The potential difference in the grid is an important safety index for the large grounding grid connected with the grounding grid voltage grading performance. In this paper

the requirement of the voltage grading of large grounding grid based on the safety of power frequency is discussed. It is pointed out that the control value of potential difference in the grid should use the power frequency withstand of the secondary equipment as the checking principle. Based on the power frequency withstand test of the secondary devices

the recommended value（1 000 V）of the potential difference in the grid is given

and the optimization strategy of the voltage grading is put forward

and an application case is given. It has a good guiding value for the design and operation and maintenance of large grounding grid

such as substation

power plant and converter station.