Based on two fault cases of weak-infeed lines，where the metal grounding fault happened on line's back outlet，this article analyzed the cause of unwanted operation of grounding distance relay on non-fault phase when the value of the first stage of the grounding distance protection is based on over range setting and the line carries a light load.Furthermore，it is proved that it is reliable to prevent unwanted operation using zero sequence directional component to block the first stage of the ground distance relay.