Average Frequency of Dangerous Failure [h-1] (PFH). (Dangerous failure as defined in IEC 61508-4)
To maintain the safety function, the IEC 61508 standard requires various levels of measures for avoiding and controlling detected errors, depending on the required SIL*.
All components of a safety function must be subjected to a probability assessment to evaluate the effectiveness of the measures implemented for controlling detected faults.
This assessment determined the PFH (Average Frequency of Dangerous Failure* [h-1]) for a safety-related system. This is the probability per hour that a safety-related system fails in a hazardous manner and the safety function cannot be correctly executed.
Depending on the SIL, the PFH must not exceed certain values for the entire safety-related system.
The individual PFH values of a function chain are added. The result must not exceed the maximum value specified in the standard.
|
Safety Integrity Level |
Average Frequency of Dangerous Failure* [h-1] (PFH) at High Demand or Continuous Demand |
|---|---|
|
4 |
10-9 ≤ — < 10-8 |
|
3 |
10-8 ≤ — < 10-7 |
|
2 |
10-7 ≤ — < 10-6 |
|
1 |
10-6 ≤ — < 10-5 |