Operating Manual Owner manual

6 Safety Manual
57
2013-04-01
The diagram above shows the different possible combinations for assessing the category with DC
avg
(horizontal axis) and the
MTTF
d
of each channel (bars). The bars in the diagram show the three MTTF
d
areas of each channel (low, medium, and high)
that can be selected to achieve the required PL.
Before the simplified procedure from the diagram shown is applied (which shows the results of different Markov models on the
basis of intended architectures from section 6), the category of the SRP/CS and the DC
avg
and the MTTF
d
of each channel
must have been determined (see section 6 and Appendices C to E).
For categories 2, 3, and 4, sufficient measures against failures due to combined failures must be fulfilled (see Appendix F). Tak-
ing these parameters into account, the diagram represents a graphical procedure for determining the PL achieved by the SRP/
CS. The combination of categories (including failures due to combined failures) and DC
avg
determines which bar to choose in
Figure 5. One of the three differently shaded areas of the relevant bar must be chosen according to the MTTF
d
of each channel.
The vertical position of these areas determines the achieved PL, which can be read off the vertical axis. If the area covers two
or three possible PLs, the achieved PL is specified in Table 7. For information on accurate selection of the PL on the basis of
the precise value of the MTTF
d
for each channel, see Appendix K.