A UPS fault is generally seen as the inability of the inverter to provide the correct voltage or frequency at the UPS output terminals, the resulting actions that take place may vary between models. Usually, the UPS control logic will detect the failing output voltage/frequency as the fault occurs and immediately signal the static switch control system to transfer the load to the bypass line in a make-before-break fashion. If the inverter is not synchronized to the bypass supply when the transfer is called for it will be impossible to perform a break-free transfer operation. Consequently there will be a brief supply break while the transfer operation. Consequently there will be a brief supply break while the transfer takes place.
These are the only circumstances under which the load is subjected to a (brief) supply break in a true on-line ups system.
Although the break-free transfer to bypass is transparent to the load, it is no longer supplied with processed power once it is transferred to the bypass supply; also, if the bypass supply is unavailable when the ‘fault’ transfer is necessary a total loss of power to the critical load is unavoidable.
The static switch usually transfers the critical load back to the inverter automatically once the inverter fault clears – this feature is occasionally described as auto-retransfer.
The response of an on-line system to an output overload is usually similar to that of the UPS failure described above in that the load is transferred to bypass until the cause of the overload clears, whereupon it automatically re-transferred back to the inverter. If the bypass supply is unavailable this will lead to a total loss of load supply. Therefore some systems allow an overload condition to continue to be supplied from the inverter for a finite time- that is the UPS equipment is able to supply enough current to a faulty piece of load equipment to insure that the load protection fuse or circuit breaker will automatically disconnect it from the UPS.