The illogicality of the traditional grounding electrode protection used in the configuration of common grounding electrode lines is analyzed. Under the common grounding electrode lines configuration， the electric parameters under each type of fault according to the circuit theory are calculated. It can be concluded that the unbalanced current measured by the protection of every system becomes small when the fault on the common grounding electrode lines is near the grounding location. The unbalanced current measured by the protection equipped on the fault side becomes large when the fault is near the DC switch station in each grounding electrode line， and the unbalanced current measured by the protection equipped on the normal side is on the contrary. It is also concluded that the unbalanced current is fixed when single-line is broken， all the single-line-fault affect little to the over-voltage protection， and the unbalanced currents in the bipolar affect the electrode current unbalanced protection depend on its direction and magnitude. Finally， it is suggested that the current unbalanced protection should adopt fault location and the inverse-time function considering the characteristics of the fault circuit.