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5 Safety-related applications
Low-demand mode
The following PFD
T [PROOF]
PFD
avg
T [PROOF]
PFD
avg
High-demand mode
The following PFH value (high demand rate) results from the fault rates:
PFH = 6.21 × 10
The requirements when it comes to the PFH value for a SIL2 system are therefore met.
The values for 1, 2, and 3 years mean that the calculated PFD
for SIL2 according to table 2 from IEC/EN 61508-1. They meet the requirement of not covering more
than 10 % of the safety circuit or are better than or equal to 1.00 × 10
The values for 4, 5, 6, 7, 8, and 10 years mean that the calculated PFD
sible range for SIL2 according to table 2 from IEC/EN 61508-1. However, they do not meet the require-
ment of not covering more than 10 % of the safety circuit/are not better than or equal to 1.00 × 10
Failure limit value
The operating mode with a low-demand rate is taken as the basis. The proportion of the device at PFH/
PFD of the entire safety loop is less than 10 %.
Safety circuit according to IEC/EN 61508-1
Sensor
25 %
5.3
2-channel operation as a repeater power supply and input isolating am-
plifier for SIL3 (1oo2) applications
5.3.1
Safety/integrity requirements
Fault rates
•
Type A device (according to IEC/EN 61508-2)
•
Safety integrity level (SIL) 3 (1oo2 structure)
•
Systematic capability (SC) 3 (1oo2 structure)
•
HFT 1
•
MTTR 24 h
•
MTR 8 h
•
Mission time 10 years
•
Safety response time 2 s
•
Ambient temperature 40 °C
•
Proof test coverage (PTC) 97 %
•
Process safety time 500 ms
•
Switch-on duration 2 s
•
ß factor 2 % (Beta D)
Two devices have to be used to achieve a SIL3 level. The two outputs of the devices to the control side
must be connected to the same evaluation unit (PLC/PLS). The evaluation unit must compare the two
values with the max. permissible deviation of 2 % of the input signal.
20
values (low demand rate) result from the fault rates.
avg
1 year
2 years
-4
2.73 × 10
5.45 × 10
6 years
7 years
-3
1.64 × 10
1.91 × 10
-8
/h
Device
< 10 %
3 years
-4
-4
8.17 × 10
8 years
-3
-3
2.18 × 10
values are within the permissible range
avg
Processing
15 %
4 years
5 years
-3
1.09 × 10
1.37 × 10
10 years
-3
2.73 × 10
-3
.
values are within the permis-
avg
Actuator
50 %
-3
-3
.
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