2.1 COMAH 2015 Regulations & Regulatory Duties

The Control of Major Accident Hazards (COMAH) Regulations 2015 represent the cornerstone of major accident hazard legislation in Great Britain. Transposing the European Union Seveso III Directive (Directive 2012/18/EU) into national law, COMAH applies to process industry establishments where specified dangerous substances are present—or likely to be present—at or above qualifying threshold quantities. The fundamental objective of the COMAH framework is twofold: to take all measures necessary to prevent major accidents involving dangerous substances, and to limit the consequences of any such accidents on human health and the environment.

1. Regulatory Context & Scope

A major accident under COMAH 2015 is defined as an occurrence—such as a major emission, fire, or explosion—resulting from uncontrolled developments during operation, leading to serious danger to human health or the environment, immediate or delayed, inside or outside the establishment.

The regulations apply to any facility storing, manufacturing, or processing dangerous substances classified under the CLP Regulation (EC No 1272/2008). Facilities fall into scope based on the maximum inventory of dangerous substances present at any single point in time.

Exclusions from COMAH 2015

Certain activities are excluded from COMAH oversight because they are governed by dedicated sector-specific legislation:

• Military establishments and defense facilities
• Ionizing radiation hazards covered by nuclear site licenses
• Off-site transport of dangerous substances by road, rail, inland waterway, sea, or air
• Offshore oil and gas exploration and production facilities (governed by the Offshore Installations Safety Case Regulations)
• Waste landfill sites (excluding operational tailings ponds containing dangerous substances)

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2. Establishment Tier Classification & Schedule 1 Thresholds

COMAH 2015 establishes a two-tiered regulatory system based on substance inventory thresholds set out in Schedule 1:

1. Lower Tier (LT) Establishments: Facilities holding dangerous substances equal to or exceeding Lower Tier threshold quantities but below Upper Tier thresholds.
2. Upper Tier (UT) Establishments: Facilities holding dangerous substances equal to or exceeding Upper Tier threshold quantities. Upper Tier establishments are subject to significantly more stringent regulatory requirements, including formal Safety Report submissions and external emergency planning.

Schedule 1 Structure

Schedule 1 is divided into two distinct parts:

• Part 1 (Named Substances): Specific chemical substances with individual threshold values.
• Part 2 (Categories of Dangerous Substances): Generic hazard categories based on CLP physical, health, and environmental classifications.

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3. The Aggregation Rule (Summation Rule)

In many process plants, no single dangerous substance exceeds its individual COMAH threshold quantity. However, the presence of multiple dangerous substances with similar hazard profiles can create a cumulative major accident risk. To address this, COMAH 2015 mandates the Aggregation Rule (Schedule 1 Part 3).

Under the Aggregation Rule, an establishment must evaluate three separate hazard groups:

1. Health Hazards: Acute Toxicity Category 1, 2, or 3, STOT SE Category 1.
2. Physical Hazards: Explosives, Flammable Gases, Aerosols, Oxidizing Liquids/Solids, Flammable Liquids.
3. Environmental Hazards: Acute Aquatic Category 1, Chronic Aquatic Category 1 or 2.

Mathematical Formulation

For each hazard group, the ratio of the quantity of each substance present ($q_i$) to its corresponding threshold quantity ($Q_i$) is summed. If the sum equals or exceeds 1.0, the establishment falls into that tier:

\sum_{i=1}^{N} rac{q_i}{Q_i} \ge 1.0

Where:

• $q_i$ = Maximum inventory of substance $i$ present on site (tonnes).
• $Q_i$ = Specific threshold quantity (Lower or Upper Tier) for substance $i$ (tonnes).

                      q_1       q_2             q_N
Aggregation Sum  =  ------- + ------- + ... + -------  >= 1.0
                      Q_1       Q_2             Q_N

Worked Calculation Example

A chemical storage warehouse holds the following quantities of flammable liquids (Physical Hazard Group, Lower Tier Thresholds):

• Toluene: $q_1 = 3,000\text{ tonnes}$ ($Q_1 = 5,000\text{ tonnes}$ for Flammable Liquids P5c)
• Methanol: $q_2 = 1,200\text{ tonnes}$ ($Q_2 = 5,000\text{ tonnes}$ for Flammable Liquids P5c)
• Acetone: $q_3 = 1,500\text{ tonnes}$ ($Q_3 = 5,000\text{ tonnes}$ for Flammable Liquids P5c)

Calculating the Aggregation Ratio for Lower Tier Physical Hazards:

ext{Sum} = rac{3,000}{5,000} + rac{1,200}{5,000} + rac{1,500}{5,000} = 0.60 + 0.24 + 0.30 = 1.14

Because $1.14 \ge 1.0$, the warehouse is designated as a Lower Tier COMAH Establishment, even though no individual chemical reached the 5,000-tonne threshold on its own.

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4. Operator Duties & Regulatory Submissions

COMAH 2015 establishes progressive duties depending on the establishment's tier classification.

General Duty (Regulation 5)

Every operator of a COMAH establishment has an overarching duty to take "all measures necessary to prevent major accidents and to limit their consequences for human health and the environment." The onus of proof rests on the operator to demonstrate to regulators that adequate safety systems are active.

Notification (Regulation 6)

Operators must submit a formal notification to the Competent Authority within a reasonable time prior to starting construction or operation of a new establishment (HSE L111 typically indicates about three to four months), or whenever Regulation 6 / HSE L111 requires an updated notification after a change in inventory that alters the establishment's classification status. The notification must include:

• Name and address of the operator and establishment
• Registered office address
• Name and function of the person in charge
• Information identifying the dangerous substances and inventory present
• Physical state and immediate surrounding environment of the establishment

Major Accident Prevention Policy (MAPP) (Regulation 7)

All Lower Tier and Upper Tier operators must prepare a written Major Accident Prevention Policy (MAPP). The MAPP sets out the operator's overall philosophy, goals, and organizational commitments regarding major hazard management.

• Must include a robust Safety Management System (SMS) proportional to the major accident hazards on site.
• Covers key pillars: organization and personnel, identification and evaluation of major hazards, operational control, management of change (MOC), emergency planning, monitoring performance, and audit/review.
• Must be reviewed and updated at least every 5 years, or immediately following significant site modifications or major hazard incidents.

Safety Report (Regulation 9 - Upper Tier Only)

Upper Tier operators must produce and submit a comprehensive Safety Report to the Competent Authority. The Safety Report is a massive technical dossier demonstrating that:

1. A MAPP and Safety Management System have been fully implemented.
2. Major accident hazards and scenario pathways have been systematically identified.
3. Necessary measures have been taken to prevent major accidents and limit their consequences.
4. Adequate safety-critical infrastructure, process controls, and emergency systems are incorporated into plant design and maintenance.
5. An Internal Emergency Plan has been drawn up.

Review Timelines for Safety Reports

• Initial Submission: Within a reasonable period before construction/operation for new sites (HSE L111 typically indicates about three to four months; follow COMAH Regulation 9 for safety reports).
• Periodic Review: Must be thoroughly reviewed and updated at least every 5 years.
• Ad-hoc Review: Must be updated immediately following major modifications, new technical knowledge, or a major accident on site.

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5. Emergency Planning & Public Information

For Upper Tier establishments, emergency planning is split into internal and external responsibilities:

• Internal Emergency Plan (Reg 12): Prepared by the site operator in consultation with site personnel and emergency services. Details on-site emergency response, alarm systems, containment actions, and evacuation protocols. Tested and exercised at intervals not exceeding 3 years.
• External Emergency Plan (Reg 13): Prepared by the local authority (e.g., County Council or Unitary Authority) using technical hazard data provided by the operator. Details off-site emergency response, public warning, shelter-in-place instructions, traffic control, and environmental protection. Tested at intervals not exceeding 3 years.
• Provision of Information to the Public (Reg 16): Upper Tier operators must ensure that persons likely to be affected by an off-site major accident receive clear, proactive safety information without having to request it. Information includes the nature of hazards, emergency alarm signals, and required public response actions.

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6. The Competent Authority (CA) Role & Enforcement

In Great Britain, COMAH is enforced by the Competent Authority (CA), which operates as a joint regulatory partnership combining safety and environmental expertise:

• England: Health and Safety Executive (HSE) + Environment Agency (EA)
• Wales: Health and Safety Executive (HSE) + Natural Resources Wales (NRW)
• Scotland: Health and Safety Executive (HSE) + Scottish Environment Protection Agency (SEPA)

Regulatory Functions of the CA

• Safety Report Assessment: The CA thoroughly assesses Upper Tier Safety Reports, issuing formal decisions (Acceptable, Acceptable with Conditions, or Reject).
• Joint Inspection Programs: CA inspector teams conduct regular, structured site inspections focused on safety-critical elements, human factors, and environmental protection.
• Enforcement Powers: Inspectors hold extensive legal powers under the Health and Safety at Work etc. Act 1974 and COMAH 2015, including:
  • Improvement Notices: Statutory notices requiring corrective action within a specified timeframe.
  • Prohibition Notices: Directives ordering immediate cessation of high-risk activities where an imminent risk of serious personal injury or environmental damage exists.
  • Prohibition of Operation (Reg 20): The CA can prohibit the operation of an establishment or part thereof if the measures taken by the operator for prevention and mitigation are seriously deficient.
  • Prosecution: Criminal prosecution in court leading to unlimited fines and potential imprisonment for corporate officers.

Establishing a Process Safety Management System (NEBOSH Element 2.1)

COMAH duties are the UK legal expression of major-hazard control, but the syllabus first asks for the purpose and elements of a process safety management system itself:

• Purpose: Provide a structured way to identify major accident hazards, implement barriers, assure barrier health, and continually improve—so low-frequency, high-consequence events are managed with the same discipline as day-to-day production.
• Core elements in practice: Process safety leadership and policy; hazard identification and risk assessment; management of change; operating procedures and safe operating limits; asset integrity and maintenance; permit-to-work and isolation; contractor control; competence; performance monitoring (leading/lagging); audit and management review; emergency preparedness; and organisational learning.
• Application: Map each element to plant reality (who owns it, what evidence shows it works, how failures are detected). Frameworks such as CCPS Risk Based Process Safety pillars and COMAH MAPP/safety-report demonstrations are tools for implementing this system—not substitutes for it.

A candidate who can only recite COMAH tier paperwork without explaining why a PSM system exists will miss Element 2.1.