Regarding the connection of N and PE in the PLC operation control system

Thread Content

Today, we replaced a device at the site with one equipped with a PLC control system. The original wiring for that device was as follows: 3-phase 4-wire power supply → power side of the 4-port leakage circuit breaker (the neutral wire at 30 mA was connected to the N terminal of the leakage circuit breaker) → load side of the 4-port leakage circuit breaker → power switch for the device’s control chassis (the N terminal on the load side was connected to the N terminal of the chassis’s phase power supply; both were labeled as N). After replacing the device, the new wiring became: 3-phase 4-wire power supply → power side of the 3-port leakage protection switch → load side of the 3-port leakage protection switch → power side of the 3-phase circuit breaker (the type with fuses) → load side of the 3-phase circuit breaker → power switch for the device’s PLC control chassis (the N terminal of this switch was directly connected to the neutral wire; meanwhile, this neutral wire also connected to a single-phase lighting circuit and a single-phase water pump – so it served as the neutral wire as well). Then, we turned on the power switch for the PLC control chassis and pressed the start button. The consequence was serious: two of the three phase wires connected to the PLC control chassis’s power switch (10 MM2 aluminum wires) melted instantly, and the third wire was also nearly damaged. Two of the fuses in the 3-phase circuit breaker also blew (the other fuse was made of copper, so...). There was a strong burning smell, 555555555. We then used a multimeter to check the circuits inside the PLC control chassis, and no short circuits were found. So we decided to reconnect the device using the original wiring. I really don’t understand why this happened, but it seems that’s the only solution. The current wiring is: 3-phase 4-wire power supply → power side of the interface leakage protection switch → load side of the 3-port leakage protection switch → power side of the 4-port leakage circuit breaker (now the N terminal on the power side is connected to that neutral wire) → load side of the 4-port leakage circuit breaker → power switch for the device’s control chassis (the N terminal on the load side is connected to the N terminal of the chassis’s phase power supply). I took a deep breath – I was very nervous. I turned on the power switch for the PLC control chassis, and everything worked normally. I started the PLC-controlled drive system (motor-driven), and it also worked fine. Then I started the PLC-controlled cutting system (motor-driven cutting), but the power switch for the PLC control chassis tripped! ! I can’t figure it out; it’s confusing. Logically, things shouldn’t reach this stage. I checked and confirmed there are no problems, then restarted the PLC to control the cutting system – it worked normally. After that I turned it off again and started it up once more, and it still worked fine. At this point I’m really puzzled; I don’t understand why this is happening. The only difference in the wiring is whether the neutral wire is connected to the leakage circuit breaker or not. Additionally, this device wasn’t grounded in its original location. I’m also confused: how is it possible that water and electricity are used simultaneously without any grounding protection? Could it be that water seeping into the ground affects the grounding, which is why grounding isn’t provided? ? I’d like to ask everyone again: should such equipment be grounded or not? Is it okay to leave it ungrounded? ? Is special personnel required to handle grounding? I’m new here, and I believe the members of this forum will surely be able to give me valuable answers to learn from (whatever the answers may be, as long as they relate to electricity, I’m sure there’s something useful for me to learn). There’s an exam to enter the forum, and its management is strict; such a great forum must indeed be an excellent place to learn. I’m a newcomer to production and know very little; I hope to learn more by coming here! ! ! !