Configuration of zero-sequence inverse-time over-current protection in transmission lines and main transformers is proposed from view point of fault analysis， under the precondition that high resistance ground fault protections are needed for all power grids. Besides， the same configuration can be also applied to equipments of both 500 kV and 220 kV voltage grade. Then related setting coordination problem is discussed， and the method for selecting time constant from zero-sequence inverse-time curves is derived. In addition， the actual case of power grid is simulated with RTDS to verify the static and dynamic characteristics of the zero-sequence inverse-time over-current protection.