DEFINITION

  PFD  =  Probability of Failure on Demand

  Variables :
    PFD   dimensionless, between 0 and 1

  Rationale : measure of reliability of a Safety Instrumented Function
  (SIF). PFD = 0.01 means that on average 1 demand out of 100 finds
  the function failed. It is the central criterion of IEC 61508 / 61511
  for low demand mode (≤ 1 demand per year — typical process : ESD,
  MOC).


SIL BANDS — IEC 61508-1 TABLE 2

  SIL 4  :  10⁻⁵  ≤  PFD  <  10⁻⁴      1 / 10 000 to 100 000 demands
  SIL 3  :  10⁻⁴  ≤  PFD  <  10⁻³      1 / 1 000 to 10 000
  SIL 2  :  10⁻³  ≤  PFD  <  10⁻²      1 / 100 to 1 000
  SIL 1  :  10⁻²  ≤  PFD  <  10⁻¹      1 / 10 to 100

  Rationale : universal classification — a safety equipment supplier
  certifies its products against these bands. SIL 4 is extremely rare
  in practice (nuclear, aviation) ; pharma / petrochemical processes
  typically target SIL 2 or SIL 3.


LINK TO RRF

  RRF  =  1 / PFD

  Variables :
    RRF   Risk Reduction Factor

  Rationale : expresses the same concept on the risk side. RRF = 100
  means the safety function divides residual risk by 100.
  PFD = 0.01 ↔ RRF = 100 ↔ SIL 1. This is the quantity that comes out
  of the LOPA analysis and sets the target requirement.