Comparative reliability analysis of the AC voltage supply to critical microprocessor loads in alternative power architectures

Many manufacturers of critical power equipment, such as UPS, publish "reliability" data for their products in the form of an MTBF (mean time between failure). Allied with an estimate of the MDT (mean down time) or MTTR (mean time to repair), which should include an element of "travel time" which reflects the anticipated reaction time of the service organization to respond to a site emergency, a simple calculation can produce the commonly used (and often abused) criteria of "reliability" that of Availability. Only a minority of manufacturers produce this MTBF data from actual field measurement or experience. When redundant power architectures are taken into account these "theoretical" module MTBFs can be used to generate system level MTBFs of such magnitude that the absolute value is meaningless, or, at best, unimaginable. Claims for UPS system MTBF\´s of more than 100 times the plant life expectancy are commonplace. So, does this mean that such calculations are of no use? Not necessarily, if used with care. This paper attempts to show that reliability calculation techniques, such as the reliability block diagram (RBD) method, can be effectively used as a tool for "comparison" of alternative power architectures.