I urge the great experts to help me resolve my doubts regarding three-phase four-wire and three-phase systems!
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My question is this: whether it’s a three-phase four-wire system or a three-phase five-wire system, isn’t the neutral point on the transformer side grounded? Since it is grounded, aren’t these N wire and PE wire collinear, and do they both have a zero potential? Then, what’s the point of distinguishing between N and PE wires? Just connect the wires randomly, right? For example, in the sockets at my home, there are N and PE wires. When I use a multimeter to measure the resistance between N and PE, it should be 0. When wiring them together, there’s no need to distinguish between the two, right? So why does the circuit breaker trip if N and PE wires are connected in reverse? Also, in factories, why is it necessary to distinguish between N and PE? After all, the neutral point on the transformer side is grounded, so theoretically it should be considered as a single wire. What is the purpose of requiring such a clear distinction? What is their real function, then? I’ve seen many explanations, but none of them seem satisfactory. I hope someone with more expertise can provide clear guidance; thank you!
There are issues with ground resistance in grounding, and there is a potential difference between different grounding points； Wires also have resistance, so there is also a potential difference. Theoretically, both PE and N should be at zero potential (ground potential); however, due to the presence of resistance, a potential difference actually exists.
But why does the circuit breaker trip when the N wire and the PE wire are connected reversely? Under normal conditions, the current in the live wire and the neutral wire is almost the same, so the leakage protector does not activate. By replacing the neutral wire with the ground wire, no current flows in the neutral wire, and the leakage protector detects this leakage, which is why it trips! The difference between the neutral and ground wires is that, under normal conditions, current flows through one while there is no current in the other, which serves a protective purpose. There is also a TN-C system in which the neutral and ground wires are not separated, but protection differs from that in TN-S; that’s my understanding.
It’s simply about providing a reliable high-speed pathway for the leakage current, using the PE wire. Once there is a leakage, ensure that the leakage persists, which causes unequal currents in the neutral and phase wires.
In the three-phase five-wire power supply system, the protective neutral wire (PE) and the working neutral wire (N) connected to the electrical equipment are laid separately. The potential on the working neutral wire cannot be transmitted to the enclosure of the electrical equipment, thereby effectively eliminating the dangerous voltages that arise in a three-phase four-wire power supply system. As a result, the potential on the enclosure of the electrical equipment remains at the “ground” level, eliminating the risk of dangerous voltages being generated.
After these two days of study, I now have a much better understanding of three-wire four-wire systems, three-wire five-wire systems, pseudo-three-wire five-wire systems, and TT systems. I welcome fellow enthusiasts to exchange ideas with each other in order to improve together.
In the electrical flow within a circuit, the neutral wire plays a role in forming a complete circuit. A leakage protector detects when the current in the live wire is equal to that in the neutral wire; when a ground connection is established, this balance is disrupted, which triggers the tripping mechanism. Tripping indicates that the leakage protector is functioning properly and reliably
I thought about it again. The neutral wire is grounded at the transformer side, so the voltage between it and the ground wire is zero; this is true everywhere in the electrical network ； But this does not mean that there is no current in the neutral wire; therefore, if the neutral wire and the ground wire are connected reversely, the current in the neutral wire will meet the threshold set by the leakage protection device, causing it to trip. In a three-phase four-wire system, if the magnitudes of the currents in the three phase wires are exactly equal, the current in the neutral wire is zero; however, it is usually difficult to achieve this condition. The ground wire usually has a dedicated grounding design near the user end. The above are unprofessional views; please point out any mistakes.
Regardless of the standard, there are the following key points: 1. The purpose of repeated grounding is to ensure personal safety. 2. As long as current flows through a grounded wire, stray current is generated at the grounding point, and this stray current is equivalent to the leakage current of that wire. 3. Function of connecting to the neutral point: It allows a circuit to be formed, enabling current to flow back rapidly to the neutral point and then to the windings. 4. Function of neutral point grounding: It keeps the center point of the three-phase windings at zero potential, and it also serves as a return path for stray currents. 5. Stray currents can cause interference and metal corrosion, so they should be avoided as much as possible. 6. In a three-phase five-wire system, there is a common-mode voltage between the neutral wire and the ground wire, which can cause common-mode voltage interference. 7. It is difficult to balance three-phase voltages, and neutral current generally appears as a result. 8. Different standards have their own advantages and disadvantages. These are the key points I have summarized; by understanding the advantages and disadvantages of different formats, one can gain a deeper understanding.