Earth Loop Impedance

I have a task to design a system that can measure earth loop impedance, i know earth impedance can be messured by draining fault current to eath and measuring its voltage at that particular moment.
my problem is that by using capacitive power supply when ever i drain current to earth using IGBT ,IGBT becomes short.this problem is covered by using resistive power supply.but why this is hapenning to capacitive power supply