Chemical engineering projects: TN-S grounding system; three-phase pumps use multi-core cables
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This is a chemical engineering project with a TN-S grounding system. Three-core cables are used for the three-phase pumps; is it appropriate to connect their enclosures to the plant’s overall grounding network? A four-core cable still must be used; personally, I think a three-core cable would suffice. But could this violate the requirement of having two different grounding methods within the same power distribution system? Are there any standards that support the use of a three-core cable? Let me know if there is any! Thank you so much! Please ask an expert to help!
Dear sea friend, since your low-voltage power distribution system is designed as TN-S, low-voltage motors cannot use three-core wires; instead, a four-core power cable with a PE wire must be used! For details, refer to the relevant provisions in the design code \"GB 50052-2009 Code for Design of Power Supply and Distribution Systems\", as shown in the figure below:
Definition of the TT system: In an electrical power system, there is one point that is directly grounded, and the exposed conductive parts of electrical equipment are connected to a grounding electrode that is independent of the grounding point of the power system, via protective wires. In this project, the grounding networks across the entire plant are closely connected, sharing a single grounding system. The enclosures of electrical equipment are directly connected to the nearest main or branch grounding wires, with the connection points being in direct relation to the grounding points of the power system. From this perspective, there’s no conflict! Furthermore, according to page 21 of atlas 94D801 \"Installation of Electrical Circuits and Equipment in Explosive and Fire-Hazardous Environments\", a three-core cable is also selected.
Do you mean Figure 7? Does that mean it’s four-core? It just means to ground it to PE. I haven’t worked in the chemical industry for many years; power plants use only three-core cables.
Chemical plants design TN-S systems; motors require four-core cables, with their enclosures being grounded separately
Original poster, if the motors in the power plant use three-core cables, then the original design of the plant’s low-voltage distribution system certainly isn’t TN-S (when it comes to explosion-proof equipment, there are significant differences between power plants and chemical plants); this is a principle issue that cannot be disputed, and it’s something electrical professionals must keep in mind! If the low-voltage distribution system is designed as TN-S, the neutral wire and the protective wire of the entire system are separate (connected only at the power supply side). That is, the protective ground wire of the motor must be led from the power supply side to the dedicated grounding terminal at the motor site. Furthermore, the atlas 94D801 mentioned by the original poster is an obsolete specification that has been updated twice. Such as 94D401-3 in the 2002 version; moreover, 94D401-3 from the 2002 version has also been replaced by 12D401-3 in the 2012 version, titled \"Installation of Electrical Circuits and Electrical Equipment in Explosive Atmospheres\". I hope this information is helpful! The three-core cables described in the elimination specifications have certain defects and have been removed (to be determined through engineering design instead)! For details, see the image below:
Chemical plant design now uses 4-core cables
The TN-S system is a low-voltage power supply system; motor cables must be four-core cables, and they must be connected to the PE conductor； If a three-core cable is used, a leakage protection switch must be employed, as clearly specified in the standards.
A three-phase four-wire system with TN-S grounding is a mandatory requirement per explosion protection regulations.
When using a three-core cable, a leakage protection switch must be employed. Which standard specifies this?