Siemens SIMATIC S5-115U User Manual page 524

Reliability, Availability and Safety of Electronic Control Equipment
14.2 Availability
Availability "V" is the probability of finding a system in a functional state at a specified point in
time.
MTBF
V =
MTBF+MTTR
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
MTBF= Mean Time Between Failures;
MTTR= Mean Time To Repair;
S5-115U Manual
EWA 4NEB 811 6130-02b