Some details on the cable wiring specifications for DCS systems

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　　I. Signal Classification 　A proper layout of cables can effectively reduce interference from the external environment on signals as well as interference between different cables, thereby improving the stability of DCS system operation. 　　Signal classifications are as follows: 1. Category I signals: low-level signals such as thermistor signals, thermocouple signals, millivolt signals, and strain signals. 　　2. Class II signals: 0–5V, 1–5V, 4–20mA, 0–10mA analog input signals ； 4–20mA, 0–10mA analog output signals ； Level-type digital input signal ； Contact-type digital input signal ； Pulse quantity input signal ； A digital output signal for resistive loads with 24VDC and less than 50mA. 　　3. Class III signals: Digital output signals for inductive loads operating at 24V～48VDC, or resistive loads with a current greater than 50mA. 　　4. Category IV signals: 110VAC or 220VAC digital output signals; the wiring for such signals can be handled as if it were power lines. 　　Among them, Type I signals are easily interfered with, Type II signals are also prone to interference, while Type III and Type IV signals become strong sources of interference at the moment of switch operation, interfering with nearby signal lines through the spatial environment. 　　 　　II. General principles for the installation of signal cables 1. For Category I signal cables, shielded cables must be used; where possible, shielded twisted pair cables are preferred. 　　2. For Category II signals, shielded cables should be used as much as possible. Among the Category II signals, those that are used for control and interlocking, such as input/output signals and input signals, must be transmitted via shielded cables; where possible, shielded twisted-pair cables are preferred. 　　3. For Class IV signals, it is strictly prohibited to route them together with Class I and Class II signals; they should be treated as 220V power lines and routed alongside the power cables. Where possible, shielded twisted-pair cables are recommended. 　　4. For Class III signals, it is allowed to route them together with 220V power cables (i.e., the same as Class IV signals), or they can also be routed together with Class I and Class II signals. However, in the latter case, Category III signals must use shielded cables, preferably shielded twisted pair cables, and should be kept at a distance of more than 15 cm from Category I and II signal cables. 　　 To ensure the stable, reliable, and safe operation of the system, the signal cables connected to the DCS system must also meet the following requirements: 1. For millivolt signals and strain signals in Category I signals, shielded twisted-pair cables should be used, as this helps to **reduce electromagnetic interference and static electricity interference. 　　 2. Where conditions permit, shielded cables (or shielded twisted-pair cables) should be used as much as possible for signals of categories I to IV. It is also necessary to ensure that the shielding layer is grounded at only one point, and that the grounding is proper. 　　3. It is strictly prohibited for high-power digital output signal cables, power cables, drive cables, and other such cables to run in parallel with Category I and II signal cables that enter the DCS system directly.** 　　4. It is strictly prohibited to use some of the conductors in a multi-core cable for transmitting Class I or Class II signals, while using other conductors for transmitting Class III or Class IV signals. 　　5. It is strictly prohibited for several wires of the same signal to be distributed in different cables (such as three-wire thermal resistors). 　　Therefore, during on-site cable installation, we must effectively separate cables that are prone to causing interference, such as Category III and IV signal cables and power cables, keeping them at a sufficient safety distance (such as 15 cm) from the Category I and II signal cables installed on site. 　　 　　III. Several regulations regarding the installation of cables on-site For simplicity, we refer to Category I and II signal cables collectively as signal cables, while Category III and IV signal cables together with on-site power cables are referred to as power cables. Where conditions permit, signal cables and power cables should be routed in separate trays, and they should also be kept as far apart as possible when entering the DCS room (or cabinet). When these two types of cables cannot meet the requirement for separate routing, both must be shielded cables (or shielded twisted-pair cables), and they must satisfy the following requirements: 1. If the distance between the signal cable and the power cable is less than 15 cm, a metal partition for shielding must be placed between them, and this partition must be grounded. See Figure 1 for details. 　　Figure 1: Cable separator. 2. When the signal cable and the power cable are installed separately, either vertically or horizontally, the distance between them should be greater than 15 cm. In applications where interference is particularly severe, such as when a Sensibility load with a voltage of 220 V AC and a current of over 10 A is connected to the power cable, and the power cable lacks a shielding layer, then the vertical distance between it and the signal cable must be at least 60 cm. 　　3. When the two cables intersect perpendicularly, if the power cable does not have a shielding layer as shown by the dashed line in the figure, it is advisable to cover the intersection area with an iron plate having a thickness of 1.6 mm or more.