Abstract: The high-resistance grounding fault current in the distribution network is weak, and the measurement accuracy of zerosequence current transformers（CTs）directly affects the ability of grounding protection to withstand the transition resistance. The requirements of zero-sequence CT for grounding protection in small-current grounding system are analyzed. Based on the equivalent circuit and electromagnetic parameters of the CT, the relationship between the measurement error of the CT and the primary current is analyzed. The theoretical analysis results are verified through experiments, and the measurement accuracies of 11 types of zero-sequence CTs for weak current signals are compared. Based on the measurement error of zero-sequence CT and the zero-sequence current of high-resistance grounding fault, the impact of measurement error of zero-sequence CT on the resistance to transition resistance of grounding protection is analyzed. Finally, the guidance on the selection of zero-sequence CTs is provided, and the measures to improve their measurement accuracy and protection against transition resistance are introduced.