Staggered Testing (of redundant items)

test of several items with the same test interval but not at the same time

EXAMPLE Figure 9 shows staggered tests for two item A and B.

Note 1 to entry: When the redundant components of a system are tested at the same time (i.e. when the tests are

synchronous) their availabilities are good (just after a test) and bad (just before a test) at the same time. This

correlation means that the unavailabilities of the components peak simultaneously. This has a detrimental effect

on the system availability which can be cancelled by de-synchronizing the tests. A practical way to do that is

staggering the tests (e.g. testing one component in the middle of the test interval of the other); the unavailability

peaks are also staggered and this improves the average availability of the system.



Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global Standards

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