2
018-
02
12
Functional Safety KCD2-UT2-(Ex)1, HiC2081
Planning
3.4
Characteristic Safety Values
The characteristic safety values like PFD, PFH, SFF, HFT and T
1
are taken from the
FMEDA report. Observe that PFD and T
1
are related to each other.
The function of the devices has to be checked within the proof test interval (T
1
).
Parameters
Characteristic values
Assessment type
FMEDA report with proven-in-use assessment
1
1
For the proven-in-use demonstration, sales figures, customer returns and questionnaires filled out by customers were used which
show that no unknown systematic faults are expected. The device is based on a former device that was evaluated for
a proven-in-use statement by exida.com GmbH.
Device type
B
Mode of operation
Low demand mode or high demand mode
HFT
0
SIL (SC)
2
Safety function
Transfer of analog signal
s
2
2
For transfer from the FMEDA report the following rules apply: "Annunciation failures" have no direct influence on the safety function
and are therefore counted as "No effect failures". "No effect failures" are not influencing the safety function and are therefore not
included in SFF and in the failure rates of the safety function.
20.1 FIT
dd
293 FIT
du
69 FIT
no part
27.6 FIT
total (safety function)
2
382 FIT
SFF
2
81,8 %
PTC
90 %
MTBF
3
3
acc. to SN29500. This value includes failures which are not part of the safety function/MTTR = 8 h. The value is calculated for one
safety function of the device.
162 years
PFH
1
6.93 x 10
-8
1/h
PFD
avg
for T
1
= 1 year
4
4
Since the current PTC value is < 100 % and therefore the probability of failure will increase, calculate the PFD value according to the
following formula:
PFD
avg
= (
du
/ 2) x (PTC x T
1
+ (1
–
PTC) x T
service
)
A service time T
service
of 10 years was assumed for the calculation of PFD
avg
.
5.79 x 10
-4
1/h
PFD
avg
for T
1
= 2 years
4
8.52 x 10
-4
1/h
Reaction time
5
5
Time between fault detection and fault reaction
• for temperature input: < 1.5 s
• for potentiometer input: < 3 s
Table 3.1
