Strictly speaking, the three-phase four-wire leakage protector must be connected with all four wires at the input end, and the output end can only be connected with one phase wire, one zero wire (single phase) or two phases (such as 380V two-phase welder) or three phases (such as motor) or all three phases and four wires (such as motor plus lighting).
If the neutral wire is directly connected to the electrical equipment without leakage protector, the current (single phase) from the phase wire will not pass through the leakage protector when it "flows back" to the power supply, and the leakage protector will detect this current (equivalent to leakage current) at this time, causing the leakage protector to trip.
Also, when the neutral point in the three-phase circuit has a non-zero potential due to unbalanced load, resulting in current in the neutral line, the neutral line will also trip without a protector. But no matter what equipment is connected, the zero line of the output terminal must not be grounded, otherwise the power supply will be abnormal. If it is necessary to connect the protective grounding wire to the equipment, it must be directly connected to the ground from the equipment shell.