In standby redundancy, multiple units of the same component are available but not all units are active. Because all units are not always operating, standby redundancy is more effective than parallel redundancy.

Standby redundancy offers the ability to protect the system through the use of cold, warm, or hot standby units and junctions. For example, in a one-of-two (1:2) standby redundancy, only one of the two units is active. If the active unit fails, the non-operational unit is switched on to take the place of the failed unit, thus preventing a system failure.

Because standby units are called into action only when needed, they fail at lower rates than parallel units. This is sometimes referred to as a quiescent or shelf failure rate. If a standby unit is not operational but powered on so that is ready to activate (hot standby), its failure rate is higher than that for a standby unit that is not powered on (cold standby).

The failure rate for a standby unit can be specified either directly or as a percentage of the assembly’s failure rate. If you specify a standby failure rate of 100%, you basically make the configuration equivalent to a parallel redundant system. You can specify a different distribution for the failure rate of a standby component.