单相接地是中压配网系统中最常见的故障，发生接地后系统虽可继续带故障运行，由于非故障相对地电压升高，若不及时处理可能会发展为非故障相绝缘破坏继发相间短路的威胁。
Single-phase grounding is the most common fault in medium-voltage distribution network system. Although the system can continue to operate with fault after grounding, due to the relative increase of non-fault ground voltage, if not handled in time, it may develop into the threat of non-fault phase insulation damage and secondary inter-phase short circuit.
及时准确地判定接地回路是快速排除单相接地故障的基础，而这种实现判定接地故障回路的保护装置通常被称为小电流接地选线装置，但早期的选线装置常发生误选和漏选，效果不能令人满意。“选线准确率偏低”是长期困扰人们的难题。
Timely and accurate determination of the grounding circuit is the basis for fast elimination of single-phase grounding fault, and this kind of protection device to realize the determination of the grounding fault circuit is usually called the small current grounding line selection device, but the early line selection device often occurs misselection and omission, the effect is unsatisfactory. "Low accuracy of line selection" is a long-term problem that has puzzled people.