How to rate it, simply
Three factors, each scored from 1 to 5 and multiplied:
- Impact: what happens if it fails. Does it stop the line? Does it affect safety? Does the customer notice?
- Frequency: how often it fails, according to the history.
- Detectability: whether you see the failure coming or it strikes without warning.
The result ranks your assets. Those at the top of the list get serious preventive work; those at the bottom, corrective.
The usual mistake
Doing the analysis once, saving it in a spreadsheet and never looking at it again. Criticality changes: a redundant machine stops being redundant when its twin breaks down, and a secondary machine becomes critical when what you produce changes.