BECKHOFF TwinSAFE - Manual (Page 119)

Circuit examples
Application Guide TwinSAFE 117
results in a
ܲܨܪ ൌ
0.1 כ ݊
௢௣
כ ሺ1 െ ܦܥሻ
ܤ10
ௗ
ൌ
1 െ DC
MTTF
ୢ
S1:
ܲܨܪ ൌ
1 െ 0.99
1358.7 כ 8760
ൌ 8.40E െ 10
S2:
ܲܨܪ ൌ
1 െ 0.90
2717.4 כ 8760
ൌ 4.20E െ 09
K1/K2: actuation 1x per shift and direct feedback
ܲܨܪ ൌ
1 െ 0.99
1766.3 כ 8760
ൌ 6.46E െ 10
The following assumptions must now be made:
Safety switch S1: According to BIA report 2/2008, error exclusion to up to 100,000 cycles is possible,
provided the manufacturer has confirmed this. If no confirmation exists, S1 is included in the calculation
as follows.
Relays K1 and K2 are both connected to the safety function. The non-functioning of a relay does not lead
to a dangerous situation, but it is discovered by the feedback. Furthermore, the B10d values for K1 and
K2 are identical.
There is a coupling coefficient between the components that are connected via two channels. Examples
are temperature, EMC, voltage peaks or signals between these components. This is assumed to be the
worst-case estimation, where ß =10%. EN 62061 contains a table with which this ß-factor can be
precisely determined. Further, it is assumed that all usual measures have been taken to prevent both
channels failing unsafely at the same time due to an error (e.g. overcurrent through relay contacts, over
temperature in the control cabinet).
This produces for the calculation of the PFH value for block 1:
PFH
tot
= PFH(S1) + β* (PFH(K1)+ PFH(K2))/2 + PFH(S2) + PFH(EL1904) +
PFH(EL6900) + PFH(EL2904) + PFH (S-o-E) + PFH(S1) + PFH(EL1904) +
PFH(EL6900)
to:
PFH PFH PFH PFH
tot tot tot tot
= == = 8.40E-10+ 10%*(6.46E-10+6.46E-10) / 2 + 4.20E-09+1.11E-09 + 1.03E-09 +
1.25E-09 + 1.00E-09 + 8.40E-10+ 1.11E-09 + 1.03E-09= 1.25E 1.25E 1.25E 1.25E----08 08 08 08
The MTTF
d
value for block 1 (based on the same assumption) is calculated with:
1
ܯܶܶܨ
ௗ ௧௢௧
= ෍
1
ܯܶܶܨ
ௗ ௡
௡
௜ୀଵ