While power supplies are quite reliable, there are many applications where zero downtime is demanded. This requirement necessitates fault tolerant power systems. Uninterruptible power sources which address the power integrity question of the utility mains are not a whole solution.
Consideration has to be given to the reliability of the power converters themselves.
It is possible to employ ultra-conservative design philosophies to minimize the likelihood of failure, but it is not possible to eliminate failure entirely. A more effective approach is to design systems that are fault tolerant in that a single failure does not disable the function. In power supplies, multi-unit assemblies are frequently specified to ensure that internal failure of one power supply does not cause loss of output. Thus if one fails, the other may continue to supply the common load. It is the nature of voltage sources that only ONE unit can establish the voltage level in a system where several are paralleled.