BECKHOFF TwinSAFE - 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
ௗ ௧௢௧
= ෍
1
ܯܶܶܨ
ௗ ௡
௡
௜ୀଵ
as:
1
ܯܶܶܨ
ௗ ௧௢௧
=
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: