High Availability Theoretical Basics - Schneider Electric

Reliability Block Diagrams (RBD)
Series-Parallel Systems
High Availability Theoretical Basics
Based on basic probability computation rules, RBD are simple, convenient tools to
represent a system and its components, to determine the Reliability of the system.
The target system, for example a PLC rack, must first be interpreted in terms of series
and parallel arrangements of elementary parts.
The following figure shows a representation of a serial architecture:
are considered as a participant member of a 5-part series.
As an example, assume that one of the 5
modules (1 power supply and 4 other
modules) that populate the PLC rack
becomes inoperative. As a consequence,
the entire rack is affected, as it is no longer
100% capable of performing its assigned
mission, regardless of which module is
inoperative. Thus each of the 5 modules
Note: When considering Reliability, two components are described as in series if both
are necessary to perform a given function.
The following figure shows a representation of a parallel architecture:
sequence of the 4 other modules.
As an example, assume that the PLC rack
now contains redundant Power Supply
modules, in addition to the 4 other
modules. If one Power Supply becomes
inoperative, then the other supplies power
for the entire rack. These 2 power supplies
would be considered as a parallel sub-
system, in turn coupled in series with the
Note: Two components are in parallel, from a reliability standpoint, when the system
works if at least one of the two components works. In this example, Power Supply 1
and 2 are said to be in active redundancy. The redundancy would be described as
passive if one of the parallel components is turned on only if the other is inoperative
only, for example in the case of auxiliary power generators.
19