Vega VEGAFLEX 80, Safety Manual

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7
4 Safety-related characteristics
VEGAFLEX 80 series • 4 … 20 mA/HART - two-wire
42960-EN-140924
•
Continuous application temperature > 60 °C (140 °F) by factor 2.5
Similar factors apply if frequent temperature fluctations are expected.
•
The failure rates are constant. Take note of the useful service life of
the components according to IEC 61508-2.
•
Multiple errors are not taken into account
•
Wear on mechanical parts is not taken into account
•
Failure rates of external power supplies are not taken into account
•
The environmental conditions correspond to an average industrial
environment
The values for PFD
AVG
specified above were calculated as follows for a
1oo1 architecture:
PFD
AVG
= + λ
DD
x MTTR +
PTC × λ
DU
× T1
2
(1 – PTC) × λ
DU
× LT
2
•
T1 (Proof Test Interval)
•
MTTR = 8 h
•
PTC = 90 %
•
LT = 10 years
A connected control and processing unit must have the following
properties:
•
The output circuit of the transmitter is judged according to the idle
current principle
•
" fail low " and " fail high " signals are interpreted as a failure, which
triggers a fault message
If this is not the case, the respective percentages of the failure rates
must be assigned to the dangerous failures and the values stated in
chapter Safety-related characteristics “ redetermined!
In multiple channel systems for SIL3 applications, this measuring sys-
tem can also be used in a homogeneously redundant configuration.
The safety-related characteristics must be calculated especially for
the selected structure of the measuring chain using the stated failure
rates. In doing this, a suitable Common Cause Factor must be consid-
ered (see IEC 61508-6, appendix D).
Assumptions of the
FMEDA
Calculation of PFD
AVG
Boundary conditions re-
lating to the configuration
of the processing unit
Multiple channel archi-
tecture