PLC grounding procedures - Solutions - Huaqiang Electronic Network

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In order to ensure the implementation level of the grounding of the plc control system and ensure the safe and reliable use of the control system at the site, this procedure is specially formulated.
First, the grounding classification grounding can be mainly divided into protective grounding, working grounding, intrinsically safe system grounding, anti-static grounding and lightning protection grounding.
1, protective grounding
1) Protective earthing (also known as safety grounding) is the grounding provided for personal safety and electrical equipment safety. The metal casing of the electrical equipment such as cabinets, consoles, instrument cabinets, power distribution cabinets, and relay cabinets of the control system and the metal parts of the control equipment that are normally uncharged may be dangerous due to various reasons (such as insulation damage). Voltage should be used as protective grounding.
2) Field instruments that are powered below 36V may be protected from grounding, except those that may be in contact with devices above 36V.
3) When the instruments installed on the metal instrument panel, box, cabinet, and frame are in good electrical contact with the grounded metal instrument panel, box, cabinet, and frame, the protection grounding may not be performed.
2, work ground
1) Instrument and control system working grounding includes: instrument signal circuit grounding and shielding grounding.
2) The isolated signal can be ungrounded. Here, "isolation" means that the circuit of each input signal (or output signal) is insulated from the circuit of other input signals (or output signals) and is insulated from the ground, and the power sources are independent and isolated from each other.
3) Non-isolated signals are usually referenced to the negative pole of the DC power supply and grounded. Signal distribution is used as a reference point.
4) The principle of working grounding of the instrument is single point grounding. The grounding circuit should be avoided in the signal circuit. If the signal source and receiving instrument on one line are unavoidably grounded, the isolator should be used to isolate the two points from ground.
3. Intrinsically safe system grounding
1) Intrinsically safe system with isolated safety barrier, no special grounding is required.
2) Intrinsically safe systems with Zener barriers shall be provided with a ground connection system.
3) The grounding of the intrinsically safe system of the Zener barrier should not be separated from the grounding of the instrument signal loop.
4, anti-static grounding
1) Install the control room of equipment such as dcs, PLC, SIS, etc., and consider anti-static grounding. These indoor static conductive grounds, raised floor, workbench, etc. should be grounded with anti-static.
2) Instruments and equipment for protective grounding and working grounding have been made, and no additional anti-static grounding is required.
5, lightning protection grounding
1) When the signal line of the instrument and control system enters the room from the outside and needs to be installed with lightning protection grounding connection, lightning protection grounding connection should be implemented.
2) The lightning protection grounding of the instrument and control system should be shared with the electrical professional lightning protection grounding system, but the grounding device must not be shared with the independent lightning protection device.
Second, the grounding form and grounding principle System grounding form is mainly divided into equipotential grounding and separate grounding. The grounding principle is a single point grounding, that is, a unique grounding reference point ERP is combined into the grounding system.
1. The system recommends grounding by equipotential single-point grounding. This requires an equipotential grounding grid around the process unit (or plant area).
2. In the case that the equipotential grounding cannot be satisfied, allow the system working ground to be grounded separately, and connect the system protection ground to the electrical ground. In the case where the system ground and the protection ground cannot be separated, the system protection ground and the working ground can be grounded separately.
Third, the ground connection method
1. When using equipotential grounding, it is required to connect the metal structure of the building (or device), the foundation steel bar, the metal equipment, the pipeline, the PE (protective grounding wire) busbar of the incoming distribution box, and the lightning receptor down the line. Forming an equipotential bonding, the control system protective grounding and working grounding should be classified into the total grounding plate to achieve equipotential bonding, and the grounding device is used in conjunction with the electrical device and connected to the earth. However, the access point of the control system on the grounding network shall be maintained at a distance of not less than 5 m from the access point of the lightning protection, high current or high voltage equipment.
2. When using separate grounding, the grounding resistance should be less than 4 ohms at this time, and the separate grounding body should be more than 5m away from other electrical professional grounding bodies, and the independent and anti-direct lightning strike grounding body must be more than 20 meters away. The specific grounding form is selected according to the field conditions and in the following cases. (The following conditions are the connection of the working ground. Under normal circumstances, the protective grounding should be connected to the electrical ground. If the working grounding and protective grounding cannot be separated, the protective grounding and working ground can be connected to the same separate grounding body.)
1 Under normal conditions, it is recommended to use 4 2m long 50*50 angle steels, and the square with a side length of 5m is driven into the ground more than 70cm, and then welded with galvanized flat iron (recommended by surfacing). 16mm2 wire (general control wire length ≤ 20m) is introduced into the grounding copper bar of the control room, which basically meets the requirements of grounding resistance less than 4 ohms. In special geographical situations, a resistance reducing agent is needed to reduce the grounding resistance.
2 For the case where there is no condition to separate the pile, the electrical ground can be used as the grounding of the system. At this time, the working connection and the protective ground are connected to the electrical ground, but it should be noted that the access point should be as far away as possible from the large motor. The distance between the access point and the access point of the lightning protection ground should also be greater than 20m.
3 The low-voltage electrical cabinets such as the operating console and external cabinet of the system should be regarded as protective grounding, and the grounding wire should be connected to a protective grounding copper strip. If a safety barrier is installed in the outer cabinet, the safety barrier grounding shall be regarded as the working ground, and the grounding wire shall be connected to the working grounding grounding copper strip. Then connect to the grounding body as appropriate.
4 For the distance between two control stations or between the control station and the console (generally based on whether it is within the same building or the distance between them is more than 30m), grounding can be grounded separately. .
5 If an electrical isolation device or opto-isolation device is used between the remote cage and the main control cage, the remote cage can be grounded in place.
6 The grounding of the UPS should generally be based on the electrical ground of the factory.
Fourth, grounding system wiring and grounding resistance
1. The conductor of the grounding system shall be made of stranded copper core insulation or cable.
2. Each grounding busbar of the grounding system can be made of copper strip with a section of 25mm×6mm.
3. The grounding summary boards of the grounding system shall be made of copper plates with a thickness of not less than 6mm, and the length and width dimensions shall be determined as required.
4. The protective earthing busbar in the cabinet should be reliably connected to the cabinet.
5. The working ground busbar and the working grounding summary board shall be fixed by insulating brackets.
6. The various connections of the grounding system should be firm and reliable, and should ensure good electrical conductivity. The grounding wire, grounding trunk, and grounding main trunk should be connected to the grounding busbar and the grounding summary board with copper lugs and galvanized steel bolts, and use anti-loose parts or welding.
7. It is strictly forbidden to connect switches or fuses in all kinds of grounding connections.
8. The cross section of the grounding wire can be selected according to the number of connected instruments and the length of the grounding wire according to the following values:
a) Grounding wire: 1mm2 ~ 2.5mm2;
b) Grounding trunk: 4mm2 ~ 16mm2;
c) Grounding mains of the grounding terminal block: 10mm2 ~ 25mm2;
d) Grounding main trunk: 16mm2 ~ 50mm2;
e) Lightning surge protector grounding wire: 2.5mm2 ~ 4mm2.