Glossary · Risk estimation methods
Severity
In machine safety risk estimation, severity is the magnitude of possible harm. It considers how serious the injury or damage to health can be and how many persons can be affected.
- Machine safety
- Risk assessment
In one sentence
Severity rates how bad the possible harm is, from slight and reversible to serious, irreversible or fatal, and is estimated without regard to likelihood.
Example
A pinch point on a slow-moving feed roller may cause bruising, rated slight, while the closing tool of a press can amputate fingers, rated serious and irreversible.
How it applies
- Risk estimation: ISO 12100 considers the severity of injuries or damage to health (slight, serious, death) and the extent of harm (one person or several). Rate the most severe credible harm, not the average outcome.
- Functional safety: In the ISO 13849-1 risk graph, S1 stands for slight, normally reversible injury and S2 for serious, normally irreversible injury including death. Severity strongly drives the required Performance Level.
- Technical documentation: Signal words in safety messages are tied to severity and likelihood: ANSI Z535 reserves DANGER for hazards that will result in death or serious injury if not avoided. A warning message should match the rated severity.
- Evidence: Severity ratings rarely change with design measures that only reduce likelihood. If a rating drops, document the design change that justifies it.
Severity vs. likelihood
Severity is how bad the harm could be; likelihood is how probable it is. Keep them separate: a low likelihood does not reduce severity.