The connection method of the current transformer is the wiring form of a single current transformer. It can only reflect the situation of single-phase current. It is suitable for the situation where one-phase current or three-phase load balance needs to be measured, and three-phase can be known by measuring one phase. Most of them are connected with ammeters. Three-phase complete star connection and delta connection form. The three-phase complete star connection can be used for current collection of electric energy meters in the neutral point directly grounded system. The three-phase three-relay wiring method can not only reflect various types of interphase short circuits, but also reflect single-phase grounding short circuits. Therefore, this wiring method is used as interphase short circuit protection and single-phase grounding short circuit protection in neutral point directly grounded systems. Two-phase incomplete star connection form. It is most used in actual work. Two-phase difference current wiring form. It is also only used in three-phase three-wire circuits, and the neutral point is not grounded. The advantage of this wiring is not only to save a current transformer, but also to use a relay to reflect various interphase short-circuit faults in the three-phase circuit, saving investment .