Reliability Considerations in Systems Applications of Uninterruptible Power Supplies

A redundant static inverter is applied as an uninterruptible power supply (UPS). A reliability analysis is presented showing that the introduction of a static bypass transfer switch brings about a reliability improvement of 8.5 to 1 in the mean time between failure (MTBF). This gives MTBF greater than 600 000 h. The static bypass transfer switch provides a synchronized automatic transfer in less than 1 ms from the UPS to the normal ac input power source upon a failure of the UPS.