Most three phase systems are of the "on line" type, using the same technologies found in single phase systems.  However, Ferroresonant three phase systems are becoming somewhat rare due to cost and weight.  Many three phase systems can be paralleled to provide growth capability and redundancy for critical systems.  Two things set three phase systems apart from the single phase systems.  First, and foremost, is their ability to spread the load they place on a building's power source evenly.  Typically, if a building has a three phase power source, and the required UPS is larger than 15 - 20 KVA,  a three phase system is desirable to avoid overloading any one phase of the source.  Secondly, three phase systems produce three phase output power, which may be required in some applications.  Some disk drive systems or other loads with three phase motors require three phase power.  Three phase systems are usually more expensive than single phase systems, but this cost differential decreases as the size of the UPS increases.  The cost per KVA in three phase systems approaches that of the single phase systems in sizes above 30 KVA, and actually becomes much lower in the larger sizes......