6 Safety Manual
39
6.5.5 Calculation of PFD
avg
The operator defines the following topics:
- Proof test interval T
i
- Planned operating duration mission time T
M
and
- the PTC-value for the performed Proof test (A,B or C)
The mission time T
M
has to be at least equal the time for the proof test interval T
i
, but not exceed the Lifetime of 10 years.
This must be taken into consideration when evaluating the probability of a dangerous failure PFD
avg
of the sensor system.
In the case of a single-channel system architecture, the mean probability of a dangerous failure PFD
avg
of the transmitter can
be calculated from the following formula:
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