Three-phase electrical connection methods include: three-phase four-wire system (three fires and one zero) and three-phase five-wire system (three fires, one zero and one ground).
General notes:
The metal casing of electrical equipment (such as motors) should be connected to the neutral line of the three-phase four-wire circuit.
Three-phase five-wire lines use protective grounding (that is, connecting the metal shell of the electrical equipment to the ground body) which is more reliable.
The metal shell of electrical equipment (such as motors) is connected to the neutral line of the three-phase four-wire line, called zero protection.
Zero protection connection: Connect the lead wire of the electrical equipment shell to the neutral line of the power supply. Since the three-phase charging currents of three-phase electrical equipment are fondally balanced and the neutral current is low, factories often use protection to connect to zero. However, the three-phase current of the civil power grid fluctuates significantly, so it is not suitable for protective earthing. Generally, protective earthing is more reliable.
Or another protection way is to connect the metal shell of the electrical equipment to the ground body, which is called grounding protection.
Electrical equipment (such as motors) is dangerous if not connected to zero or ground.