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