Siemens SIMATIC S5-115U, User Manual

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S5-115U Manual Reliability, Availability and Safety of Electronic Control Equipment
14.4 Summary
• In electronic control systems, failures of any kind can occur at any point in the system.
• Even when the greatest efforts are made to obtain maximum reliability, the probability of
such a failure occurring can never be zero.
• The following is decisive for the effects of such failures: depending on the job a control
system has to do, active or passive failures may be fatal or non-fatal.
• When safety requirements are very high, fatal failures must be recognized by taking
additional measures and prevented from affecting other parts of the system.
• In the case of single-channel systems, the means available to do this are relatively limited.
For this reason, safety-oriented functions should generally be implemented outside the
electronics by interposing conventional components.
• In order to satisfy safety functions, electronic control systems should be of the multi-channel
(redundant) type.
• These fundamental considerations are independent of
-
the type of control systems (hard-wired or programmable)
- the vendor
- the country of origin (Europe, US, etc.).
EWA 4NEB 811 6130-02b
14-7