Two schemes of generator rotor earth fault protection（REFP）， named“ping-pong” and“voltage injection”， are often applied. The secondary circuit of the REFP must be connected to the positive and negative terminals of rotor field windings. The rated field voltage of 600 MW or larger generator is commonly higher， and there are various over-voltages in the process of operation. Therefore， it is worried that the low-level insulation of the secondary circuit cannot withstand over-voltages. The over-voltages in the secondary circuit under the effects of common-mode and differential-mode voltages， while one-point rotor earth fault occurs， are analyzed. The results show that the demands on voltage withstanding capability of the secondary circuits， which are directly and non-directly connected to rotor field windings， are different. It suggests installing an independent relay of REFP in the cabinet of excitation system， so that the length of the directly connecting secondary circuit can be shortened. It is proper to use a transducer with galvanic insulation to measure the field voltage.