Location of a fault in underground system can be done by TDR (time domain reflectometry) .

Basics in transmission line theory :

Zterm refers to load .

If there is a fault in line , if ,line is open → characteristic impedance of line is -1 i.e., the reflected wave add to incident wave in 180 deg out of phase . Outcome is no wave .we see a straight line after some time to an input step . The time indicates the distance of fault .