What is a preliminary hazard analysis?
A preliminary hazard analysis is the first safety analysis of a new system. Done early, it is cheap and powerful: hazards designed out at concept stage cost nothing to control later. It covers the whole life cycle: normal operation, start-up, shutdown, maintenance, emergencies, installation and disposal. Its output, a list of safety requirements, flows into the design specification and later into the detailed hazard analysis.
MIL-STD-882E risk assessment matrix
| Probability | 1 Catastrophic | 2 Critical | 3 Marginal | 4 Negligible |
|---|---|---|---|---|
| A Frequent | High | High | Serious | Medium |
| B Probable | High | High | Serious | Medium |
| C Occasional | High | Serious | Medium | Low |
| D Remote | Serious | Medium | Medium | Low |
| E Improbable | Medium | Medium | Medium | Low |
PHA example
The example is a PHA for an automated guided vehicle (AGV) fleet in a warehouse, at concept stage. Collision with a pedestrian is rated catastrophic and occasional: a high risk. Safety-rated scanners, speed limits at crossings and marked aisles are specified to bring it to improbable (medium). Battery thermal runaway, unexpected movement during maintenance and two vehicles routed to the same crossing start as serious and are reduced to medium with battery management, a lockout point and zone reservation logic.
After the PHA
- Turn safety requirements into design requirements and verify them.
- Analyse the detailed design with FMEA, HAZOP or fault tree analysis.
- Keep a hazard log: close each hazard only when its controls are verified.
How to use this Preliminary Hazard Analysis tool
- Describe the system. Name the system, its design phase and the team.
- Break it into subsystems. List subsystems or functions and the modes in which they operate.
- Identify hazards. For each subsystem, record hazards, their causes and their effects.
- Rate the initial risk. Use MIL-STD-882E severity categories and probability levels, or drag the markers on the matrix.
- Define safety requirements. Write the requirements that will eliminate or reduce each hazard, and rate the target risk.
- Track and export. Track the status of each hazard and export the PHA to Excel or PDF.
Frequently asked questions
What is a preliminary hazard analysis (PHA)?
A preliminary hazard analysis is an early, broad identification of the hazards of a system, done at the concept or preliminary design stage when there is still freedom to design hazards out. It lists hazards, causes and effects for each subsystem, rates the initial risk, and defines safety requirements for the design.
When should a PHA be done?
As early as possible: during concept development or preliminary design, before detailed drawings exist. It is then updated and evolves into the subsystem and system hazard analyses, FMEA and the detailed hazard analysis as the design matures.
What is the MIL-STD-882E risk matrix?
MIL-STD-882E, the US Department of Defense standard practice for system safety, rates severity in four categories (1 catastrophic, 2 critical, 3 marginal, 4 negligible) and probability in levels A (frequent) to E (improbable), plus F (eliminated). Combinations map to high, serious, medium or low risk, which determine who must accept the risk.
What is the difference between PHA and HAZOP?
PHA is a top-down, early screening of hazards for a whole system. HAZOP is a detailed, bottom-up study of deviations from design intent, node by node, usually done when piping and instrumentation diagrams are available. A PHA is often a prerequisite for a HAZOP.
What goes in the recommended safety requirements?
Design requirements, safety features and procedures that eliminate the hazard or reduce its risk, following the order of precedence: eliminate the hazard by design, reduce risk through design changes, add engineered safety features, add warning devices, and only then rely on procedures and training.