Uninterruptible power supply. Power supply that employs automatic switching of main power supply from primary to secondary (usually battery and/or diesel generator) upon failure of the primary. Providing uninterrupted power to critical instrumentation for a limited period of time even in the event of a plant power failure. Usually accomplished by an inverter, transfer switch and batteries. Alternatives means may include motor generators. A type of power supply that can provide electrical power even when line power is lost.