1. The shell, instrument panel, cabinet, box, box, cable tray, protective tube, bracket, base, and other normally uncharged metal parts of electrical instruments that may carry dangerous voltage due to insulation damage should be protected and grounded. For local instruments, switches, etc. with a power supply voltage not exceeding 36V, protective grounding may not be required when there are no special requirements in the design documents.
2. The metal casing of small low-voltage electrical appliances such as buttons, signal lights, relays, etc. installed on metal plates and plates in non explosive hazardous areas may not be protected from grounding when in good contact with grounded metal plates and plates.
3. The instrument protective grounding system should be connected to the protective grounding network of low-voltage electrical equipment in electrical engineering, and the connection should be firm and reliable, and should not be grounded in series.
4. The grounding resistance value of protective grounding should comply with the provisions of the design documents.
5. Cable trays and cable protection pipes installed on buildings can be repeatedly grounded.
6. The instrument and control system should be grounded for work, including signal circuit grounding and shielding grounding, as well as intrinsically safe circuit grounding with special requirements. The connection method and grounding resistance value of the grounding system should comply with the design documents.
7. The signal circuit grounding and shielding grounding of the instrument and control system should share a grounding device.
8. Each instrument circuit should only have one signal circuit grounding point, unless an isolator is used to separate the DC signal circuit between the two grounding points.
9. The grounding point of the signal circuit should be on the display instrument side. When using grounded thermocouples and instruments with grounded detection elements, it should not be grounded on the display instrument side again.
10. The shielding layer of instrument cables and wires should be grounded on the side of the control room instrument panel cabinet. The shielding layer of the same circuit should have reliable electrical continuity, should not be floating or repeatedly grounded, and should not be grounded on the display instrument panel. The shielding layer of the on-site instrument end cable shall not be exposed outside the protective layer.
11. When there are anti-interference requirements, the spare core wire in multi-core cables should be grounded at one point, and the spare core wire of shielded cables should be grounded on the same side as the cable shielding layer.
12. The various grounding of each circuit in the instrument panel, cabinet, and box should be led to the grounding busbar or grounding terminal board through their respective grounding branch lines. The grounding main line should be led out from the grounding busbar or grounding terminal board, and then connected to the grounding main line and grounding electrode. Each grounding branch line, busbar, or grounding terminal board should be insulated from each other at non connecting points.
13. The connection of the grounding system should use copper core insulated wires or cables, fixed with galvanized bolts, and the grounding busbar inside the instrument panel, cabinet, box, etc. should be made of copper material and fixed with insulation brackets. Welding should be used between the main grounding line and the grounding body.
14. Unless otherwise specified in the design documents, intrinsically safe circuits should not be grounded. When using diode safety barriers, their grounding should be connected to the common terminal of the DC power supply.
15. The anti-static grounding should comply with the design documents and can be carried out simultaneously with the anti-static engineering of equipment, pipelines, and electrical equipment.
16. The grounding connection should be made of stranded copper insulated wires or cables, connected with galvanized or copper bolts. The grounding busbar should be made of copper material and fixed by insulation brackets.
17. The cross-sectional area of the grounding wire should comply with the design documents and the manufacturer’s regulations. When there are no regulations in the design documents and the manufacturer, the grounding wire should be greater than 2mm;, The grounding branch line should be greater than 6mm², The grounding main should be greater than 20mm², The main grounding line should be greater than 30mm;.
18. Welding should be used between the main grounding line and the grounding body. The buried grounding main line can be welded with hot-dip galvanized flat steel or round steel with the same effective cross-sectional area. The joint can be overlapped, with a length of 2 times the width for flat steel and 6 times the diameter for round steel. The contact should be good, the connection should be firm, and the welding should be treated with anti-corrosion.
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The grounding resistance value should comply with the design documents and the manufacturer’s regulations. If not specified in the design documents or the manufacturer, the following regulations should be met:
a. The protective grounding resistance value of the instrument system is generally 4 Ω, and the maximum should not exceed 10 Ω. When a high sensitivity grounding automatic alarm device or grounding automatic cut-off device is installed, the grounding resistance value can be greater than 10 Ω, but should be within 100 Ω. When a lightning protection system is installed, the grounding resistance should not be greater than 1 Ω;
b. The shielding grounding and working grounding should be within 10 Ω;
c. The grounding resistance of intrinsically safe circuits and other instruments should comply with the requirements of the product technical documents;
d. The grounding resistance of DCS, PLC, and PCS systems should be less than 4 Ω;
e. The distance between grounding plates should be greater than 5m.
20. During the construction of the grounding system, concealed engineering records should be kept in a timely manner.