Preliminary Hazard Analysis

The Preliminary Hazard Analysis (PHA) is a risk tool that is used during initial design and development, thus the name “preliminary”, to identify systematic hazards that affect the intended function of the design to provide an opportunity to modify requirements that will help avoid issues in the design.

Like a fair amount of tools used in risk, the PHA was created by the US Army. ANSI/ASSP Z.590.3 “Prevention through Design, Guidelines for Addressing Occupational Hazards and Risks in Design and Redesign Processes” makes this one of the eight risk assessment tools everyone should know.

Taking the time to perform a PHA early on in the design will speed up the design process and avoid costly mistakes. Any identified hazards that cannot be avoided or eliminated are then controlled so that the risk is reduced to an acceptable level.

PHAs can also be used to examine existing systems, prioritize risk levels and select those systems requiring further study. The use of a single PHA may also be appropriate for simple, less compelx systems.

Main steps of PHA

A. Identify Hazards

Like a Structured What-If, the Preliminary Hazard Analysis benefits from an established list of general categories:

  • by the source of risk: raw materials, environmental, equipment, usability and human factors, safety hazards, etc.
  • by consequence, aspects or dimensions of objectives or performance

Based on the established list, a preliminary hazard list is identified which lists the potential, significant hazards associated with a design. The purpose of the preliminary hazard list is to initially identify the most evident or worst-credible hazards that could occur in the system being designed. Such hazards may be inherent to the design or created by the interaction with other systems/environment/etc.

A team should be involved in collecting and reviewing.

B. Sequence of Events

Once the hazards are identified, the sequence of events that leads from each hazard to various hazardous situations is identified.

C. Hazardous Situation

For each sequence of events, we identify one or more hazardous situations.

D. Impact

For each hazardous situation, we identify one or more outcomes (or harms).

E. Severity and occurrence of the impact

Based on the identified outcomes/harms the severity is determined. An occurrence or probability is determined for each sequence of events that leads from the hazard to the hazardous situation to the outcome.

Based on severity and likelihood of occurrence a risk level is determined.

From hazard to a variety of harms

I tend to favor a 5×5 matrix for a PHA, though some use 3×3, and I’ve even seen 4×5.

Intended outcomes

Likelihood of Occurrence

Severity Rating

Impact to failure scale

1

Very unlikely

2

Likely

3

Possible

4

Likely

5

Very Likely

5

Complete failure

5

10

15

20

25

4

Maximum tolerable failure

4

8

12

16

20

3

Maximum anticipated failure

3

6

9

12

15

2

Minimum anticipated failure

2

4

6

8

10

1

Negligible

1

2

3

4

5

Very high risk: 15 or greater, High risk 9-14, Medium risk 5-8, Low risk 1-4

 

F. Risk Control Measures

Based on the risk level risk controls and developed and applied. These risk controls will help the design team create new requirements that will drive the design.

On-going risks should be evaluated for the risk register.