To diagnose the corrosion condition of the horizontal conductors and the down lead lines of a grounding grid

a novel approach is presented.A hierarchical model of grounding grid is established

and an intrinsic grounding grid with only accessible nodes is hence achieved based on the model.The testability of the grounding grid is analyzed with Monte-Carlo method.As a result

the branches are divided into clear branches and uncertain branches.The resistances of the clear branches can be exactly determined while the resistances of the uncertain branches can not.A method to judge the sufficiency of test is suggested

and it is pointed out that in sufficient test the resistances of branches in the intrinsic grounding grid can all be exactly determined.The influence of down lead lines on corrosion diagnosis results is analyzed

and an approach to evaluate the resistances of down lead lines based on the diagnosis results of intrinsic grounding grid is proposed.It is believed that the resistances of all down lead lines can be exactly determined.An experimental grounding grid with sixty branches is adopted to verify the proposed approach.The results show that the maximum relative errors of the resistances are 4.86% for the clear branches and 8.43% for the down lead lines

respectively

which demonstrates that the proposed approach can be used to examine corrosion condition of the horizontal conductors and the down lead lines.