2
02
1-
09
32
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:
For the corresponding surge protection barrier P-LB-1.A the following characteristics
are given:
The combination of both devices is done by simply adding the characteristics again as before.
The SFF and PFD
avg
values can be calculated by using the equations given above.
This means that if compared with table 1 for this specific combination the requirements
for a SIL 2 safety loop are still easily fulfilled.
Total failure rate
348 FIT
Dangerous undetected failure rate
67 FIT
PFD
avg
for T
1
= 1 year
2.93 x 10
-4
SFF
80 %
Table 7.9 Characteristics of isolator KCD2-STC-Ex1
Total failure rate
16 FIT
Dangerous undetected failures
0 FIT
Table 7.10 Failure rates of surge protection barrier P-LB-1.A
Total failure rate
364 FIT (rounded)
Dangerous undetected failure rate
67 FIT
PFD
avg
for T
1
= 1 year
2.93 x 10
-4
SFF
81 %
Table 7.11 Sum of failure rates of isolator KCD2-STC-Ex1 and surge protection barrier P-LB-1.A
