2
019-
02
22
Functional Safety M-LB-(Ex-)2000-System
Application Examples
Example 2 - Digital Output
–
Solenoid Drivers
The Fuchs digital output devices can be categorized into two groups
–
loop powered
devices and bus powered devices.
The calculation for both groups is identical except that values of the solenoid driver
may be different. The method for calculation is identical to the method in the previous example.
Basic parameters:
1. Signal characteristic of the safety loop: 2-wire application
2. Signal direction of the safety loop as seen from the perspective of the safety-rated
programmable logic controller (SPLC): output
3. Safe state of the field device allocated to the surge protection barrier: de-energized
4. Required SIL level of the safety loop: SIL 2
The isolated barrier 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 2 safety loop are still fulfilled for this specific combination.
total
714 FIT
du
10.3 FIT
PFD
avg
for T
1
= 1 year
4.51 x 10
-5
SFF
98.5 %
Table 7.4
total
18.3 FIT
du
1.1 FIT
Table 7.5
total
732 FIT
du
11.4 FIT
PFD
avg
for T
1
= 1 year
4.97 x 10
-5
SFF
98.4 %
Table 7.6
