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Fig. 8.
Bridge reliability structure a) RBD model without elements multiplication; b)
RBD model with elements duplication
E
3
and
E
4
are the output elements and
E
5
element is a bridging element (bridge).
Such a system is safe if and only if
E
1
,E
3
or
E
2
,E
4
or
E
1
,E
5
,E
4
and
E
2
,E
5
,E
3
are fit and works properly.
The bridge structure is an irreducible one, it cannot be reduced to a simple
form of the structure of a series or parallel without duplicating specific blocks
in the model (Fig. 8b).
5Fin lRem rks
Apart from the described reliability structures the redundant elements can be
found in the literature [4, 6, 10, 14]. The redundant elements are defined as
not currently active in the system and waiting for a task. These elements are
activated while the base element is failed. Taking into consideration different
reservation types the following methods can be stated: general, mixed and dis-
tributive.
Among the reliability structures with the redundant elements one can identify
following ones: relay (cascade), series-relay, parallel-relay and relay-threshold -
which respectively use the basic previously presented structures. The redundant
system with a coherent reliability structure is characterized by the fact that the
path of any element in the down state causes inadequacy of the whole system.
This is the case of a series reliability structure. However, the series reliability
structure is usually an approximation of the actual reliability structure and has
a theoretical character. This fact allows to determine the reliability of the system
taking into account the worst case scenario [6, 8]. In real technical objects, there
are many redundant structures both known and unidentifiable. In practice, we
always have to deal with the redundant structure [1, 11].
Definitions of redundant and unredundant structures are not accurate and
should be made more precisely in their analysis [2]. This applies both to the
many kinds of redundancy and the growing complexity of today's systems.
Apart from implementations of the theoretical elaborations [15, 22, 23] the
shown reliability structures will be implemented in applications connected with
transportation safety [12, 13]. A very important thing is to provide proper safety
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