Distance protection III is backup protection of own line and adjacent line. Its action zone often has impedance offset characteristics. Because the PT breakage discriminating element will exit once the protection starts, the distance protection III of non-fault line will trip incorrectly when external fault occurs accompanied by AC voltage losing, even leading to all blackout of substation bus in serious case. To prevent malfunction of distance protection III, two measures are put forward based on voltage circuit and action characteristics:1) DC power supply, reinitiated circuit and switch circuit of public voltage are all dual-designed which can avoid AC voltage losing from the source; 2) the impedance offset characteristics is opened conditionally according to the current value which improve the tolerance ability of distance protection III.