38477BA2B
1 – 72 25.5.09
9 Functional Safety Volume 1
1. For common cause ß = 2%
PFD
1002
= (PFD
1001
)
2
+ ß x PFD
1001
2. For common cause ß = 5%
Time-dependent trend of PFD
avg
The time trend of PFD
avg
is nearly linear to the operating time in
the period up to 10 years. The above mentioned values apply only
to the T
Proof
interval, according to which a recurrent performance
check has to be carried out.
Time-dependent trend of PFD
avg
1 PFD
avg
= 0
2 PFD
avg
after 1 year
3 PFD
avg
after 5 years
It is in the interest of the operator of a facility to verify the safety
function quantitatively.
PFD
avg
T
Proof
= 1 year
T
Proof
= 2 years
T
Proof
= 5 years
<6.5 x 10
-6
<13.2 x 10
-6
<34.5 x 10
-6
PFD
avg
T
Proof
= 1 year
T
Proof
= 2 years
T
Proof
= 5 years
<1,6 x 10
-5
<3,2 x 10
-5
<8.2 x 10
-5
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