Question

Why there is low voltage applied than rated voltage in short circuit

Answer 1

That would be a short circuit.If rated voltage is applied to transformer in short circuit test it would produce currents so high which transformer is not designed to withstand. It will destroy it.I don't recollect the exact motive for short circuit test but objective is to circulate full load current with secondary short circuited.In normal conditions the current is inhibited by transformers own impedance and load as well.Since,in Sc test there is no load ,the requirement of drive voltage drastically reduces for the full load current to flow