2
02
1-
09
30
Functional Safety K-LB-*.*, F*-LB-I, P-LB-*.*.*
Application Examples
The IEC/EN 61508 characteristics of the KCD2-SR-Ex1.LB are:
For further calculation
du
of the KCD2-SR-Ex1.LB has to be calculated by using
the PFD
avg
formula given in IEC/EN 61508:
•
PFD
avg
= 1/2 x
du
x T
1
(1)
•
du
= 2 x PFD
avg
/ T
1
= 2 x 2.05 x 10
-4
/ 8760 [h] = 47 FIT
For the surge protection barrier K-LB-1.30 the following characteristics are given:
The next step is to allocate the surge protection barrier to the device by adding the total failure
rates of both components.
•
total
=
Isolator
+ ... +
Surge protection barrier
(2)
•
total
= 254 FIT + 16 FIT = 270 FIT (rounded)
The same has to be done for the dangerous undetected failure rates of both devices.
•
du
=
du/Isolator
+ ... +
du/Surge protection barrier
(3)
•
du
= 47 FIT + 0 FIT = 47 FIT
These characteristics allow calculating the SFF and PFD
avg
for the combination
of both devices.
•
SFF = 1
–
(
du
/
total
) (4)
•
SFF = 1
–
(47 FIT / 270 FIT) = 82 %
•
PFD
avg_1y
= 1/2 x
du
x 8760 [h] (5)
•
PFD
avg_1y
= 1/2 x 47 FIT x 8760 [h] = 2.05 x 10
-4
The following table summarizes the results of the calculations:
This means that for this specific combination the requirements for a SIL 2 safety loop
are still fulfilled.
Total failure rate
254 FIT
PFD
avg
for T
1
= 1 year
2.05 x 10
-4
SFF
81.5 %
Table 7.3 Characteristics of isolator KCD2-SR-Ex1.LB
Total failure rate
16.1 FIT
Dangerous undetected failure rate
0 FIT
Table 7.4 Failure rates of surge protection barrier K-LB-1.30
Total failure rate
270 FIT
Dangerous undetected failure rate
47 FIT
PFD
avg
for T
1
= 1 year
2.05 x 10
-4
SFF
82 %
Table 7.5 Sum of failure rates of isolator KCD2-SR-Ex1.LB and surge protection barrier K-LB-1.30
