What Is a Preliminary Hazard Analysis?
Preliminary Hazard Analysis (PHA) is an early-stage risk assessment technique used to identify potential hazards in a system, process, or product before it is fully developed or operational. It is particularly valuable during the conceptual and early design phases, before detailed system requirements have been defined.
Unlike more detailed hazard analyses—such as System Hazard Analysis (SHA)—conducted later in the lifecycle, a PHA provides a broad overview of possible hazards and their consequences, helping teams guide design decisions and prioritize safety efforts.
How PHAs Are Used
PHAs are typically performed during conceptual design to identify hazards before detailed engineering begins. Early identification is critical: gathering hazard-related information early in the design process saves time, reduces rework, and helps de-risk design iterations.
For modified systems, a PHA can be used to assess risks introduced by changes. In fact, some level of PHA should be included in every feasibility analysis, as many safety incidents have been traced back to design changes made after the original design team had moved on.
How to Conduct a Preliminary Hazard Analysis
Conducting a PHA involves several key steps:
Step 1: Define the Scope
Identify the system, process, or concept to be analyzed.
Clarify operational boundaries and interfaces.
Step 2: Assemble a Team
Include experts from engineering, safety, operations, and design.
Step 3: Gather Background Information
Collect conceptual designs, process descriptions, and known hazard data.
Step 4: Identify Hazards
Use brainstorming, checklists, and historical data to identify potential hazards.
Step 5: Assess Risks
Estimate the likelihood and severity of each hazard.
Prioritize hazards based on risk levels.
Step 6: Recommend Controls
Suggest design changes, safety features, or procedural safeguards.
Step 7: Document and Review
Record findings, decisions, and recommendations.
Update the analysis as the design evolves.
PHAs vs. FMEAs: A Complementary Relationship
PHAs are structured, early-stage evaluations that often use an FMEA-style approach. Like Failure Mode and Effects Analyses (FMEAs), PHAs aim to systematically identify potential failure points and assess their consequences—but with a broader focus on high-level hazards rather than component-level failures.
While FMEAs drill down into specific failure modes of individual components, PHAs take a top-down approach, identifying overarching hazard categories such as collisions, fires, toxic releases, or structural failures. This allows teams to evaluate system-wide impacts and develop safeguards that address root causes rather than symptoms.
By combining the systematic rigor of FMEAs with the broad hazard identification of PHAs, organizations can build safer, more resilient systems—especially in industries like rail, aerospace, chemical processing, and power generation.
Common Top-Level Hazards in Rail PHAs
To illustrate, here are the most frequently identified top-level hazards in rail applications, based on industry guidelines and safety assurance practices:
1. Collisions
Train-to-train, train-to-vehicle, or train-to-person collisions due to signaling failures, human error, equipment malfunction, or environmental hazards.
2. Derailments
Caused by track defects, excessive speed, mechanical failure, or environmental conditions (e.g., flooding, landslides).
3. Doors Opening
Doors opening while a train is moving, or on the incorrect side, allowing passengers to fall out.
4. Fire and Smoke
Electrical faults, overheating components, or flammable materials leading to onboard or infrastructure fires.
5. Toxic or Hazardous
Material Release Spills or leaks from freight trains carrying dangerous goods, especially in urban or environmentally sensitive areas.
6. Structural Failures
Bridge, tunnel, or track bed collapses due to aging infrastructure, poor maintenance, or natural disasters.
7. Obstructions on Track
Debris, vehicles, or animals on the track leading to potential collisions or derailments.
8. Security Threats
Sabotage, terrorism, or unauthorized access to critical systems or infrastructure.
PHAs represent the highest-level hazards within a system—virtually any failure in a rail system typically falls under one of these categories. Security Threats are listed separately to emphasize the growing importance of cybersecurity and physical security in modern rail systems. The effect of a malicious attack often maps to one of the other seven hazard types.
Final Thoughts
Preliminary Hazard Analysis is a vital tool for embedding safety into the earliest stages of system development. By identifying and addressing hazards before they become part of the design, organizations can reduce risk, improve safety outcomes, and ensure compliance with industry standards.
Integrating PHAs into your development process is a proactive step toward building safer, more resilient systems from the ground up.