Most of important substations are required to be equipped with two sets of direct current systems. Loop net fault may occur in the process of operation

which will affect the safe operations of DC system

and then further influence the correct operation of the relay protection. The causes and harms of loop net fault are described

the main existing forms of loop net are analyzed

and the method and principle of locating the loop net fault are summarized in the paper.At last an locating method by analog grounding with using insulation supervisory device is proposed

and some countermeasures are put forward to locate the loop net fault.