What is PFH?
PFH is the average frequency of dangerous failures per hour, used for safety functions in high demand or continuous mode.
PFH Explained
When demands are frequent (more than once a year) or the function acts continuously, the probability of failure on demand is no longer the right measure. PFH counts dangerous failures per hour instead.
Machinery and burner safety functions are often high demand.
PFH Formula
| λDU | dangerous undetected failure rate (per hour) |
PFH: Typical Values
| SIL 1 | 10⁻⁶ to 10⁻⁵ per hour |
| SIL 2 | 10⁻⁷ to 10⁻⁶ per hour |
| SIL 3 | 10⁻⁸ to 10⁻⁷ per hour |
| SIL 4 | 10⁻⁹ to 10⁻⁸ per hour |
Calculators for PFH
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Frequently Asked Questions
What is PFH?
PFH is the average frequency of dangerous failures per hour, used for safety functions in high demand or continuous mode.
How do I choose low or high demand?
IEC 61511 treats demands more than once per year as high demand mode.
Does proof testing help in high demand mode?
Much less; the function is exercised by frequent demands, so diagnostics and failure rates dominate.
Learn more on the blog
What is SIL (safety integrity level)
A practical guide on instrumentationblog.in with more detail on PFH and how it is applied on real plants.
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