Branch scan is an important approach to locate insulation faults. Nevertheless the conventional DC leakage method often makes false alarms or omissions， which is resulted from 5 factors in the malfunction. There is a need for modification. Based on the analysis of current components through leakage sensors and voltage drops between buses， the optimized algorithm can significantly improve the measurement accuracy， without the disadvantage of zero-drift in leakage sensors， and without the leakage from sub-bus to earth voltage meters. Through analysing the pass-through current to a sub-bus and the current split-flow into a closed loop， the new method outperforms the conventional residual current warning based method which often misguides operators. Experimental results show validity of the algorithm and adaptability to diversiform omni-bus insulation monitoring methods.