PHA — Preliminary Hazard Analysis

1What is PHA?

L'Preliminary Hazard Analysis (PHA) systematically identifies facility hazards early in a project's or operation's lifecycle. It reviews every system or item of equipment to list what could go wrong, without seeking FMEA's exhaustiveness or LOPA's quantitative precision. It is a structured first pass and often the gateway to more detailed methods for the most critical scenarios.

Polyodos provides two PHA variants: Physical-risk PHA (general purpose, this page) and Safety-report PHA (the SEVESO/ICPE regulatory variant—see the dedicated page).

Unlike FMEA, which has the dedicated NF EN IEC 60812 standard, PHA has no standalone standard. It is a technique documented in ISO 31010 — Risk management — Risk assessment techniques — sous le nom Preliminary Hazard Analysis.

2Definitions

Term	Definition
Hazard	An inherent property of equipment or a substance that can cause harm, such as stored mechanical energy, a toxic substance, or electrical voltage. A hazard exists independently of any incident; it is a potential, not an event.
Hazardous event	The abnormal situation that would materialise the hazard, such as rupture of a pump casing. It is the shift from potential to incident.
Cause	What may lead to the hazardous event, such as prolonged cavitation or internal corrosion.
Consequence	What results if the hazardous event occurs, such as projected metal fragments or flooding of the room.

In practice, the Hazard field is often completed using broad categories to facilitate classification and statistics: Mechanical (rupture, projection, crushing), Chemical (leak, toxicity, reaction), Electrical (short circuit, electric shock), Thermal, Ergonomique…

3Before starting: model the system

Like FMEA, PHA relies on an initial breakdown of the facility into systems and items of equipment — voir la notion transverse Facility modelling, shared by PHA, FMEA, HAZOP, and Bow-Tie.

4Method steps

Break down the facility into systems or items of equipment (see modelling above).
Identify the hazard associated with each item of equipment (energy, substance, movement, etc.).
Describe the hazardous event : the abnormal situation that would materialise the hazard.
List the causes that may lead to the event.
List the consequences for people, the environment, and property.
Rate the scenario en severity level and probability, avant mesures (voir section suivante).
List existing and planned measures, then rate the scenario again after measures.

PHA works well as a multidisciplinary group review involving operations, safety, and maintenance. Diverse viewpoints reduce blind spots more than sophisticated tooling.

5Rate risk in PHA

The general principles for rating risk—matrix, scales, and before/after measures—are shared by all methods. See the cross-cutting concept Rate a risk. PHA combines two metrics, each rated from 1 to 4 in Polyodos:

Severity

Level	Label	Signification
1	Negligible	First aid with no time off work
2	Significative	Partial temporary incapacity under three days
3	Grave	Partial permanent incapacity or hospitalisation over one day
4	Catastrophique	Death or total permanent incapacity

Probability (likelihood)

Level	Label	Indicative frequency
1	Highly unlikely	< 10⁻⁴ / an
2	Unlikely	10⁻⁴ – 10⁻² / an
3	Likely	1 occurrence possible par an
4	Frequent	Plusieurs occurrences attendues

Criticality matrix

Criticality results from combining Severity × Probability in a configurable matrix, producing a four-level scale:

Level	Label	Signification
1	Acceptable	Maintenir la vigilance
2	Monitor	Planned treatment
3	Non acceptable	Short-term treatment required
4	Inacceptable	Urgent treatment; stop if necessary

The default matrix covers only severity and probability levels 1 to 4. If a scenario is rated beyond them, such as Severity 5 on a custom scale, criticality cannot be resolved automatically until the matrix is extended. The example below demonstrates this actual case.

6Existing and planned measures

Each scenario distinguishes existing measures (already in place and considered in the before-measures rating, often blank when the scenario is not yet covered) from planned measures (the action plan, whose implementation justifies the after-measures rating). This uses the same distinction between prevention / protection qu'en AMDEC :

Type	Acts on	Exemples
Prevention	Probability	Periodic vibration monitoring, visual inspection, preventive replacement of wear parts, and procedures governing hazardous operations (work permit)
Protection	Severity / detection	Gas detector with alarm and automatic shutdown, residual-current device, and thermal protection

7Example completed in Polyodos

The Polyodos demonstration dataset for a drinking-water treatment plant covers four scenarios across Pumping, Chemical Treatment, and Storage systems:

#	System / Equipment	Hazard	Causes	Hazardous event	Consequences	G	P	C	Planned measures	G'	P'	C'
1	Pompage — Pompe centrifuge P-101	Mechanical	Prolonged cavitation, excessive vibration, internal corrosion	Rupture de la volute de pompe	Projected metal fragments, flooding of the pump room, production shutdown	G4	P2	R3	Monthly vibration monitoring (piezo sensors), replacement of mechanical seals every two years	G3	P1	R2
2	Traitement chimique — Cuve chlore CL-301	Chemical	HDPE wall corrosion, rupture of the filling connection, overpressure during unloading	Fuite de chlore liquide	Toxic atmosphere (chlorine gas), severe respiratory irritation, site evacuation	G5	P2	R?	Chlorine-gas detector with audible alarm and automatic shutdown, unloading procedure with work permit, annual tank inspection	G4	P1	R2
3	Pumping — Electric motor M-101	Electrical	Wiring degraded by moisture, prolonged motor overload, circuit-breaker failure	Short circuit causing a switchboard fire	Electrical-room fire, loss of pumping power, operator electric-shock risk	G3	P2	R2	Annual electrical-installation inspection, 30 mA residual-current device, and motor thermal protection	G2	P1	R1
4	Storage — Treated-water tank CS-301	Mechanical	Reinforced-concrete ageing, differential ground settlement, hydraulic overpressure	Concrete tank rupture (structural crack)	Major overflow (500 m³), downstream flooding, groundwater contamination	G4	P1	R2	Quarterly visual inspection (cracks and deformation), geotechnical ground assessment every ten years	G3	P1	R2

PHA table — Polyodos demonstration dataset for a drinking-water treatment plant (before / after measures)

Line 2: the chlorine tank is rated Severity 5, outside the default scale (1 to 4), so before-measures criticality cannot be read directly from the standard matrix until it is extended to this level. This is an actual demonstration-data case, not an error in the example; it illustrates the pitfall described above, where a custom scale is not carried into the matrix.

Section to be expanded — Polyodos screenshots showing entry of this table and the matrix view.

8Common mistakes

Mistake	Why it is a problem
Confusing a hazard with a hazardous event	A hazard is an inherent property, such as high pressure; a hazardous event is an abnormal situation, such as tank rupture. Mixing them makes the causes inconsistent.
Rating only after measures	Without a before-measures rating, the actual effectiveness of barriers cannot be assessed and the most critical scenarios cannot be prioritised.
Extending a scale without extending the matrix	Adding a custom severity level, such as 5, without updating the criticality matrix leaves scenarios with no calculable risk level, as shown above.
Seeking exhaustiveness at the expense of perspective	PHA loses its purpose if it becomes FMEA in disguise. A broad overview is more useful, followed by deeper HAZOP or FMEA analysis of critical scenarios.

9Further reading

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Facility modelling

The initial breakdown used by PHA

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Rate a risk

General rating principles

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PHA — Hazard study

SEVESO regulatory variant

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FMEA

Analyse a critical system in greater depth

🎀

BowTie

Visualise causes, barriers, and consequences

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All methods

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