2
019-
02
20
Functional Safety M-LB-(Ex-)2000-System
Application Examples
Example 1 - Digital Input - NAMUR NE 22 Signals
When using a standard isolated barrier from Fuchs, it is possible to implement low
demand safety loops with SIL 2 with a standard NAMUR NE 22 digital input signal.
A sample configuration would be the isolated barrier KCD2-SR-Ex1.LB with surge protection
barrier M-LB -Ex-2***.
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): input
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
Figure 7.2
Sample configuration consisting of a surge protection barrier and an isolated barrier
SIL level calculation of the safety loop
•
SFF value of the safety loop is as demanded for type A components > 60 %.
•
As an optimum, the combination of surge protection barrier and isolated barrier claims
10 % of the overall PFD
avg
maximum 1 x 10
-2
and therefore has a PFD
avg
<1 x 10
-3
The isolated barrier KCD2-SR-Ex1.LB has the following performance values:
Use for calculation of
du
of the isolated barrier the PFD
avg
formula given in IEC 61508:
•
PFD
avg
= 1/2 x
du
x T
1
•
du
= 2 x PFD
avg
/ T
1
= 2 x 2.05 x 10
-4
/ 8760 [h] = 47 FIT
total
254 FIT
PFD
avg
for T
1
= 1 year
2.05 x 10
-4
SFF
81.5 %
Table 7.1
24 V
Input
Non-hazardous area
Hazardous area
Isolated barrier
Surge protection
barrier
