Standards for working ground and protective ground of instruments

Grounded automatic control equipment such as dashboards, instrument cabinets, instrument boxes, cabinets and operation stations for DCS/PLC/EDS, instrument power supply equipment, cable trays, conduit pipes, junction boxes and the shielding layer of armored cables, as well as the anti-static flooring in the control room. Generally speaking, field instruments, transmitters, etc. that use DC24V as power supply and have no special requirements do not need to be protected by grounding. Methods for protective grounding: On-site instrument trays and conduit pipes should be connected to grounded metal components using grounding wires every 30 meters. It should be particularly noted that on-site grounding must never be carried out using metal equipment and pipelines used for storing or transporting flammable media, as well as the metal components connected to them. The instrument automation equipment, cabinets, dashboards, etc. in the control room should be equipped with separate protective grounding busbars. Its grounding electrode can be shared with the grounding electrode of the power system. The identification color for the instrument protection ground connection wire is green. II. Working Grounding Working grounding includes signal circuit grounding, shielding grounding, and intrinsically safe grounding. 1. Grounding of the signal circuit: In non-isolated signal systems, a unified signal reference point should be established. That is, ground the signal circuit. It is usually grounded to the negative pole of the DC power supply. Using a non-isolated signal system is generally my preferred approach in design. During operation, it is extremely rare for the system to be disrupted. In isolated signal systems, the isolated signals can be ungrounded. The isolation referred to here means that the circuit for each input/output signal is insulated from other input/output signals. Achieve independent power supplies, isolation between sections on page 1/5, and an open reference point. I think this method should not be used lightly in systems with many loops. Signal and shield grounding busses should be provided within the control cabinet. The ground wire is identified by yellow/green wires. 2. Shield grounding: The shield layer of the cable and the shielding wires should be grounded for shielding purposes. In areas with severe lightning strikes, for ordinary multi-core cables installed outdoors without shielding, the spare cores should be shielded and grounded. It is mainly to prevent lightning from inducing high voltages in the signal lines. Inside the on-site junction box, the cable shielding wires on both sides of the terminals should be bonded together within the box. For the same signal circuit, the same shielding layer should be grounded at a single point. Generally, the shielding ground should be connected to ground on the control room side. Signal and shield grounding busses should be provided within the control cabinet. The ground wire is identified by yellow/green wires. 3. 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 guide rail is grounded intrinsically safe. An intrinsically safe grounding busbar should be installed in the control section. The ground wire is identified by blue/green wires. Method of working ground connection: The signal and shielding ground busses, as well as the intrinsically safe ground bus, are connected to the working ground bus through their respective grounding wires. Page 2/5: What is mentioned below is a reference, more in detail: There are two purposes for instrument grounding – one is protection and the other is operational grounding. The differences are as follows: [Protective grounding] Lightning protection grounding is a grounding system that prevents damage when struck by lightning (directly, through induction, or via power lines). There is often a distinction between lightning protection grounds for signals (low-voltage systems) and those for power supplies (high-voltage systems). The reason for this distinction lies not only in the different requirements regarding grounding resistance, but also in practical engineering practices, where the lightning protection ground for signals is usually connected to an independent ground dedicated to signals, and is constructed separately from the lightning protection ground for power supplies. Safe grounding of the enclosure involves establishing a good electrical connection between the metal parts of the system that are not normally charged (such as the cabinet enclosure and control panel enclosure) and the ground, in order to protect both the equipment and human safety. The reason is that the system is powered by high-voltage electricity (380, 220, or 11V). Under normal circumstances, the casing and other components are not electrified. However, when a fault occurs (such as a problem with the main power supply or some other issue) that causes a short circuit between the live wire supplying power and conductive metal parts like the casing, these metal parts or the casing become electrified. If there is no proper grounding, a high voltage difference is created between these electrified parts and the ground. If a person accidentally touches these electrified parts, an electric current can flow through their body, posing a danger. Therefore, a good connection must be established between the metal enclosure and ground to make the enclosure and ground at the same potential. Furthermore, protective grounding can also prevent the accumulation of static electricity. Safe grounding involves establishing a good electrical connection between the metal parts of a system that are not normally energized (such as cabinet enclosures and control panel casings) and the ground, in order to protect both the equipment and human safety. The reason is that the system is powered by high-voltage electricity (380, 220, or 11V). Under normal circumstances, the casing and other components are not electrified. However, when a fault occurs (such as a problem with the main power supply or some other issue) that causes a short circuit between the live wire supplying power and conductive metal parts like the casing, these metal parts or the casing become electrified. If there is no proper grounding, a high voltage difference is created between these electrified parts and the ground. If a person accidentally touches these electrified parts, an electric current can flow through their body, posing a danger. Therefore, a good connection must be established between the metal enclosure and ground to make the enclosure and ground at the same potential. Furthermore, protective grounding can also prevent the accumulation of static electricity. 【Working Grounding】 Working grounding is a type of grounding designed to ensure the reliable operation of the system and the instruments connected to it, as well as to maintain accuracy in measurements and control. It is divided into machine logic grounding, signal circuit grounding, and shielding grounding; in petrochemical and other explosion-proof systems, there is also intrinsically safe grounding. The machine ground, also known as the host power ground, is the common ground for the negative logic levels inside a computer; it is also the output ground for power supplies such as +5V. The signal circuits are grounded, such as the negative terminals of various transmitters being grounded, and the negative terminals of digital signals being grounded as well. Shield grounding (grounding of the shield layer for analog signals). Intrinsic safety grounding refers to the grounding of intrinsic safety instruments or safety barriers. In addition to suppressing interference, this grounding is also one of the measures that confer intrinsically safe properties on instruments and systems. The intrinsically safe grounding varies depending on the specific safety measures adopted by the equipment. Below, the Zener-type safety barrier is used as an example to illustrate its grounding details. The function of the safety barrier is to ensure that the hazardous area remains within the range of safe power supply and safe voltage levels at all times. If the field side is short-circuited, the current in the wires will be limited to a safe level due to the current-limiting effect of the load resistance and the safety barrier resistor R, preventing the field side from reaching high temperatures that could cause combustion. In the second scenario, if a fault occurs on the computer side, high-voltage signals are introduced into the signal circuit; however, due to the clamping effect of the Zener diodes, the voltage remains within a safe range. It is worth noting that the introduction of Zener safety barriers increases the resistance in the signal circuit by a significant amount. Therefore, when designing the load capacity of the output circuit, it is necessary to take into account not only the actual load requirements but also the resistance of the safety barriers, leaving some margin. In addition to the aforementioned types of grounding, another grounding system that often causes confusion in many situations is the power supply ground, also known as the AC power supply working ground. It is a type of grounding established within electrical systems for operational purposes (such as neutral point grounding)

This is from the internet, with no editing.

http://bbs.hcbbs.com/thread-1346201-1-1.html Take a look at this; search on the forum

Specifications for the grounding design of instrument systems HG/T 20513-2014