Siemens SIMATIC S7-400H System Manual page 436

Characteristic values of redundant automation systems
A.1 Basic concepts
Mean Down Time (MDT)
The MDT of a system is determined by the times outlined below:
● Time required to detect an error
● Time required to find the cause of an error
● Time required for troubleshooting and to restart the system
The system MDT is calculated based on the MDT of the individual system components. The
structure in which the components make up the system also forms part of the calculation.
Correlation between MDT and MTBF: MDT << MTBF
The MDT value is of the highest significance for the quality of system maintenance. The
most important factors are:
● Qualified personnel
● Efficient logistics
● High-performance tools for diagnostics and error recognition
● A sound repair strategy
The figure below shows the dependency of the MDT on the times and factors mentioned
above.
Figure A-1
436
MDT
System Manual, 03/2012, A5E00267695-11
S7-400H