This article elaborates on the basic definition and use of HFT, SFF, and PFDavg terms which are widely used during the SIL verification process.
IEC 61511 defines the safety life cycle where in SIL verification is part of phase 4 (SIS design and engineering).
Before this step Hazard and risk analysis, allocation of safety functions to protection layers, SIS safety requirement specification phases are completed.
SIL (Safety Integrity Level)
A quantitative target for measuring the level of safety in a process.
Defining a target SIL level for the process should be based on the assessment of the likelihood that an incident will occur and the consequences of the incident.
HFT (Hardware Fault Tolerance)
HFT is the ability of equipment to continue to perform the required function in presence of faults or errors.
HFT of device indicates the quality of safety system.
HFT is N means N+1 faults could result into loss of entire safety function.
HFT is 0 means 1 fault can cause loss of entire safety function
(e.g. 1oo1 pressure transmitter used in SIF). Loss of this transmitter will result in the loss of the entire safety loop.
HFT is 1 means 2 faults can cause loss of entire safety function
(e.g. 1oo2 voting)
Following table illustrate the HFT of various voting configuration. So the HFT of XooY = Y-X
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