Electrical regulations require a UPS to be able to deliver a sufficient current to clear Fuses or MCB’s in the event of a short circuit on the output.
When the system is online this is not an issue as system switches to bypass until fault is cleared, but in battery operation there is a current limiter which limits the current to 150% and this would not necessarily be sufficient for the to trip the required protection in the specified time. However we also specify a subcycle 300% current limit for fault clearing for the Silcon.
This means that 300% of the max rated current will be supplied for up to 20 mil.sec. (0.02 Sec) If the MCB or Fuse is selected correctly, this will be sufficient to clear the MCB / Fuse.