Functional Safety 2016
November, 2016 - London
Page 5 of 17
How to Calculate PFD
AVG
and PFD
AVG
including
PFD
AVG
for a basic 1oo1 system including C
PT
:
!"#
=
!"
!"#$%
×
!"
× 2
+
!"
!"#$%
× 1 −
!"
× 2
(1)
(Abdelrhafour, Bajaj, & Boily, 2012)
Where:
!"
!"#$%
is the total dangerous undetected failure rate;
!"
is the proof test coverage;
is the proof test interval
is the mission time of the equipment
C
PT
is calculated by:
!"
=
!"
!"#$%&'&#" !" !"
!"
!"#$%
(2)
(Abdelrhafour, Bajaj, & Boily, 2012)
Where:
!"
!"#$%&'&#" !" !"
is the dangerous undetected failure rate identified by the proof test.
!"
is undetected dangerous failure rate
!!
is dangerous systematic failure rate
Table 1: Simplified 1oo1 PFD
AVG
Equations
Architecture
Simplified PFD
AVG
1oo1
!
!"
×!" !
+
!
!!
×!" !
(ISA, 2002)
(3)
1oo1 including
C
PT
!"#
=
!
!" !"#$%
×!
!"
×!"
!
+
!
!" !"#$%
× !!!
!"
×!"
!
(Abdelrhafour, Bajaj, & Boily, 2012)
(4)