Vega VEGABAR 54 Operating Instructions Manual page 57

Low demand mode
High demand mode
Assumptions
VEGABAR 54 - 4 ... 20 mA/HART
If this is not the case, the appropriate shares of the failure rates
must be assigned to the dangerous failures. Therefore the
stated values in chapter "Safety-relevant characteristics" can
deteriorate.
If the demand rate is only once a year, then the measuring
system can be used as safety-relevant subsystem in "low
demand mode" (IEC 61508-4, 3.5.12).
If the ratio of the internal diagnostics test rate of the measuring
system to the demand rate exceeds the value 100, the
measuring system can be treated in the way it is executing a
safety function in the mode with low demand rate (IEC 61508-2,
7.4.3.2.5).
Corresponding characteristics is the value PFD
Probability of dangerous Failure on Demand). It is dependent
on the test interval T
protective function.
Numbers see paragraph "Safety-technical characteristics".
If the "low demand mode" does not apply, the measuring
system must be used as safety-relevant subsystem in "high
demand" (IEC 61508-4, 3.5.12).
The fault tolerance time of the complete system must be higher
than the sum of the reaction times or the diagnostics test
periods of all components in the safety chain.
Corresponding characteristics is the value PFH (failure rate).
Numbers see paragraph "Safety-technical characteristics".
The following assumptions form the basis for the implemen-
tation of FMEDA:
Failure rates are constant, wear of the mechanical parts is
l
not taken into account
Failure rates of external power supplies are not included
l
Multiple errors are not taken into account
l
The average ambient temperature during the operating
l
time is +40°C (104°F)
The environmental conditions correspond to an average
l
industrial environment
The lifetime of the components is around 8 to 12 years
l
(IEC 61508-2, 7.4.7.4, remark 3)
The processing unit can interpret "fail low" and "fail high"
l
failures as interferences and output a suitable fault signal
Functional safety acc. to IEC 61508-4 (SIL)
between the function tests of the
Proof
(average
avg
57