AbstractIn communication line systems, in general, failure correlation (dependency of communicational line (path) composing a system due to the existence of common‐caused failure mode) often occurs and dependent failure correlation between communication lines (paths) must inevitably be considered in many cases. A concrete example is in reliability analysis between communication terminals in communication cable lines linking two central terminals, a characteristic quantity analysis of reliability when r‐fold common‐caused failures of neighboring lines occur. Thus, in this paper, we take a parallel redundant system given by common‐caused failure (r) and the number of communication lines (n), etc.