BECKHOFF TwinSAFE User Manual
Page 137
Circuit examples
Application Guide TwinSAFE
135
2.23.3 Calculation
2.23.3.1 PFH / MTTF
d
/B10
d
– values
Component
Value
EL1904 – PFH
1.11E-09
EL2904 – PFH
1.25E-09
Days of operation (d
op
)
230
Hours of operation / day (h
op
)
8
Cycle time (minutes) (T
cycle
)
60 (1x per hour)
Lifetime (T1)
20 years = 175200 hours
2.23.3.2 Diagnostic Coverage DC
Component
Value
EL1904/EL2904
DC
avg
=90%
2.23.3.3 Calculation for block 1
This produces for the calculation of the PFH value for block 1:
PFH
tot
= PFH(EL1904) + PFH(EL2904)
to:
PFH
PFH
PFH
PFH
tot
tot
tot
tot
=
==
=
1.11E-09 + 1.25E-09 = 2.36E
2.36E
2.36E
2.36E----09
09
09
09
The MTTF
d
value for block 1 (based on the same assumption) is calculated with:
1
ܯܶܶܨ
ௗ ௧௧
=
1
ܯܶܶܨ
ௗ
ୀଵ
as:
1
ܯܶܶܨ
ௗ ௧௧
=
1
ܯܶܶܨ
ௗ
(ܧܮ1904) +
1
ܯܶܶܨ
ௗ
(ܧܮ2904)
If only PFH values are available for EL1904 and EL6900, the following estimation applies:
ܯܶܶܨ
ௗ
(ELxxxx) =
(1 െ ܦܥ(ܧܮݔݔݔ))
ܲܨܪ(ܧܮݔݔݔ)
Hence: