier COMAH establishment storing bulk chlorine must submit a safety report to the Competent Authority showing the major-accident hazards are controlled to ALARP. The SIL-rated trips protecting the chlorine system, designed to IEC 61511, are cited in that report as independent protection layers that lower the release frequency.
See Also: Seveso, ALARP, major accident, PSM
Cited Sources
- The Control of Major Accident Hazards Regulations 2015 (SI 2015/483), legislation.gov.uk
- HSE, Control of Major Accident Hazards (COMAH)