we can't say that the fault current lessthan the normal 
current ,fault current depend upon the system parameters 
i.e resistance ,inductance

thanks for the reply, but i meant to ask that why is the 
earth fault current lesser than other type of fault 
currents.

earth falt current current is nothing but the leakage 
current and leakage current is alwase less than line or 
neutral current.therefore earth fault current is always 
less than normal current.
there should not be the common conductor for the earth and 
neutral for the safety and to minimizing the ground 
potential differances.

earth fault current will be less because the voltage 
difference between line and ground is V/1.732 and that of 
line to line is V. since the voltage is less, the earth 
leakage current will be 57.7% of line to line faults.