Protection for single-phase high-resistance ground faults is a long term concern in the field of relay protection. Because characteristic of single-phase high-resistance ground faults is unobvious, the traditional distance protection can't deal with it. Meanwhile adaptive distance protection uses memory values so that it only has ability of instantaneously tripping and can't become the backup protection of single-phase highresistance ground faults. This paper analyzes the relationship between measured impedances at both ends of the line on the impedance plane and puts forward the concept of summation impedance. Then summation impedance relay with simple criterion is proposed. The simulation results prove that it has higher sensitivity on single-phase high-resistance ground faults and can be used as backup protection.