VEGA VEGAPULS 56K - Manual (Page 60)

60 VEGAPULS 56K
22915-EN-041227
Supplement
2 Planning
2.1 Low demand mode
If the demand rate is only once a year, then the measuring system can be used as safety-
relevant subsystem in „low demand mode“ (see IEC 61508-4, 3.5.12).
Corresponding characteristics : PFD
avg
(average probability of dangerous failure on demand).
It is dependent on the test interval T
Proof
between the function tests of the protective function.
2.2 High demand or continuous mode
If the demand rate is more than once a year, the measuring system must be used as safety-
relevant subsystem in „high demand or continuous mode“ (see IEC 61508-4, 3.5.12).
The fault reaction time of the measuring system must be set to less than or at the most, equal
to the fault tolerance time of the total system. The smallest fault reaction time of the measuring
system is 60 sec.
Corresponding characteristics: PFH (probability of a dangerous failure per hour)
2.3 General
The safe condition of the measuring system in case of failure is defined for both modes of
operation at an output current > 21 mA. An output current < 3.6 mA and > 21 mA must be
configured as alarm in the processing unit.
Definition of a dangerous undetected failure:
- the instrument does not react to the demand of the process
- the output current deviates by more than 2 % from the actual value.
Otherwise the tolerance data in the operating instructions manual are applicable. Make sure
that the measuring system is used as it was intended for the application (see operating in-
structions manual). The application-specific limits must be complied with and the specifications
must not be exceeded (see operating instructions manual). Use only in products against
which the materials of the antenna system are sufficiently chemically resistant.