Because the brush's bad contact has some impacts on the correct measurement for the excitation loop grounding resistance of the rotating excitation generator， this paper sets up a uniform model to the excitation loop grounding protection. Based on this model， this paper analyzes the impacts of the brush touching resistance on the measurement of the grounding resistance when the impressed DC voltage method， the impressed AC voltage method and the Ping-Pang switch method are adopted respectively， obtaining the following conclusions: for the impressed DC voltage method， the measurement value of the grounding resistance will ascend， which may cause relay failure to operate; for the impressed AC voltage method， the measurement value of the grounding resistance will firstly descend， and then ascend， which may cause relay misoperation firstly， and then failure to operate; for the Ping-Pang switch method， the measurement value of grounding resistance is related to the grounding position and is uncertain.