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Functional Safety M-LB-(Ex-)4000-System
Planning
3.2
Assumptions
The following assumptions have been made during the FMEDA:
•
The device will be used under average industrial ambient conditions comparable
to the classification stationary mounted according to MIL-HDBK-217F.
Alternatively, operating stress conditions typical of an industrial field environment similar
to IEC/EN 60654-1 Class C with an average temperature over a long period of time of 40 ºC
may be assumed. For a higher average temperature of 60 ºC, the failure rates must
be multiplied by a factor of 2.5 based on experience. A similar factor must be used
if frequent temperature fluctuations are expected.
•
External power supply failure rates are not included.
•
Propagation of failures is not relevant.
•
Failure rates are constant, wear is not considered.
•
The control loop has a hardware fault tolerance of 0 and it is a type A device.
A SFF value for this device is not given, since this value has to be calculated in conjunction
with the connected field device, as described in the following section.
Application
The surge protection barrier and the connected device (field device, isolator or actuator) have
to be considered in combination. The PFD
safety loop is:
•
Actuator (valve) 40 %
•
Transmitter (sensor) 25 %
•
Isolator 10 %
As an overview for the SIL2 or SIL3 safety loop this means:
Device category
Total
Actuator (40 %)
Transmitter (25 %)
Isolator (10 %)
Table 3.1
Overview PFD
SIL 2
PFH
-6
10
-7
4 x 10
-7
2.5 x 10
-7
10
/PFH budget
avg
/PFH budget of the device categories in the entire
avg
SIL 3
PFD
PFH
avg
-2
-7
10
10
-3
-8
4 x 10
4 x 10
-3
2.5 x 10
2.5 x 10
-3
-8
10
10
PFD
avg
-3
10
-4
4 x 10
-8
-4
2.5 x 10
-4
10
11
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