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Redundancy may either be active, in which case the redundant items operate simultaneously in performing the same function (as for the parallel structure), or standby, such that the redundant items are only activated when the primary item fails. With standby redundancy, the standby items may be in cold standby or in partly loaded standby. With cold standby, the redundant item is considered to be as‐good‐as‐new when activated. With partly loaded standby, the item may be failed or worn when activated.

A simple standby structure with two blocks is shown in Figure 2.15. Initially, block 1 is functioning. When block 1 fails, a signal is sent to the switch to activate block 2 and a repair action of block 1 may be started. The switch may be automatic, or a manual action to connect and start block 2. Depending on the operating rules, block 1 may be activated again as soon as the repair action is completed, or block 2 may run until it fails.

Figure 2.15 Standby structure.