Definition:
Human Error Probability (HEP) quantifies the likelihood of a human failing to perform a required action within a given timeframe. As a probability it is unitless and between 0 and 1. It is a foundational concept to human reliability analysis (HRA).
It can be a failure of omission which is a failure to perform a required action. Or a failure of commission which the action was done, but incorrectly.
Within IEC 61511-1, HEP would come into play during the H&RA in the determination of risks, and would feed the decision if a SIF is needed or not. Once a SIF is needed, everything is automated (no operator action) and an HEP is not relevant.
Key Points:
- Critical in LOPA and risk assessments.
- Can always be improved with better procedures and training.
- Can be impacted by fatigue, stress, and competence.
Example:
Failure to manually shut down a pump within 5 minutes in a specific facility is decided to have an HEP of 0.10 (or 10% chance the valve will not be shut by the operator). The reason it is so high is because the training program for the last few years has been lacking. The facility hopes to improve the training program and then they could lower the HEP.
See also: H&RA
Cited Source:
- CCPS Guidelines for LOPA.