FCU 500, FCU 505 · Edition 02.17
126
Project planning information
14 .4 .5 PFH
D
value for STM/STL temperature module
and thermocouple
This is determined pursuant to standard
EN 13611:2007+A2:2011, Section J, with a 1oo2 calculation.
The dangerous failure rate for a thermocouple is as follows:
λ
DU
thermocouple
=
FIT
discontinuity
×
(1 - DC
discontinuity
)
+
FIT
short-circuit
× (1 - DC
short-circuit
)
+
FIT
drift
× (1 - DC
drift
)
λ
DU
: Dangerous failure rate (
×
10
-9
1/h)
FIT
discontinuity
, FIT
short-circuit
, FIT
drift
: see information
from the manufacturer
DC
discontinuity
= 0.99 (99%),
DC
short-circuit
= 0.6 (66%),
DC
drift
= 0.6 (66%)
The total failure rate for the thermocouple and STM/STL
temperature module of the FCU..H1 is calculated as follows:
PFH
D total
=
λ
DU
thermocouple
+ PFH
D STM/STL temperature module
PFH
D
STM/STL temperature module
= 5.84 × 10
-9
1/h
See also page 141 (Mean probability of danger-
ous failure PFHD of individual safety functions) and
page 147 (Probability of dangerous failure PFHD)
Calculation
Enter the manufacturer’s information for the FIT values of
the thermocouple for discontinuity, short-circuit and drift
(0 = FIT value unknown):
Determine the designed lifetime (service life) of the
thermocouple:
FIT
discontinuity
FIT
short-circuit
FIT
drift
Thermocouple
lifetime
years
×
10
-9
1/h
×
10
-9
1/h
×
10
-9
1/h
×
10
-9
1/h
×
10
-9
1/h
λ
DU
thermocouple
PFH
D total
250
133
0
10,00
55,7
7,24