Overview of E+H Instrument Grounding Precautions
In industrial automation systems, the grounding design of E+H instruments is directly related to equipment safety, signal stability, and system reliability. Improper grounding may cause equipment damage, signal interference, or even safety accidents. Based on relevant specifications, this article systematically summarizes 16 core precautions for E+H instrument grounding for the reference of engineering technicians and procurement personnel.
Instrument grounding is mainly divided into two categories: protective grounding and working grounding, and the two have clear differences in function, connection method, and resistance requirements.
1. Protective Grounding Requirements
Protective grounding is intended to prevent dangerous voltages on instrument metal enclosures due to insulation damage and to ensure personal safety. The specific specifications are as follows:
1.1 Which Parts Require Protective Grounding
- The enclosures, instrument panels, cabinets, boxes, and cases of electrical instruments and other normally non-live metal parts that may become dangerously live after insulation failure shall all be protectively grounded.
- Metal components such as cable trays, protective conduits, supports, and bases also require protective grounding.
- Local instruments, switches, etc. with a supply voltage not higher than 36V may be exempted from protective grounding when the design documents have no special requirements.
1.2 Cases Where Protective Grounding May Be Omitted
- Small low-voltage electrical apparatus such as buttons, signal lamps, and relays installed on metal panels or plates in non-hazardous areas may be exempted from protective grounding when their metal enclosures are in good contact with the already grounded metal panel or plate.
1.3 Connection Requirements for the Protective Grounding System
- The instrument protective grounding system shall be connected to the protective grounding network of low-voltage electrical equipment in electrical engineering.
- The connection shall be firm and reliable, and series grounding shall not be used.
- The grounding resistance value of protective grounding shall comply with the provisions of the design documents.
- Cable trays and cable protective conduits installed on buildings may be repeatedly grounded.
2. Working Grounding Requirements
Working grounding is the grounding that ensures the normal operation of instruments and control systems, including signal circuit grounding, shielded grounding, and intrinsically safe circuit grounding with special requirements.
2.1 Basic Provisions for Working Grounding
- Instruments and control systems shall be provided with working grounding, and the connection method and grounding resistance value of the grounding system shall comply with the provisions of the design documents.
- Signal circuit grounding and shielded grounding shall share the same grounding device.
- Each instrument loop shall have only one signal circuit grounding point, unless an isolator is used to isolate the DC signal circuit between two grounding points.
2.2 Signal Circuit Grounding Location
- The grounding point of the signal circuit shall be on the display instrument side.
- When grounded thermocouples and instruments whose detecting elements are already grounded are used, grounding on the display instrument side shall no longer be performed.
2.3 Shielded Grounding Requirements
- The shielding layer of instrument cables and wires shall be grounded on the instrument panel and cabinet side in the control room.
- The shielding layer of the same loop shall have reliable electrical continuity and shall not be left floating or repeatedly grounded.
- When anti-interference requirements exist, the spare cores in multi-core cables shall be grounded at one point, and the spare cores of shielded cables and the cable shielding layer shall be grounded on the same side.
2.4 Intrinsically Safe Circuit Grounding
- The intrinsically safe circuit itself shall not be grounded unless otherwise specified in the design documents.
- When a diode safety barrier is used, its grounding shall be connected to the common terminal of the DC power supply.
3. Grounding System Installation and Material Requirements
Correct installation methods and qualified materials are the basis for ensuring the effectiveness of the grounding system.
3.1 Grounding Connection Method
- Various types of grounding for each loop in instrument panels, cabinets, and boxes shall be led from their respective grounding branch lines to the grounding busbar or grounding terminal board, then led from the grounding busbar or terminal board to the grounding main line, and finally connected to the grounding main trunk line and grounding electrode.
- Each grounding branch line, busbar, or terminal board shall be insulated from each other at non-connection points.
3.2 Grounding Materials and Fastening
- The wiring of the grounding system shall use copper-core insulated wires or cables.
- Galvanized bolts shall be used for fastening.
- The grounding busbar in instrument panels, cabinets, and boxes shall use copper material and be fixed with insulating supports.
- Welding shall be used between the grounding main trunk line and the grounding body.
3.3 Grounding Wire Color and Anti-static Requirements
- The color of the grounding wire shall comply with the provisions of the design documents, and green and yellow markings shall be provided.
- Anti-static grounding shall comply with the provisions of the design documents and may be carried out simultaneously with anti-static works for equipment, piping, and electrical systems.
4. Key Parameters and Comparison
To facilitate comparison between engineering design and on-site construction, the following table summarizes the core differences between protective grounding and working grounding:
| Item | Protective Grounding | Working Grounding |
|---|---|---|
| Purpose | Prevent electric shock and ensure personal safety | Ensure signal stability and suppress interference |
| Connection Object | Protective grounding network of low-voltage electrical equipment | Shared grounding device (signal circuit, shielding) |
| Grounding Resistance | Comply with design document provisions | Comply with design document provisions |
| Connection Method | Series grounding shall not be used | One-point grounding, avoid repeated grounding |
| Typical Scenarios | Instrument enclosures, cable trays, supports | Signal circuits, shielding layers, intrinsically safe circuits |
5. Recommended Construction Steps
To ensure the correct implementation of the E+H instrument grounding system, the following steps are recommended:
- Read and understand the specific grounding requirements in the design documents.
- Distinguish between protective grounding and working grounding, and plan grounding paths separately.
- Prepare compliant materials such as copper-core insulated wires, galvanized bolts, and copper busbars.
- Install grounding busbars or terminal boards, ensuring insulation between branch lines.
- Connect protective grounding to the low-voltage electrical protective grounding network, avoiding series connection.
- Connect working grounding, ensuring that the signal circuit is grounded at only one point and the shielding layer is grounded on the control room side.
- Check the grounding resistance to ensure compliance with design provisions.
- Identify the grounding wire color (green/yellow) and record the construction results.
6. Common Problems and Precautions
- Avoid repeated grounding: The shielding layer of the same signal circuit should not be grounded at both ends; otherwise, interference current may be introduced.
- Intrinsically safe circuit grounding: Unless special provisions are made in the design, intrinsically safe circuits should not be grounded, so as not to compromise intrinsic safety performance.
- Grounding resistance testing: After construction, the grounding resistance must be tested to ensure compliance with design document requirements.
- Material compliance: Grounding busbars must use copper material, and connecting wires must be copper-core insulated wires; aluminum must not be used as a substitute for copper.
- Anti-static grounding: Anti-static grounding may be carried out simultaneously with anti-static works for equipment and piping, but must comply with design provisions.
Following the above E+H instrument grounding precautions can effectively improve the safety and reliability of industrial automation systems and reduce signal interference and fault risks. Correct grounding is not only a requirement of technical specifications but also the key to ensuring long-term stable operation.