Some suggestions from Siemens regarding the grounding of DCS control systems

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Numerous field experiences have shown that DCS systems using a \"floating ground\" approach have a higher failure rate compared to those with a \"grounded\" system; therefore, the Siemens Automation Systems Grounding Guide recommends that, except in special circumstances, DCS control systems should be grounded using the \"grounded approach\". 1. Grounding requirements for S7-400H: The grounding requirements for the S7-400H are the same as those described in Changhui Instrument Network’s technical article “Grounding Requirements for S7-400 PLC”, as shown in Figure 1. http://yunrun.com.cn/upload/201708/08/201708081549163252.png Figure 1: Grounding principles for the S7-400H. Additionally, the grounding requirements for the ET200M rack and IM153-2 are the same as those for the S7-300 PLC, as shown in Figure 2. http://yunrun.com.cn/upload/201708/08/201708081552558088.png Figure 2: Schematic diagram of grounding for the ET200M. As can be seen from Figure 2, in the Siemens system, the PE conductor serves both as a protective ground and, due to the characteristics of the ET200M series modules, it also acts as the reference potential for the system’s operation. Thus, the PE conductor performs the function of a system reference point; the quality of its grounding has a direct impact on the stability of the system. The grounding busbars in the system cabinet should be made of tinned copper bars, and the cable connections for the grounding wires should also be properly pressed, as shown in Figure 3. http://yunrun.com.cn/upload/201708/08/201708081554257004.png Figure 3: PE grounding bus inside the electrical cabinet. 2. Grounding principles for DCS: For the grounding method of DCS, please refer to Figure 4. http://yunrun.com.cn/upload/201708/08/201708081556336301.png Figure 4 Schematic diagram of grounding for the DCS control system. ① Requirements for grounding resistance ◆ The total resistance of the conductors and connection points from the grounding terminals of the instrumentation equipment to the main grounding plate is referred to as the connection resistance. The grounding connection resistance of the instrument system should not be greater than 1Ω. ◆The ratio of the potential of the grounding electrode to the current flowing into the ground through it is called the resistance of the grounding electrode to ground. The ground resistance is defined as the sum of the resistance of the grounding electrode to the ground, and the resistances of the total grounding plate, the main grounding conductor, as well as the connection points at both ends of the main grounding conductor. The grounding resistance of the instrumentation system should not exceed 4Ω. ②The recommended colors for grounding wires are listed in Table 1. Table 1: Colors of Grounding Wires in DCS Control Systems. Purpose of the grounding wire: Grounding wires used in the DCS control system. Colors: Green – for grounding connections, busbars, branch wires, and main grounding lines related to protective grounding. Green + Yellow – for branch wires and busbars related to intrinsically safe grounding. Green + Yellow again – for the main ground plate, main grounding wires, and grounding electrodes. Green. ③ Specifications for grounding connection wires in DCS control systems: The wires in the grounding system should be multi-strand twisted copper-core insulated wires or cables. The wires for the grounding system should be selected according to the number and length of the instruments connected, using the following values: ◆ Grounding wires > 2.5–4.0 mm2 ； ◆Ground branch cable > 16-35mm2 ； ◆Ground main > 35-75mm2 ； This excerpt is taken from the book \"Siemens Automation System Grounding Guide.\" Changhui Instrument Network shares this book free of charge; it can be obtained by contacting customer service.