DEFINITION

  RRF  =  Risk Reduction Factor

  Variables :
    RRF   dimensionless, typically between 10 and 10⁵

  Rationale : expresses by how much the safety function divides the
  residual risk. A SIS with RRF = 1 000 reduces the frequency of the
  hazardous event by a factor of 1 000. It is the quantity that comes
  out of LOPA analyses and risk graphs.


LINK TO PFD

  RRF  =  1 / PFD

  Rationale : two ways of expressing the same thing. PFD is the
  failure probability ; RRF is its inverse, more intuitive on the
  risk side ("by how much it is divided"). PFD = 0.001 ↔ RRF = 1 000.


SIL BANDS — IEC 61508, LOW DEMAND MODE

  SIL 1  :       10  ≤  RRF  <      100
  SIL 2  :      100  ≤  RRF  <    1 000
  SIL 3  :    1 000  ≤  RRF  <   10 000
  SIL 4  :   10 000  ≤  RRF  <  100 000

  Rationale : direct equivalent of the PFD bands, but inverted —
  higher = better. SIL 2 covers common functions in petrochemical
  plants ; SIL 3 is typical of critical ESDs (toxic gases, BLEVE) ;
  SIL 4 extremely rare (nuclear, civil aviation).


REQUIRED vs ACHIEVED — DO NOT CONFUSE

  Required RRF   →  from LOPA / risk graph                  (the target)
  Achieved RRF   →  from the PFD calculation of the SIS     (the result)

  Rationale : LOPA sets the requirement ("we need RRF ≥ 1 000 for this
  risk"). The PFD calculation gives what the designed SIS actually
  achieves. Achieved RRF must be ≥ Required RRF to validate the
  claimed SIL level. This is the foundation of the IEC 61511 Safety
  Lifecycle.