UPS fed from two sub stations
The need for multiple supplies when planning a UPS installation is dependent on local needs. Sometimes it is desirable to provide supply switching so that alternate sources may be used. When the supplies are out of phase care must be taken when connecting the UPS. If separate sub-stations supplies are used then isolation MUST be provided. (This is because the UPS usually has the bypass and mains input connections commoned at the neutral terminal inside the UPS).
| The suggested arrangement shows prime components for two sources with essential isolating transformer when two sub-station transformers are available. This arrangement permits the load to run from either supply if the
UPS is out of service. Care must be taken to ensure that switch type wiring and position cannot be unsafe.
The transformer may sometimes be better placed in the UPS supply so that it's size is determined by the UPS full load needs not the possible bypass power requirement.
Some types of bypass switch require care and attention when operated. Particular care must be taken to ensure that a UPS is in-phase with the bypass line before reconnecting the load. If in doubt check before use