Table of Contents
Advertisement
Determination of the
failure rates
Assumptions of the
FMEDA
Calculation of PFD
AVG
Configuration of the pro-
cessing unit
Multiple channel archi-
tecture
Parameter
Performance Level
4.3
Supplementary information
The failure rates of the instruments were determined by an FMEDA
according to IEC 61508. The calculations are based on failure rates of
the components according to SN 29500:
All figures refer to an average ambient temperature of 40 °C (104 °F)
during the operating time. For higher temperatures, the values should
be corrected:
•
Continuous application temperature > 50 °C (122 °F) by factor 1.3
•
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 failures 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
•
To avoid a fusing of the relay contacts, these must be protected by
an external fuse
The values for PFD
AVG
1oo1 architecture:
PTC × λ
PFD
=
AVG
2
Parameters used:
•
T1 = Proof Test Interval
•
PTC = 90 %
•
LT = 10 years
•
MTTR = 8 h
A connected control and processing unit must have the following
properties:
•
The failure signals of the measuring system are judged according
to the idle current principle
•
"fail low" and "fail high" signals are interpreted as a failure, where-
upon the safe state must be taken on
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!
Due to the systematic capability SC3, this instrument can also be
used in multiple channel systems up to SIL3, also with a homogene-
ously redundant configuration.
SITRANS LVL200H - Safety Manual
Value
3.60 x 10
1/h
-8
specified above were calculated as follows for a
× T1
DU
+ λ
x MTTR +
DD
(1 – PTC) × λ
× LT
DU
2
7
Advertisement
Table of Contents
