The invention discloses a 400 volt open type current transformer assembly. A P1 phase current loop wire, a P2 phase current loop wire and a voltage loop wire in a low voltage current transformer are led out respectively through a wiring phase separation lead-out device, the P1 phase current loop wire, the P2 phase current loop wire and the voltage loop wire are connected with a copper rod to forman integral bus of a measuring loop, then the bus is connected with a bus plug through a phase separation connection device, the cross connection problem of phase voltage and phase current is avoided,meanwhile the position relationship of phase current wire is clear, P1 and P2 wiring errors are effectively prevented, and thus the low voltage current transformer is easy to install and convenient to maintain. When an external-connection current lead-out wire of the low voltage current transformer is pulled out, a short-connection metal contact makes phase separation holes of a current loop short connected mutually, and thus secondary current of the current transformer is in a short-connection state in a charged state.