S5-115F Manual
Reliability, Availability and Safety of Electronic Control Systems
9.1.3
Failure Distribution
Despite the extensive measures described above, one must still reckon with the occurrence of
failures. Experience has shown that, in installations with programmable controllers, failures can be
distributed approximately as follows:
Enhancement of
availability by pro-
grammed diagnostic
functions
90%
10%
95%
5%
25%
each
Failures
Internal
failures
Central
functions
Input/out-
put modules
Bus
system
External
failures
Processor
Memory
Power
supply
Central
unit
Control
system
Plant
Figure 9-2. Distribution of Failure Occurrences in Installations Incorporating Programmable
Controllers
Significance of failure 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.
9.2
Availability of Electronic Control Systems
9.2.1
Overview
Availability ”V” is the probability of findig a system in a functional state at a specified point in
time.
V =
MTBF
MTBF+MTTR
MTBF=
Mean time between failures;
MTTR=
Mean time to repair;
Ideal availability, i.e. V=1, can never be attained owing to the residual failure probability that
always exists.
EWA 4NEB 811 6148-02
9-3
