When considering the theory of reliability in the design, operation an
d maintenance of water supply systems, the assessment of system reliab
ility, in addition to the establishment of reliability criteria, must
also be made. Reliability assessment is based on the elaboration of a
reliability block diagram of the system, which represents the connecti
ons between its elements to determine the dependence of a system's fai
lure on the failures of its elements. There are general formulae given
for establishing the reliability of a system with series, series-para
llel or series-threshold, parallel-series or threshold-series structur
es and special methods are applied for establishing the reliability of
systems with complex structures. An example of a reliability model of
a water network transit mains is presented.