In practical work, some beginners will simply think that the zero line is the ground line, and they don't know much about the differences between PE line and pen line often used in power system. In the low-voltage installation, maintenance and repair, the above protective wiring of household appliances is often confused, resulting in confusion and error of protective wiring, unnecessary trouble, and even electric shock accidents. Therefore, Xiaobian will briefly introduce the meaning of various "lines" in 380V / 220V power system for reference.
Phase line, commonly known as live line, is relative to neutral line (zero line), which refers to any of the three phase lines L1, L2 and L3 in three-phase AC circuit. Usually, household electricity only uses one phase of three-phase electricity. The voltage of phase line to ground (neutral line) is 220V, which is called phase voltage. It forms a loop with neutral line to make the current work through household appliances. The voltage between phase lines of different phases is 380V, which is called line voltage. Generally, the three phase lines L1, L2 and L3 in the three-phase circuit are marked with yellow, green and red respectively.
In three-phase four wire system or three-phase five wire system low-voltage power supply system, neutral line is also known as neutral line, zero line, working zero line and N line, and the line is marked in black. Usually, the neutral line refers to a line led out from the neutral point of the transformer. For grounding power grid, the neutral point of transformer shall also be grounded, and its grounding resistance is generally not allowed to be greater than 4 Ω.
The function of neutral line is to keep the phase voltage of star connected three-phase asymmetric load symmetrical. For three-phase symmetrical load, the neutral line can be omitted because the neutral line current is zero. For three-phase asymmetric load, due to the existence of neutral line current, there must be a neutral line to balance the voltage of three-phase load. Otherwise, the voltage of each phase of three-phase asymmetric load will no longer be equal (balanced). The load voltage of the phase with smaller impedance is low, and the load voltage of the phase with larger impedance is high, which is easy to cause damage to household appliances. Therefore, in the low-voltage power supply system with asymmetric three-phase load, it is not allowed to install fuses and separately controlled switching devices on the neutral line, and the neutral line is usually made of steel cored aluminum strand to avoid accidents due to neutral line disconnection. The difference between phase line and neutral line is that their voltage to ground is different. The voltage of phase line to ground is 220V. Under normal conditions, the voltage of neutral line to ground is zero.
The protective wire (also known as PE wire, the full English name is protective earthing), which is a conductor short circuited with the earth through a deeply buried electrode, that is, the ground wire, which is specially used to ground the exposed conductive part of the electrical device.
The protective wire can be divided into working grounding wire and protective grounding wire. In low-voltage power system, the connecting line between transformer neutral point and working grounding body is called working grounding wire; The connecting line between the metal shell of household appliances and the protective grounding body is called the protective grounding wire, which is represented by yellow and green. In TT system, PE line is directly connected to the grounding electrode irrelevant to the working grounding of the power point, also known as pee line; In TN-C system, PE line and neutral line of the whole system are integrated; In TN-S system, the PE line of each electric appliance is directly connected with the special public PE line in the low-voltage power supply system, and the public PE line shall be reliably connected with the working grounding of the transformer; In TN-C-S system, a section of line from the low-voltage side of the transformer to the incoming point of the user's power supply, the PE line is connected with the public neutral line, the incoming point of the user's power supply to each electrical appliance is provided with a public PE line, and the PE line of each electrical appliance is connected with the public PE line.
The protective neutral line (also known as pen line and protective neutral conductor) is marked with vertical bar interval light blue. It is a conductor that combines PE line and N line into one and has the functions of protective line and neutral line. TN-C system in low-voltage power system combines PE line and N line in the whole line; TN-C-S system, a section of line from the low voltage side of transformer to the incoming point of user power supply, combines PE line and N line into one; TN-S system separates PE line and N line. The power supply system adopts pen line, which is usually to save materials, but reduces the reliability and safety of power supply. Where conditions permit, TN-S system shall be used for low-voltage power supply to the greatest extent, that is, fully independent PE protection line shall be deployed, rather than TN-C system that binds N line and PE line together. Special reminder: the requirements for power supply safety are relatively high, especially in places with explosion hazards, TN-S system must be selected for power supply.