In the power supply system, what is the role of neutral line and zero line

"The negative U2, 3 voltage source V2, W2 connected together to form a contact, denoted as N, called the power of the mid point; from the positive voltage source U1, 3 V1, 3 transmission lines W1 leads out of the called line commonly known as FireWire, power leads to the midpoint of the line called as the middle commonly known as the zero line, this is the three-phase power supply connected in star mode."

This is an excerpt from an electric teaching material in paragraph. There are some articles, can often see this problem: in the power supply system, what is the role of neutral line and zero line? From the question on the surface it itself contains two concepts: the neutral line and the zero line. "Electrical Engineering" in say: winding transformer, generator has a point, this point and the external terminal voltage between the absolute values are equal, this is called the neutral point, but when the neutral point grounding, the point is called the zero. Wire by the neutral point, called the neutral wire; wire leads from zero, is called the zero line. From the above comparisons, we can draw the safety measures of different operation mode of neutral point in grid environment, the neutral point insulated neutral point operation mode and operation mode of the direct grounding.

Isolated neutral point operation mode should be:
(1) all electrical equipment must be in the protective earthing, and does not allow the use of protective zero terminating;
(2) the mechanical strength of the neutral line and phase lines should be the same, not allowed to disconnect the neutral line;
(3) the neutral line current shall not exceed two times of rated power transformer coil 25%, a three-phase load current should not be too large, so as not to affect the balance of three-phase voltage;
(4) to neutral directly grounding, low voltage distribution panel must be set three-phase insulation monitoring device, in order to discover and eliminate in the low voltage grid grounding fault;
(5) with 4 lightning arrester shall be two side of distribution transformer, in order to prevent lightning overvoltage.

Neutral grounding mode should be:
(1) all the metal parts of electrical equipment under normal circumstances do not live, must use protective zero terminating or protective earthing;
(2) in the same low voltage power distribution system of the three-phase four wire system, then zero protection and grounding protection can not mix, which is a part of the protection to zero, while the other part of the protective earth, but in the same device with zero protection and grounding protection is allowed because of its safety, better;
(3) require grounded neutral line must be repeated, because in the neutral line broken, earthing device housing with 220 volts of voltage, which is not allowed.

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