Beckhoff EL72 1-901 Series Manual page 184

Integrated safety
The following assumptions must now be made:
The door switches S1/S2 are always actuated in opposite directions. Since the switches have different
values, but the complete protective door switch consists of a combination of normally closed and normally
open contacts and both switches must function, the poorer of the two values (S1) can be taken for the
combination!
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, overtemperature in the
control cabinet).
This produces for the calculation of the PFH value for block 1:
PFHtot = β* (PFH(S1)+ PFH(S2))/2 + PFH(EL1904) + PFH(EL6900) + PFH(EL2904) + PFH(EL7201-9014)
to:
PFHtot = 10%* (1.68E-09+1.68E-09)/2 +1.11E-09 + 1.03E-09 + 1.25E-09 + 0.00 = 3.558E-09
The MTTF value for block 1 (based on the same assumption) is calculated with:
d
as:
with:
If only PFH values are available for EL1904, EL2904 and EL6900, the following estimation applies:
184 Version: 1.9 EL72x1-901x