Abstract: It is introduced that in a substation capacity increase project, the failure of B phase insulation sheath of main transformer differential current circuit caused by unreasonable inner wiring of sample plate of protection device leads to incomplete contact between B phase insulation sheath and protection device Shell, there is incomplete two-point grounding in the current loop, during the construction personnel installing the protective panel cabinet and fixing the protective panel cabinet with the electric welding machine, the accident of transformer differential protection maloperation caused by short-circuit current flowing into main transformer b protective device through primary grounding grid of electric welding machine, the influence of ground potential difference and the cause of Maloperation of two points grounding in current loop are analyzed, finally, the corresponding measures are put forward.