Because the total Power is same, and under fault condition (Short circuit)…There is no load (Impedance (Z), or Reactance ( XL)= Resistance, and in case of no load, there will be no reactance or resistance, so current will be high) in this condition…So current will be too high, and when power is same, and current increases, voltage will be decrease.
Suppose, (In normal condition)
P= 10 watt, V = 5 Volts, and Current = 2 Amp.
But in Short circuit Condition, (When current is too high)
P = 10 Watts, I= 10 A, so
V = P/I….. 10 Watts/10A=1 V.
In case of short circuit, there will be no load (load = may be inductive (XL) or resistive) so when XL (We can say it resistance or opposition of current) = Zero, then Current will be too high.
So we can see that, in case of short circuit, (Faults condition) XL (inductive Reactance) =0, so Current increase, voltage decreases.