Preliminary Hazard Analysis – Step By Step Guide

What Is Preliminary Hazard Analysis

Preliminary hazard analysis (PHA) is usually the first attempt in the system safety process to identify and categorize hazards or potential hazards associated with the operation of a proposed system, process, or procedure; it is used in the early stages of system design.


It is a semi-quantitative analysis that is performed to identify all potential hazards and accidental events that may lead to an accident, rank the identified accidental events according to their severity and identify required hazard controls and follow-up actions.


It also provides rationale for hazard control and indicates the need for more detailed analyses, such as the Subsystem Hazard Analysis (SSHA) and the System Hazard Analysis (SHA). The PHA is usually developed using the system safety techniques known as Failure Modes and Effects Analysis (FMEA) and/or the Energy Trace and Barrier Analysis (ETBA).

Preliminary hazard analysis that can be used under different names, such as Rapid Risk Ranking and Hazard Identification (HAZID). 


Read Also8 Important Qualitative Risk Analysis Methods


Characteristics Of PHA

  • It relies on brainstorming and expert judgment to assess the significance of hazards and assign a ranking to each situation.
  • It is typically performed by one or two people who are knowledgeable about the type of activity in question.
  • It is applicable to any activity or system
  • It can be used as a high-level analysis early in the life of a process.
  • It is used to generates qualitative descriptions of the hazards related to a process. Provides a qualitative ranking of the hazardous situations; this ranking can be used to prioritize recommendations for reducing or eliminating hazards in subsequent phases of the life cycle.
  • Quality of the evaluation depends on the quality and availability of documentation, the training of the review team leader with respect to the various analysis techniques employed, and the experience of the review teams.

Advantages And Disadvantages Of PHA


  • Helps ensure that the system is safe
  • Modifications are less expensive and easier to implement in
    the earlier stages of design
  • Decreases design time by reducing the number of surprises


  • Hazards must be foreseen by the analysts
  • The effects of interactions between hazards are not easily


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Steps To Carrying Out Preliminary Hazard Analysis (PHA)

1. PHA prerequisites: This involves, establishing a PHA team, description of the system to be analysed, and collection of risk information from previous system.

2. Hazard identification: This is where all hazards and possible accidental events must be identified. All part of the system should be considered at this stage. All findings should be recorded.

Note: No hazards are too insignificant to be recorded.

Hazards can be identified by –

  • Examining similar existing systems
  • Review of previous hazard analyses for similar systems
  • Review of hazard checklists and standards
  • Considering energy flow through the system
  • Considering inherently hazardous materials
  • Considering interactions between system components
  • Review of operation specifications, and consider all
    environmental factors
  • Brainstorming as a team
  • Considering human/machine interface
  • Considering usage mode changes
  • Using small scale testing, and theoretical analysis
  • Using a “Think through a worst case what-if analysis”.
  • Checking through records like accident statistics, Near miss/ dangerous occurrence reports, Reports from authorities or governmental bodies, etc.

3. Consequence and frequency estimation: To determine the risk level, we have to estimate the frequency and the severity of each accidental event. At this stage, the consequence and frequency of every hazard is considered.

4. Risk ranking and follow-up actions: Risk is established as a combination of a given event/consequence and a severity of the same event/consequence. This will enable a ranking of the
events/consequences in a risk matrix. This ranking level will determine the follow up actions necessary for the risk.


NOTE: The Preliminary hazard analysis is not a static document, it should be review and updated under certain condition. Some of the conditions are:

  • When the system equipment is modified
  • When maintenance or operating procedures change
  • After a mishap or near-miss occurs
  • Environmental conditions change
  • When operating parameters or stress change.