The grounding of instrumentation system is divided into protective grounding and working grounding.
I. Protective grounding
Methods of protective grounding:
The field instrument bridge and threading pipe should be connected to the grounded metal components with grounding wires every 30m. It should be pointed out in particular that the field grounding of instruments and meters must not be grounded using metal equipment, pipelines and metal components connected to them that store and transport flammable media. The instrument automatic control equipment, cabinets, instrument panels, etc. in the control room should be equipped with a separate protective grounding bus. Its grounding body can be shared with the grounding body of the power system.
II. Working Grounding
Instrumentation and meter working grounding includes signal circuit grounding, shielding grounding, and intrinsic safety grounding.
1. Signal Circuit Grounding
In non-isolated signal systems, a unified signal reference point should be established, i.e., signal circuit grounding. Typically, this involves grounding the negative terminal of the DC power supply. When the instruments are in operation, interference in the system is extremely rare.
2. Shielding Grounding
The shielding layer of the cable and the noise-reduction wires should be grounded for shielding. In areas prone to strong lightning strikes, unused cores of ordinary unshielded multi-core cables used outdoors should be grounded for shielding. This is primarily to prevent high voltage from being induced in the signal lines by lightning. In the field junction box, the cable shielding wires on both sides of the terminal should be bridged inside the box. For the same signal circuit, the same shielding layer should be grounded at a single point. Generally, shielding grounding should be done on the control room side.
III. Intrinsic Safety Grounding
The busbar of the Zener safety barrier must be connected to the common terminal of the DC power supply (primarily to ensure protection of hazardous areas in the event of a power failure). Its busbar or rail is used for intrinsic safety grounding. An intrinsic safety grounding busbar should be set up within the control room.
Method for Working Grounding:
The signal and shield grounding busbar, and the intrinsic safety grounding busbar, should be connected to the working grounding busbar through their respective grounding wires.