The function of a static switch in the UPS is to provide an automatic transfer from the output of the inverter to an alternate or backup source in the event of an overload or fault on the UPS output. An overcurrent transfer circuit is included in the static switch for this purpose.
This circuit provides an overcurrent transfer to the alternate source due to inrushes from the load or faults on the load. Without this feature, the inverter could be driven into current limit prior to clearing a fault. This would most likely cause all the loads to be lost. The static switch, therefore, transfers to the alternate source at 110% to 125% (depending on manufacturer) of rated load, where fault clearing capabilities should exist. Because this circuit cannot differentiate between an inrush and a fault, it is common for the initial energisation of a load to cause a transfer and be energized from the alternate or backup source