PFH (Probability of dangerous Failure per Hour) is the probability that a system will fail dangerously, and not be able to perform its safety function when required. PFH can be determined as a probability or maximum probability over a time period of an hour. IEC 61508 and IEC 61511 use PFH as the system metric upon which the SIL is defined. Each SIL rating has an associated PFH which increases an order of magnitude for each increase in SIL rating.

The PFH is based on the dangerous failure rate, and automatic diagnostics.  

If Automatic Diagnostics are effective:

If Automatic Diagnostics are NOT effective:

Here you are assume your automatic diagnostics will not find the failure before a demand.

PFH calculation is an extremely important part of safety engineering in high and continuous demand applications as it is probably the most difficult of the three barriers to meet if realistic assumptions are made and if realistic failure rates are used (like failure rates from www.SILSafeData.com). 

Fun facts:

• PFH is defined for high or continuous demand mode of operation (for low demand mode see PFDavg)
• Target levels for PFH are defined in IEC 61508 for each of the 4 levels of SIL
• It’s one of the 3 design barriers that must be met for certification