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Functional Safety K-LB-*.*, F*-LB-I, P-LB-*.*.*
Application Examples
Example 3 – Analog Inputs – 4 mA to 20 mA Signals
The basic requirements are:
1. Signal type of the safety loop = analog
2. Signal direction of the safety loop as seen from the perspective of the safety DCS = input
3. Safe state of the field device allocated to the surge protection barrier = output signal
< 4 mA or respectively > 20 mA
4. Desired SIL level of the safety loop = SIL 3
A standard KCD2-STC-Ex1 device has the following characteristics:
Total failure rate
Dangerous undetected failure rate
PFD
avg
SFF
Table 7.9
For the corresponding surge protection barrier P-LB-1.A the following characteristics
are given:
Total failure rate
Dangerous undetected failures
Table 7.10 Failure rates of surge protection barrier P-LB-1.A
The combination of both devices is done by simply adding the characteristics again as before.
The SFF and PFD
Total failure rate
Dangerous undetected failure rate
PFD
avg
SFF
Table 7.11 Sum of failure rates of isolator KCD2-STC-Ex1 and surge protection barrier P-LB-1.A
This means that if compared with table 1 for this specific combination the requirements
for a SIL 2 safety loop are still easily fulfilled.
32
for T
= 1 year
1
Characteristics of isolator KCD2-STC-Ex1
values can be calculated by using the equations given above.
avg
for T
= 1 year
1
348 FIT
67 FIT
-4
2.93 x 10
80 %
16 FIT
0 FIT
364 FIT (rounded)
67 FIT
-4
2.93 x 10
81 %
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