2
02
1-
10
14
Functional Safety HiD282*, HiD284*
Planning
HiD2842, HiD2844
The characteristic safety values like PFD, SFF, HFT and T
1
are taken
from the SIL report/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 and
documentation
Full assessment
Device type
A
Mode of operation
Low demand mode or high demand mode
HFT
0
SIL
2
SC
3
Safety function
Output is de-energized.
1 output repeats the input signal.
2 outputs repeat the input signal
(HiD2842 only).
s
76 FIT
89 FIT
dd
19.5 FIT
19.5 FIT
du
31.3 FIT
16.8 FIT
total (safety function)
1
1
"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.
127 FIT
125 FIT
no part
34 FIT
34 FIT
SFF
1
75.3 %
86.6 %
MTBF
2
2
acc. to SN29500. This value includes failures which are not part of the safety function/MTTR = 24 h.
The value is calculated for one safety function of the device.
367 years
311 years
PFH
3.13 x 10
-8
1/h
1.68 x 10
-8
1/h
PFD
avg
for T
1
= 1 year
3
3
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 20 years was assumed for the calculation of PFD
avg
.
1.63 x 10
-4
8.74 x 10
-5
PFD
avg
for T
1
= 2 years
3
2.99 x 10
-4
1.60 x 10
-4
PFD
avg
for T
1
= 5 years
3
7.06 x 10
-4
3.78 x 10
-4
PTC
99 %
Reaction time
4
4
Time between fault detection and fault reaction
< 20 ms
Table 3.3
