Functional Safety M-LB-(Ex-)5000-System
Application Examples
201
8-03
21
The corresponding surge protection barrier M-LB-Ex-5*** has the following performance
values:
The next step is to allocate the surge protection barrier to the isolated barrier by adding the
total failure rates of both components.
•
total
=
isolated barrier
+ to +
surge protection barrier
•
total
= 254 FIT + 17.8 FIT
272 FIT
The same has to be done for the dangerous undetected failure rates of both devices.
•
du
=
du/isolated barrier
+ to +
du/surge protection barrier
•
du
= 47 FIT + 1.1 FIT
48 FIT
Use these values for calculating the SFF and PFD
avg
for the combination of both devices.
•
SFF = 1
–
(
du
/
total
)
•
SFF = 1
–
(48 FIT / 272 FIT) = 82 %
•
PFD
avg_1y
= 1/2 x
du
x 8760 [h] (5)
•
PFD
avg_1y
= 1/2 x 48 FIT x 8760 [h] = 2.10 x 10
-4
The following table summarizes the results of the calculations:
The requirements for a SIL 2 safety loop are still fulfilled for this specific combination.
total
17.8 FIT
du
1.1 FIT
Table 7.2
total
272 FIT
du
48 FIT
PFD
avg
for T
1
= 1 year
2.10 x 10
-4
SFF
82 %
Table 7.3
