Cascade or Parallel redundancy?
UPS systems can be designed to operate in parallel to increase reliability (redundant systems) or to increase load handling capability (full capacity systems). These modules would include extra paralleling circuitry and would usually share the load during normal operation. Each individual unit is sized to support the load in the event of a single system malfunction and naturally a seamless transfer is ensured. Under mains failure conditions all units would feed the load until the batteries were exhausted or until a generator takes up the supply.
Cascade Redundancy uses totally standard units with one feeding the load whilst the other sits in the Bypass Line of the main unit ready to support the load in the event of a system malfunction. Since the second is brought online by activation of the first Bypass Switch a seamless transfer is ensured. Under mains failure conditions the first unit would feed the load via battery until the batteries are exhausted and then the load would be transferred to the second unit via the static bypass and the second battery bank would then feed the load.