Definition:
Probability of Failure on Demand (PFD) measures the likelihood that a safety function will fail when required to operate.
It is the most core mathematical concept of all of Functional Safety.
Key Points:
- Primary risk metric for low-demand SIFs.
- Calculated based on failure rates, proof testing, diagnostics, and architecture.
Example:
If PFDavg is 1E-2, the safety function has a 1% chance of failing on demand.
See also: probability of failure per hour (PFH), failure rate (λ), risk reduction factor (RRF)
Cited Source:
- IEC 61508-6:2010, Annex B.