Event Tree Analysis (ETA) — What It Is and How It Works

Event Tree Analysis (ETA) is a forward-looking risk assessment tool used to explore the different outcomes that can result from a single initiating event. It maps possible scenarios step by step, showing how protective systems and human actions influence the outcome.

What is Event Tree Analysis?

ETA is a structured, visual method that helps you answer the question: if this goes wrong, what happens next? Starting from an initiating event, the tree branches at each point where a safety function or human action may succeed or fail — leading to different end states, each with its own risk level and probability.

The key question is: if a hazard occurs, can you map every possible outcome — and show which barriers determine the result?

Why use ETA?

ETA is widely used across industries including:

• Process industry
• Nuclear plants
• Chemical plant safety
• Process and system hazard analysis (HAZOP follow-ups)
• Functional safety (e.g. IEC 61508, ISO 26262)
• Natural disaster modelling (e.g. rockslides and tsunamis)

Benefits of ETA

• Easy to understand and communicate
• Helps identify system weaknesses
• Supports compliance and safety design
• Can be qualitative or quantitative

Key concepts: initiating and pivotal events

• Initiating Event (IE): The first significant failure or disturbance (e.g. fire, system error)
• Pivotal Events (PEs): Key points in the scenario where a safety function may succeed or fail
• End States: The possible outcomes (e.g. no damage, partial damage, catastrophic loss)

Each path through the tree represents a different scenario — each with its own risk level and probability.

Step-by-step: how to build an event tree

1. Define the initiating event — e.g. fire in a control room.
2. List the pivotal events — e.g. fire detection, alarm, sprinkler, operator response.
3. Map out each success/failure point. Each decision branches the tree.
4. Create all paths to end states. Each path equals a scenario.
5. (Optional) Assign probabilities using historical data or expert judgement.
6. Analyse the results. Identify critical failures, success paths, and improvement areas.

Real-world example: reactor coolant loss

This example is based on nuclear risk modelling:

• A small pipe crack causes coolant to leak
• The High Pressure Injection System (HPIS) must activate
• If it fails, the Low Pressure Injection System (LPIS) is next
• If LPIS also fails, the operator must act quickly
• Depending on what works and what fails, the outcome ranges from full recovery to core damage

This scenario is often used in training and simulation across safety-critical industries.

ETA vs. Fault Tree Analysis (FTA)

Feature	ETA	FTA
Logic Direction	Inductive (forward-looking)	Deductive (root cause focus)
Starts With	A trigger or failure event	A top-level system failure
Focus	Scenario outcomes	Cause diagnosis
Best For	Understanding consequences	Tracing failure origins

How ETA supports better decision-making

ETA helps you:

• Design more resilient systems
• Visualise worst-case and best-case scenarios
• Support functional safety assessments (SIL, LOPA)
• Improve operator training and simulations
• Prioritise mitigation strategies
• Communicate complex risk scenarios to stakeholders

Final thoughts

Event Tree Analysis is a simple yet powerful tool. Whether you are working in process safety, energy, manufacturing, or infrastructure — ETA helps you prepare for the unexpected, communicate risk, and make better decisions.

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