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Reliability, Availability and Safety of Electronic Control Equipment
S5-115U Manual
14.2
Availability
Availability "V" is the probability of finding 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.
However, it is possible to get near this ideal state by using, for example, voter systems. Such
systems include the following:
•
Standby sytems
•
2-out-of-3 voter systems
•
Multi-channel voter systems with mutual check functions (for maximum safety requirements).
Availability can also be enhanced by reducing the mean time to repair. Such measures include, for
instance:
•
the stocking of spare parts
•
the training of operating personnel
•
fault indicators on the devices
•
higher memory and software overhead for implementing programmed diagnostic functions.
14-4
EWA 4NEB 811 6130-02b
Summary of Contents for SIMATIC S5-115U
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