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First, the reasonable arrangement of signal classification cables can effectively reduce external signal interference and interference between various cables, and improve the stability of DCS system operation.
The 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 signal; 4 ~ 20mA, 0 ~ 10mA analog output signal; level switch input signal; contact type switch Input signal; pulse input signal; 24VDC is less than 50mA resistive load switching output signal.
3. Class III signals: 24V to 48VDC inductive load or switching output signal with a current greater than 50mA.
4. Class IV signals: 110VAC or 220VAC switch output signals. The feed of such signals can be regarded as the problem of power line processing and wiring. Among them, Class I signals are easily disturbed, Class II signals are easily disturbed, and Class III and Class IV signals become a strong source of interference at the moment of switching action, and interfere with nearby signal lines through the space environment.
Second, the overall layout of the signal cable 1, for the I signal cable, must use shielded cable, shielded twisted-pair cable is the best condition.
2. For type II signals, shielded cables should be used as much as possible. Type II signals used for control and interlocking of the molded-in signal and the open signal must be shielded. It is best to use shielded twisted-pair cables when conditions are met. .
3. For Type IV signals, it is forbidden to be wired together with Type I and Type II signals. It should be treated as a 220V power cord and run along with the power cable. When conditions permit, shielded twisted-pair cables are recommended.
4. For Class III signals, allow to be routed with the 220V power line (that is, the same as the Class IV signal), or with the I and II signals. However, in the latter case, Category III signals must use shielded cables, preferably shielded twisted-pair cables, and they must be more than 15cm away from Category I and II signal cables.
In order to ensure the stable, reliable and safe operation of the system, the signal cables connected to the DCS system must also ensure that:
1. The shielded twisted pair cable should be used for the millivolt signal and strain signal in the Class I signal. In this way, electromagnetic interference and static interference can be greatly reduced.
2. When conditions permit, I-IV signals should be shielded cables (or shielded twisted-pair cables) as much as possible. It should also ensure that the shield is grounded only at one point and grounded.
3. It is absolutely forbidden that high-power switch output signal lines, power lines, and power lines are bundled in parallel with the type I and II signal cables that directly enter the DCS system.
4. It is absolutely forbidden to use some core wires in a multi-core cable to transmit Class I or Class II signals, and other core wires to transmit Class III or Class IV signals.
5. It is forbidden to distribute several cores of the same signal in several different cables (such as a three-wire RTD).
Therefore, in field cable laying, we must effectively separate III, IV signal cables, power cables, and other cables that are likely to cause interference, so that they can be kept at a certain safety distance from the cables of Class I and II signals laid on the site (such as 15cm) or more.
Third, on-site cable layout of several provisions For the convenience of description, we will be I, II signal cable collectively referred to as the signal cable, the III, IV signal cable and field power cable collectively referred to as the power cable. In case of conditions, signal cables and power cables should be routed in different cabling channels. When entering DCS equipment rooms (or cabinets), they should be far away from each other. When these two kinds of cables can not meet the separate wiring requirements, they must all use shielded cables (or shielded twisted pair cables), and meet the following requirements:
1. If the distance between the signal cable and the power cable is less than 15cm, you must install a metal partition for shielding between the signal cable and the power cable, and ground the partition. See Figure 1 (omitted) for details.
2. When the signal cable and the power cable are installed separately in the vertical or horizontal direction, the spacing between the signal cable and the power cable should be greater than 15cm.
For applications where the interference is particularly large, such as when the power cable is connected to a 220VAC voltage and the current is more than 10A inductive load, and the power cable is not shielded, the vertical distance between the signal cable and the signal cable must be 60cm. the above.
3. When two sets of cables intersect perpendicularly, if the power cable is not shielded, as shown by the dashed line in the figure, it is better to cover the intersection with an iron plate with a thickness of 1.6 mm or more.
DCS Computer Cable Installation Specifications: