The wiring form of a single current transformer 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, three-phase current transformer can timely and accurately understand the change of three-phase load, and it is mostly used in transformer differential protection wiring. The two-phase incomplete star connection form is most used in actual work. It saves a current transformer and uses the combined current of phase A and phase C to form an inverted phase B current. The two-phase difference current wiring form is also only used in three-phase three-wire circuits. The neutral point is not grounded and there is no neutral wire. The advantage of this wiring is that it not only saves a current transformer, but also can use a relay to reflect the three Various phase-to-phase short-circuit faults in the phase circuit, that is, three-phase overcurrent protection is completed with the least relay, saving investment.