A question about a step-down transformer with its secondary winding shorted
Under real-world conditions no transformer is 100% efficient at converting power. If the secondary is shorted then the primary 'sees' more watts being dissipated than the secondary. In fact the primary always 'sees' more power dissipated regardless of the load. Many fused transformers have fuses on the primary only, and they are usually the slow-blow type because of inrush currents when the transformer is turned on. Most 50HZ to 60HZ transformers are only 40% to 60% efficient at converting power, so for a given known maximum continuous load the transformer is likely over-rated by 50%. Some transformers have short-circuit protection, usually those called 'wall-packs'. So called power transformers and industrial transformers have a fused primary. Those with extreme high power/high voltage may have secondary fuses as well. I have seen pole mounted transformers explode from a lightning strike maybe 50 yards from me, only to see a huge fuse explode about 100 yards away on another pole. For major power distribution, it pays to fuse both sides of a transformer.