An uninterruptible power supply provides a source of ac power if utility power should fail. Typically, uninterruptible power supplies have a finite backup time.
An uninterruptible power supply supplies power to the system and all associated disk unit controllers and devices during a utility power failure. The runtime of the uninterruptible power supply should be sized appropriately for a power loss controlled shutdown.
Three system values affect an uninterruptible power supply. These system values define the action the system takes in response to a change in the power supply signal when an uninterruptible power supply is attached: