The goals of this paper are to present an efficient redundancy scheme for highly reliable systems, to give a method to compute the exact reliability of such systems and to compare this scheme with other redundancy schemes. This redundancy scheme is self-purging redundancy, a scheme that uses a threshold voter and that purges the failed modules. Switches for self-purging systems are extremely simple: there is no replacement of the failed modules and module purging is quite simply implemented. Because of switch simplicity, exact reliability calculations re possible. The effects of switch reliability are quantitatively examined. For short mission times, switch reliability is the most important factor: self-purging systems have a probability of failure several times larger than the figure obtained when switches are assumed to be perfect. The influence of the relative frequency of the diverse types of failures (permanent, intermittent, stuck-at, multiple,...) is also investigated. Reliability functions, mission time improvements, and switch efficiency are computed and displayed. Self-purging systems are compared with other redundant systems, like hybrid or NMR, for their relative merits in reliability gain, simplicity, cost, and confidence in the reliability estimation. The high confidence in the reliability evaluation of self-purging systems makes them a standard for the validation of several models that have been proposed to take into account switch reliability. The accuracy of the models using coverage factors can be evaluated in this way.