BECKHOFF TwinSAFE - Manual (Page 125)

Circuit examples
Application Guide TwinSAFE 123
The following assumptions have to be made now:
Safety switch S1: According to BIA report 2/2008, error exclusion to up 100,000 cycles is possible, provided
the manufacturer has confirmed this. If no confirmation exists, S1 is included in the calculation as follows.
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)+ PFH(EL1904) + PFH(EL6900) + PFH(EL2904) + β*
(4*PFH(AX5801)+ 4*PFH(AX5801))/2
to:
PFH PFH PFH PFH
tot tot tot tot
==== 2.10E-09 +1.11E-09 + 1.03E-09 + 1.25E-09 + 10%*(4*2.70E-10 + 4*2.70E-
10)/2 = 5.60E 5.60E 5.60E 5.60E----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
ௗ
(ܵ1) +
1
ܯܶܶܨ
ௗ
(ܧܮ1904) +
1
ܯܶܶܨ
ௗ
(ܧܮ6900) +
1
ܯܶܶܨ
ௗ
(ܧܮ2904)
+
1
ܯܶܶܨ
ௗ
(ܣܺ5801)
+
1
ܯܶܶܨ
ௗ
(ܣܺ5801)
+
1
ܯܶܶܨ
ௗ
(ܣܺ5801)
+
1
ܯܶܶܨ
ௗ
(ܣܺ5801)
with:
ܯܶܶܨ
ௗ
(S1) =
ܤ10
ௗ
(ܵ1)
0.1 כ ݊
௢௣
ܯܶܶܨ
ௗ
(AX5801) =
ܤ10
ௗ
(ܣܺ5801)
0.1 כ ݊
௢௣
If only PFH values are available for EL1904 and EL6900, the following estimation applies:
ܯܶܶܨ
ௗ
(ELxxxx) ൌ
(
1 െ ܦܥ(ܧܮݔݔݔ))
ܲܨܪ(ܧܮݔݔݔ)