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Functional Safety K-LB-*.*, F*-LB-I, P-LB-*.*.*
Application Examples
7
Application Examples
All characteristics used in the following chapter were actual when this safety manual
was released but may vary. The main purpose of this section is to show how to include
a surge protected device into a safety loop.
Integration of a Surge Protection Barrier into a Safety Loop
The general considerations given above lead to the conclusion that it is mandatory
before starting any safety loop calculation to have a clear understanding of:
1. the signal type of the safety loop (analog, digital),
2. the signal direction of the safety loop as seen from the perspective of the safety DCS
(input or output),
3. the safe state of the field device allocated to the surge protection barrier,
4. the desired SIL level of the safety loop.
After the safety loop under consideration is defined, a surge protection barrier can be
integrated into this safety loop by allocating it to a field device. It should be possible to get
a principled overview as shown below.
Sensor
Sensor
1
2
3
4
5
6
7
8
+
PFD
1
25 %
35 %
Sensor system and signal path
Figure 7.1
In principle the IEC/EN 61508 characteristics of the surge protection barrier have to be added
to the IEC/EN 61508 characteristics of the field device. By doing so it is assumed
that the surge protection barrier becomes an integral part of this device.
After that the Probability of Failure on Demand (PFD
(SFF) have to be recalculated. With these new characteristics it can be verified if the desired
SIL level can be reached.
28
Binary
Logic system
Analog
input
1
2
3
4
5
6
1
2
3
4
7
8
9
10
11
12
13
14
15
5
6
7
8
+
PFD
2
10 %
Signal path
Example of a complete safety loop with allocated surge protection barriers
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
+
PFD
3
Signal path
15 %
Safety PLC
) and the Safe Failure Fraction
avg
Binary
Actuator
Analog
Actuator
output
1
2
1
2
3
4
3
4
5
6
5
6
7
8
7
8
+
PFD
PFD
4
5
10 %
40 %
50 %
Actuator and signal path
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