Siemens SIMATIC S5-115U, User Manual

Page: 523 / 676

S5-115U Manual Reliability, Availability and Safety of Electronic Control Equipment
14.1.3 Failure Distribution
Despite the extensive measures described above, one must still reckon with the occurence of
failures. Experience has shown that, in installations with programmable controllers, failures can
be distributed approximately as follows:
Figure 14-2. Distribution of Failure Occurrences in Installations Incorporating Programmable
Controllers
Enhancement of
availability by
programmed
diagnostic functions
90 %
10 %
95 %
5 %
each
25 %
Failures
Internal
failures
Central
functions
Input/Out-
put modules
Bus
system
External
failures
Processor
Memory
Power
supply
Central
unit
Control
system Plant
Meaning of error distribution:
• Only a small number (approx. 5 %) of failures occur inside the electronic control system. These
can be broken down as follows:
-
CPU failures (about 10 %, i.e. only 0.5 % of all failures);
these failures are evenly divided among the processor, memory, bus system and power
supply.
- I/O module failures (about 90%, i.e. only 4.5 % of all failures)
• The highest number of all failures (about 95 %) occur in the sensors, actuators, drives, cabling
etc.
EWA 4NEB 811 6130-02b
14-3