Functional Safety K-LB-*.*, F*-LB-I, P-LB-*.*.*
Application Examples
20
21
-0
9
29
Basically the calculations are always very similar. Nevertheless it is important to understand
the application completely and to interpret the figure above correctly. The calculation
has to show that the subpart of the safety loop under consideration allows the overall safety
loop to get into the range given in the following table:
Examples of various combinations are given in the following text.
Example 1
–
Digital Input - NAMUR NE 22 Signals
When using a switch amplifier from Fuchs it is possible to implement safety loops up
to SIL 2 with a standard NAMUR NE 22 digital input signal. A sample configuration
of a surge protected device would be KCD2-SR-Ex1.LB and K-LB-*.**.
The basic requirements are:
1. Signal type of the safety loop = digital
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 = de-energized
4. Desired SIL level of the safety loop = SIL 2
Figure 7.2 Combination of K-LB-1.30 and an isolator, e. g. KCD2-SR-Ex1.LB
Has the Safety Loop SIL 2 after the Surge Protection Barrier is inserted?
•
SFF of the safety loop is as demanded for type A components > 60 %
•
As an optimum the combination of surge protection barrier and field barrier claims
10 % of the overall PFD
avg
maximum 1 x 10
-2
and therefore has a PFD
avg
< 1 x 10
-3
.
SIL level
PFD
avg
SFF
SIL 2
10
-2
to < 10
-1
60 %
SIL 3
10
-3
to < 10
-2
90 %
Table 7.1 IEC/EN 61508 characteristics for type A subcomponents
SIL level
PFD
avg
SFF
SIL 2
10
-3
to < 10
-2
90 %
SIL 3
10
-4
to < 10
-3
99 %
Table 7.2 IEC/EN 61508 characteristics for type B subcomponents
K-LB-*.*
24 V
Input
Non-hazardous area
Isolator
