BECKHOFF TwinSAFE - Manual (Page 11)

Circuit examples
Application Guide TwinSAFE 9
Inserting the values, this produces:
S1:
݊
௢௣
ൌ
230 כ 16 כ 60
10080
ൌ 21.90
ܯܶܶܨ
ௗ
ൌ
100000
0.1 כ 21.90
ൌ 45662.1y ൌ 399999120h
K1/K2:
݊
௢௣
ൌ
230 כ 16 כ 60
10080
ൌ 21.90
ܯܶܶܨ
ௗ
ൌ
1300000
0.1 כ 21.90
ൌ 593607.3y ൌ 5199997320h
and the assumption that S1, K1 and K2 are in each case single-channel:
ܯܶܶܨ
ௗ
ൌ
1
ߣ
ௗ
results in a
ܲܨܪ ൌ
0.1 כ ݊
௢௣
כ ሺ1 െ ܦܥሻ
ܤ10
ௗ
ൌ
1 െ DC
MTTF
ୢ
S1:
ܲܨܪ ൌ
1 െ 0.99
45662.1 כ 8760
ൌ 2.50E െ 11
K1/K2:
Actuation 1x per week
ܲܨܪ ൌ
1 െ 0.60
593607.3 כ 8760
ൌ 7.69E െ 11
K1/K2:
Actuation 1x per shift
ܲܨܪ ൌ
1 െ 0.90
593607.3 כ 8760
ൌ 1.92E െ 11
The following assumptions have to be made now:
Safety switch S1: According to BIA report 2/2008, error exclusion 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 signal. 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).