A UPS fault is generally seen as the inability of the inverter to provide the correct voltage or frequency at the UPS output terminals, and 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. However, if the inverter is not synchronised 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 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.
Note that 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-transfers 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 ensure that the load protection fuse or circuit breaker will automatically disconnect if from the UPS.
While feeding the overload under these circumstances, the inverter operates in a current-limit mode and its output voltage may be reduced deliberately, but in most cases this is preferable to total power loss, and of course conditions will return to normal if the overload is cleared during the allotted time.
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