Yes I would have thought that the KVA rating of the transformer is sort of a maximum. If the secondary is shorted the voltage would be almost zero but the current would be very high. If it were possible to determine the exact voltage even at milllivolts or microvolts and then multiply it by the short circuit curent which would be on the order of thousands of amps then you would come up with the rated KVA value. But the primary voltage would not be zero because it is isolated. I would've thought that the primary current would be rated current which in this case is the maximum. I wish there was some technical material that specifically addresses the subject of short circuited secondaries with rated voltage applied to the primary.