Functional Safety M-LB-(Ex-)2000-System
Application Examples
201
9-02
23
Example 3 - Analog Inputs
–
4 mA to 20 mA Signals
Basic parameters:
1. Signal characteristic of the safety loop: analog 2-wire application
2. Signal direction of the safety loop as seen from the perspective of the safety-rated
programmable logic controller (SPLC): input
3. Safe state of the field device allocated to the surge protection barrier:
output signal < 4 mA or > 20 mA
4. Required SIL level of the safety loop: SIL 3
The isolated barrier KCD2-STC-Ex1 has the following performance values:
The corresponding surge protection barrier M-LB-Ex-2*** has the following
performance values:
Use these values for calculating the SFF and PFD
avg
for the combination of both devices.
Use for calculation the PFD
avg
formula given in IEC 61508, see above.
The following table summarizes the results of the calculations:
The requirements for a SIL 3 safety loop are still fulfilled for this specific combination.
total
348 FIT
du
67 FIT
PFD
avg
for T
1
= 1 year
2.93 x 10
-4
SFF
80.8 %
Table 7.7
total
18.3 FIT
du
1.1 FIT
Table 7.8
total
366 FIT
du
68 FIT
PFD
avg
for T
1
= 1 year
2.98 x 10
-4
SFF
81.4 %
Table 7.9
