Availability - Siemens SIMATIC S7-400H Manual

Availability

Availability is the probability of a system being capable of operation at a specified
point of time. It can be enhanced by means of redundancy – for example, by using
redundant I/O modules or by using multiple sensors at one sampling point.
Redundant components are arranged such that system operability is not affected
by the failure of a single component. Here, again, an important element of
availability is a detailed diagnostics display.
The availability of a system is expressed as a percentage. It is determined by the
mean time between failure (MTBF) and the mean repair time (MTTR). The
availability of a two-channel (2-out-of-2) fault-tolerant system can be calculated
from the following formula:
MTBF
V +
MTBF
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
Characteristic Values of Redundant Programmable Logic Controllers
1v2
100%
) MDT
1v2
A-3