Three-phase four-wire system: phase lines A, B and C protect the neutral line PEN, and the working current flows through the PEN line. When PEN enters the electrical building, it should be grounded repeatedly; Belongs to TN-C grounding system.

In TN-C system, the protection line and neutral line are combined into PEN line, which has the advantages of simplicity and economy. When a grounding fault occurs, the fault current is large, so general overcurrent protection appliances can be used to cut off the power supply to ensure safety.

However, for lines with single-phase load or three-phase unbalanced load and harmonic current load, the normal PEN line has current, and the voltage drop generated by it appears on the metal shell of electrical equipment and the metal sleeve of the line, which is unfavorable to sensitive electronic equipment.

Three-phase five-wire system: phase wires A, B, C, neutral wire N, protective grounding wire PE, N wire has working current, and PE wire has no current at ordinary times; TN-S grounding system is adopted in the distribution of civil buildings in China.

In TN-S system, the protection line is separated from the neutral line (the five-wire power supply is taken from the transformer), which has the advantages of TN-C system, but it is more expensive. Under normal circumstances, the PE line does not pass the load current, and the metal shell of the electrical equipment connected with the PE line has no potential, which is suitable for data processing and power supply of precision electronic instruments and equipment, and can also be used in the environment with explosion danger.

Extended data:

Three-phase four-wire consists of three live wires and one zero wire. There are two kinds of connections: star connection and triangle connection, and generally star connection is adopted.

Five wires in the three-phase five-wire system refer to: three-phase wires plus a grounding wire and a zero wire. The most widely used low-voltage transmission mode is three-phase four-wire system, which uses three-phase wire and a zero line to supply power. The neutral line leads from the neutral point of the transformer and is grounded at a voltage of 380/220V. Any one-phase line and the neutral line form a 220V power supply line for ordinary households, and the voltage between three-phase lines is 380V, which is generally used by power supply units.

References:

Baidu encyclopedia-three phases and four lines

Baidu encyclopedia -tn-s