Distributed control systems (DCS) connect to field detection devices and actuators via numerous low-level I/O signal cables, 110V and 220V AC/DC switching signal cables, and then communicate with distributed control equipment in the field via a network. Therefore, a well-organized and orderly layout of various cables in the field, along with effective grounding techniques, will improve the DCS system's anti-interference capability, reduce its impact on the external environment, and enhance its operational safety and reliability.
I. Signal Type
Class I signals: Low-level signals such as RTD signals, thermocouple signals, millivolt signals, and strain signals.
Class II signals: 0～5V, 4～20mA, 0～10mA analog input/output signals, level-type digital input/output signals, contact-type digital input/output signals, and pulse input/output signals.
Class III signals: Switch output signals for inductive loads of 24-48VDC or resistive loads with operating current greater than 50mA.
Class IV signals: 110VAC or 220VAC switch output signals.
Among them, Class I and Class II signals are easily interfered with, while Class III and Class IV signals will become strong interference sources at the moment of switching action, interfering with nearby signal lines through the spatial environment.
II. Precautions for cable laying
1. First, choose products with high anti-interference capabilities, including electromagnetic compatibility, especially the ability to resist external interference. Understand the anti-interference specifications provided by the manufacturer, such as common-mode rejection ratio, differential-mode rejection ratio, and withstand voltage.
2. During the project implementation process, proper layout can effectively reduce external environmental interference to signals and mutual interference between various cables.
(1) For Class I signal cables, shielded cables must be used, and shielded twisted-pair cables are preferred when conditions permit. The cable shielding layer must be grounded at one end. When the shielded twisted-pair cables of multiple measurement point signals are connected to the multi-core twisted-pair shielded cable, each shielding layer should be connected to each other, and an appropriate grounding point should be selected to achieve single-point grounding.
(2) For Class II signals, shielded cables should be used as much as possible. Among them, the analog input and output signals and switching signals used for control and interlocking in Class II signals must be shielded cables. Shielded twisted pair cables are preferred when conditions permit.
(3) Class III signals must not be bundled together with Class I and II signals. They should be routed together with 220V power lines and cables. If possible, shielded twisted-pair cables are recommended.
(4) For Class III signals, it is permissible to run them together with 220V power lines (i.e., the same as Class W signals), or they can be run together with Class I and II signals. However, in the latter case, Class III signals must use shielded cables, preferably shielded twisted-pair cables, and must be kept at least 15cm away from Class I and II signal cables.
It is absolutely forbidden to use only some of the cores in a multi-core cable to transmit Category I or Category II signals, and use the remaining cores to transmit Category III or Category IV signals.
It is strictly forbidden to distribute the same signal cores in different cables (such as a three-wire RTD).
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