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What are the characteristics of the three-phase four-wire system?

Views : 1575
Update time : 2024-08-09 11:20:20
The characteristics of the three-phase four-wire system are: the neutral point of the power transformer is grounded, and the protective neutral line (PE) and the working neutral line (N) are shared (referred to as PEN).
It is suitable for situations where the three-phase load is basically balanced. If the three-phase load is unbalanced, there will be an unbalanced current in the PEN line. In addition, the harmonic current caused by some load equipment will also be injected into the PEN, so that the neutral line N is charged and extremely dangerous. It may be higher than 50V. It not only makes the equipment case electrified and is unsafe for people, but also makes it impossible to obtain a stable reference potential;
The PEN line should be grounded repeatedly. Its function is to effectively reduce the voltage of the neutral line to ground when the phase of the equipment connected to zero comes into contact with the shell.
defect:
(1) When the three-phase load is unbalanced, an unbalanced current will appear on the neutral line, and a voltage will appear between the neutral line and the ground. Touching the neutral line may cause an electric shock accident.
(2) The neutral line passing through the leakage protection switch can only be used as the working neutral line and cannot be used as the protective neutral line of electrical equipment. This is determined by the working principle of the leakage switch.
(3) Single-phase electrical equipment connected to a two-pole leakage protection switch, such as the protective neutral line of its metal shell used in a TN-C system, is strictly prohibited from being connected to the working neutral line of the circuit, nor is it allowed to be connected to a leakage current The PEN line in front of the protection switch, but it is easy to be misconnected during use.
(4) The connecting wire of the repeated grounding device is strictly prohibited from being connected to the working neutral line passing through the leakage switch. The TN-S power supply system completely separates the working neutral line and the protective neutral line, thereby overcoming the shortcomings of the TN-C power supply system. Therefore, the TN-C system is no longer used at construction sites, and the TN-S system is recommended.
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