DCS computer cable installation specifications:
I. Signal Classification
Proper cable routing can effectively reduce external environmental interference with signals and mutual interference between various cables, thereby improving the stability of DCS system operation.
Signals are classified as follows:
1. Class I signals: Low-level signals such as RTD 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 switch input signals; contact-type switches.
Input signals; pulse input signals; 24VDC switching output signals for resistive loads less than 50mA.
3. Class III signals: Switch output signals for inductive loads of 24V～48VDC or resistive loads with current greater than 50mA.
4. Class IV signals: 110VAC or 220VAC switch output signals. The feeder for this type of signal can be treated as a power supply line. Among them, Class I signals are easily interfered with, Class II signals are easily interfered with, while Class III and IV signals become strong interference sources at the moment of switching action, interfering with nearby signal lines through the spatial environment.
II. General Principles for Signal Cable Laying
1. For Class I signal cables, shielded cables must be used, and shielded twisted-pair cables are preferred if possible.
2. For Class II signals, shielded cables should be used whenever possible. Specifically, shielded cables must be used for control, interlocking, and analog input/output signals within Class II signals; ideally, shielded cables should be used whenever possible.
Twisted pair cable.
3. Class IV signals must not be bundled with Class I or II signals. They should be treated as 220V power lines and routed together with power cables. Shielded twisted-pair cables are recommended when possible.
4. For Class III signals, they are permitted to be routed along with 220V power lines (i.e., the same as Class IV signals), or along with Class I and II signals. However, in the latter case, Class III signals must be routed with shielded cables.
The cable should preferably be a shielded twisted-pair cable, and should be kept at least 15cm away 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 ensure:
1. For millivolt signals and strain signals in Class I signals, shielded twisted-pair cables should be used. This can greatly reduce electromagnetic interference and electrostatic interference.
2. Where conditions permit, shielded cables (or shielded twisted-pair cables) should be used for Class I to IV signals whenever possible. It should also be ensured that the shielding layer is grounded at only one point and that the grounding is good.
3. High-power switch output signal lines, power lines, and other cables are prohibited from being bundled in parallel with Class I and II signal cables that directly enter the DCS system.
4. It is prohibited to use some of the cores in a multi-core cable to transmit Category I or Category II signals, and the remaining cores to transmit Category III or Category IV signals.
5. It is strictly forbidden to distribute the same signal cores in different cables (such as a three-wire RTD).
Therefore, during on-site cable laying, we must effectively separate Category III and IV signal cables, power lines, and other cables that are prone to interference, ensuring they maintain a certain distance from the Category I and II signal cables laid on-site.
Maintain a safe distance (e.g., 15cm or more).
III. Several Regulations for On-site Cable Laying
For ease of description, we will refer to Category I and II signal cables collectively as signal cables, and Category III and IV signal cables and field power cables collectively as power cables. Where possible, signal cables and power cables...
Cables should be routed in separate cable trays and kept as far apart as possible when entering the DCS room (or cabinet). If separate routing is not possible, both types of cables must be shielded cables.
(or shielded twisted-pair cable), and meets the following requirements:
1. If the distance between the signal cable and the power cable is less than 15cm, a shielding metal partition must be installed between the signal cable and the power cable, and the partition must be grounded. See Figure 1 for details (omitted).
2. When signal cables and power cables are installed separately in the vertical or horizontal direction, the distance between them should be greater than 15cm.
For applications with particularly high interference, such as when a power cable is connected to an inductive load with a voltage of 220VAC and a current of over 10A, and the power cable is unshielded, then it is required to ensure proper operation of the signal cable.
The vertical spacing between cables must be at least 60cm.
3. When two sets of cables intersect perpendicularly, if the power cables do not have a shielding layer as shown by the dotted line in the figure, it is best to cover the crossing part with an iron plate with a thickness of more than 1.6mm.
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