Given a set of units in parallel and a specified minimum number of required units (m), this tool enumerates all possible states (up to a maximum of 2,000 for browser output and 10,000 for Microsoft Excel csv file output). The tool can
generate the system reliability function, R(t), assuming either an exponential failure distribution (i.e., constant unit failure rate) or use the Weibull distribution to model non-constant failure rates. The tool also calculates the effective system mean time
between failure (MTBF) if option "b" or "c" is selected for input #2. The effective MTBF calculation assumes that the system is restored to "as good as new" at the start of each new mission of time "T." The tool also calculates the system mean time to failure (MTTF) for the case of a non-repairable system.
The specific calculation methods are described in paragraphs 2 and 3 here.
The tool can also be useful for modeling situations where there is partial "m of n" redundancy. For example, if three units are almost fully redundant (1 of 3 required), except for one critical state where units #2 and #3 both fail and unit #1 cannot deliver the full specified performance by itself, this tool allows calculating system reliability and/or effective MTBF by initially assuming full "1 of 3" redundancy, but then allowing the user to explicitly define this single state as an unsuccessful state.
Example Excel input template