Most of what is said concerning active redundant systems holds true for stand-by systems as well. For the latter, however, there is an additional mechanism by which the theoretically achievable system reliability parameters can be severely compromised: namely the failure of the switch-over mechanism to operate successfully when required to do so (and to some extent the failure of the switch-over mechanism to remain inactive when required to be so).

It is the existence of the control and switch-over mechanism that can often make a stand-by system less reliable than its active counterpart. Note that for the non-repairable case, the MTTF of a two sub-system active redundant system is 1.5 times the MTTF of the individual sub-systems; but for a standby system, the multiplier is 2 and not 1.5. Thus the standby system appears to be superior; however, this may not be true in practice on account of any unreliability of the control and switch-over mechanism.