Definition:
Probability of Failure per Hour (PFH) measures the rate of dangerous failures occurring per hour for continuous or high-demand SIFs. Interestingly, PFH is not unitless, it is probability PER hour. Thus has units of 1/sec.
This is distinct from PFDavg which is for on-demand applications.
The way PFH is calculated is different than PFD and is generally much simpler. The specifics of that are beyond the scope here.
Key Points:
- Used when demand is frequent or continuous.
- SIL targets are defined by PFH limits in IEC standards.
Example:
A continuous safety function with PFH = 1E-7 failures/hour may meet SIL 3 requirements.
See Also: PFD
Cited Source:
- IEC 61508-1:2010, Clause 7.6.2.