The requirements of functionality， startup value， limiting value， limiting course and setting for anti-time over-excitation limiter and ceiling current limiter of generator excitation systems are provided. The relationship between over-excitation limiter and generator protections， or limiter of stator current， or excitation transformer protections are confirmed. The technical requirements of over-excitation protection of generator excitation systems are also presented. Finally， recommendations for the functionality of over-excitation limiter using exciter field current as the input signal are also given.