Question

When is the transformer becomes really necessary in short circuit

Answer 1

In a transformer short circuit test is done to find the copper loss at rated load current, if possible. The maximum current allowed to flow during short circuit is the rated current. Theoretically any one winding can be shorted and power, voltage and current measured in the other winding. Practically it is more convenient to supply a smaller current and use a wattmeter of lower current rating if the low voltage winding is shorted. Note that the voltage required to pass rated current is about 5 to 10% of rated voltage.