A.1 Basic Concepts - Siemens SIMATIC S7-400H Manual

Characteristic Values of Redundant Programmable Logic Controllers
A.1 Basic Concepts
The parameters normally used for a quantitative assessment of redundant
programmable logic controllers are reliability and availability, which are described in
further detail below.
Reliability
Reliability is the characteristic of a technical device to fulfill its function during its
operating period. This is usually no longer possible when a component fails.
The criterion frequently specified for reliability is therefore the MTBF (Mean Time
Between Failures). It can be determined statistically by systems that are operating
or by calculating the failure rates of the components used.
Reliability of modules
The reliability of SIMATIC components is extremely high as a consequence of
wide-ranging quality assurance measures in development and manufacture.
The following average values apply to SIMATIC modules:
MTBF of a central processing unit: 15 years
MTBF of an I/O module: 50 years
Reliability of programmable logic controllers
The use of redundant modules prolongs the MTBF of a system to a very large
extent. In connection with the high-quality self-tests and the mechanisms for error
detection, which are integrated in the CPUs of the S7-400H, virtually all errors are
discovered and localized. The diagnostic coverage (dc) is approximately 95
percent.
Starting from the reliability of a single system (1-out-of-1 systems having an
MTBF ), it is possible to calculate the reliability of the S7-400H as a two-channel
1oo1
(1-out-of-2) fault-tolerant system from the following formula:
MTBF
1oo2
The MTBF of the S7-400H is determined by its MDT (Mean Down Time). This time
consists essentially of the time for error detection and the time required to repair or
replace defective modules.
The error detection time is half the configured test cycle time (by default, 90 min.).
The repair time for a modular system such as the S7-400H is normally four hours.
A-2
2
MTBF
1oo1
2MDT 2(1 dc)·MTBF
1oo1
S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04