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.

Random Failure Probability

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 

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

Related Items

Back to Basics 01 - Functional Safety

Back to Basics 02 - Safety Integrity Level (SIL)

Back to Basics 03 - Safety Instrumented Function (SIF)

Back to Basics 04 - Safety Instrumented System (SIS)

Back to Basics 05 - What is a Safety Function?

Back to Basics 06 – IEC 61508

Back to Basics 07– Safety Lifecycle – IEC 61508

Back to Basics 08 – IEC 61511

Back to Basics 09 – Safety Lifecycle – IEC 61511

Back to Basics 10 – How Does a Product Get a SIL?

Back to Basics 11 – How is SIL Used by an End User?

Back to Basics 12 – What is IEC 61508 Certification?

Back to Basics 13 - How Do I Start IEC 61508 Certification?

Back to Basics 14 - Systematic Capability

Back to Basics 15 - Architectural Constraints

Back to Basics 16 - PFDavg

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