More and more large-sized generators are installed 100% stator earth fault protection with low-frequency alternating-voltage injection. The field adjustment methods of the voltage injection scheme against the stator earth fault protection are summarized and the experimental results are given. The equipment measuring accuracy（relative error is not greater than±5%） of the earth fault resistors is verified by the experimental results. Moreover， some suggestions are given by analyzing a two-point earth fault in potential transformers circuit， which can cause stator earth fault protection warning.