

MILHDBK338B: Electronic Reliability Design Handbook 
 

7.5.3.1 Levels of Redundancy
7.5.3.1 Levels of
Redundancy
Redundancy may be applied at the system level (essentially two
systems in parallel) or at the subsystem, component, or part level within a
system. Figure 7.56 is a simplified reliability block diagram drawn to
illustrate the several levels at which redundancy can be applied. System D is
shown with its redundant alternative D’, at the system level. D’ is in turn
built up of redundant subsystems or components (C_{1} and
C_{2}) and redundant parts within
components (b_{1}
and b_{2} within
Component B). From the reliability block diagram and a definition of block or
system success, the paths which result in successful system operation can be
determined. For example, the possible paths from Input to Output
are:
(1) A, a, b_{1} ,
C_{1}
(2) A, a, b_{1} ,
C_{2}
(3) A, a, b_{2} ,
C_{1}
(4) A, a, b_{2} ,
C_{2}
(5)
D
FIGURE 7.56: RELIABILITY
BLOCK DIAGRAM DEPICTING REDUNDANCY AT THE SYSTEM, SUBSYSTEM, AND COMPONENT
LEVELS
The success of each path may be computed by determining an
assignable reliability value for each term and applying the multiplicative
theorem. The computation of system success (all paths combined) requires a
knowledge of the type of redundancy to be used in each case and an estimate of
individual element reliability (or
unreliability).




 
 